The Go Modem Protocol

This Go modem protocol was developed by Bruce Wilcox, with input from David Fotland, Anders Kierulf, and other computer Go Programmers. It will be avilable in the next releases of NEMESIS and Many Faces of Go [ It is included in Many Faces of Go. Bruce Wilcox is no longer associated with NEMESIS. ]. We hope that all Go programs will implement this protocol so that all Go program users will be able to play go by phone, no matter which program they own, and so that computer/computer competitions can be played without needing an operator to type moves back and forth.

The protocol is followed by the code used to implement it from Nemesis and Many Faces. We make this code available royalty free, for use for any purpose, commercial or otherwise.

NEMESIS and Many Faces will use this protocol in the upcoming North American Computer Go Championship at the Go Congress. We hope that other programs at this competition will also implement it.

Bruce volunteers to help with final testing of your modem protocol code.

— David Fotland (Author of Many Faces of GO)

Some files for using the Go Modem Protocol in Windows can be downloaded from here.

Standard Go Modem Protocol – Revision 1.0

The Standard Go Modem Protocol is intended to facilitate computerized remote Go play and discussion between two players. Tranmission requirements are: 8 data bits, 1 stop bit, no parity. Baud rate to be agreed upon by the players. The protocol consists of two parts: a message format and a usage protocol.

The protocol is maintained by Bruce Wilcox of Toyogo, Inc. If you need new codes (e.g., query, extended command) assigned, call or fax for them. In North America call toll-free 800-869-6469. Otherwise call 808-396-5526 or Fax 808-396-4126. Or write to: PO Box 25460, Honolulu, HI 96825

Features of the protocol:

  1. Easy parsing. Independent of any prior characters, you can distinguish talk text, start of packet message, and other packet bytes. Knowing when a message is done is easy. All required commands take 4 bytes. Required input buffer size is only 4 bytes. Additional incoming characters can be discarded or, if talk, echoed to user's talk window. Format does not conflict with control characters (e.g., Ctrl-S and Ctrl-Q).
  2. Strong text support. Text can be optionally checksummed, if getting it correct is important. Support for multi-byte foreign languages.
  3. High reliability. Checksum is large relative to packet size, and additional checks exist on some critical commands. Packets are small. Programs can also autoverify board and time correspondence.
    Programs detecting errors of other program or unable to handle messages of some kind can tell foe via DENY.
  4. Minimal modem traffic. No continuous token passing, only passing signals to execute and acknowledge commands.
  5. Extensibility. Provisions for program specific extensions (via Extension command).
  6. Future Compatibility . Ability to recognize limitations of foe program and retain backward compatibility in the face of protocol revisions via QUERY and ANSWER comands.
  7. Simple code. The minimal set is easy. Send and receive {OK, MOVE, TAKEBACK, NEWGAME}, respond with ANSWER of 0 to any QUERY, and cancel any command you have pending if you receive a DENY. Don't do anything with incoming talk, or transmit outgoing talk.

Message Format

The standard packet message is 4 bytes, consisting of a header byte with sequencing information, a checksum byte, and a two-byte command. Individual text bytes intended as optional TALK for the other's talk window may be sent outside of the packet and are not subject to synchronization and checksum protection. However if the language involved requires multiple bytes per character (e.g., Japanese), such messages must be transmitted using the extended String command to insure character sequencing is preserved. All packet bytes but the packet start byte have their top (sign) bit turned on. The discussion below assumes the two machines/players are labelled you and him.

       1. Start Byte – 0000 00hy
The start byte has 6 fixed bits, his sequence bit (h) and your
sequence bit (y). The sequence bits  determine if his incoming
command is old or new, and whether he has seen or not seen your
most recent command sent to him. Maximizing the fixed bits
minimizes mistaken start byte recognition. If a new start byte is
detected before the full 4 bytes of a previous packet are read, the
old bytes are discarded.

       2. Checksum Byte – 1sss ssss
Top bit always on. Checksum (s)  of any message (including the
variable length Extended command)  is created by summing into
unsigned int  the 1st, 3rd and 4th message bytes, and using only the
bottom 7 bits. If a program receives a command with a bad
checksum, it should just discard the message.

       3,4 Command 2-byte – 1ccc rvvv 1vvv vvvv
Top bit of each byte is always on. ccc is 3-bit command (8 basic
commands). vvv vvv vvvv is a 10-bit value for use with commands. A
reserved (r) bit of 0 separates the two fields. TEST FOR IT. Future
commands or data or whatever may use it and break your code if you
don't.  Types are:

       0. OK – 1000 0111 1111 1111
Value must be all 1's.  OK means I got your command ok, and is given
whether or not the command is executed without error by recipient.
This releases sender, who cannot transmit  any new commands until
he receives it. ANYTIME you have been sent a command and are
expected to send an OK, you may instead send a command with a new
sequence number for you and his current sequence number.  This acts
as an implicit OK, and a command to him which cannot be in conflict
with a new command from him (see CONFLICT at the end of command
list).  This property is explicitly used in the Query-Answer protocol.
DENYis an alternate response instead of OK. OK is not a true
command. You do not send back an OK in response to OK. No response
is needed or expected. You also do not send back OK in response to a
QUERY. You must send an ANSWER command.

       1. DENY command – 1001 0000 1000 0000
If you wish to reject a command (can't do it or whatever), you can
deny it by sending back the DENY command instead of an OK. He
should treat your deny just as he would conflict, by retracting his
command. However he does not revert his or your sequence ids. As a normal
command, DENY is rebroadcast until he sends you an OK (explicit or
implicit), and he will undo his command and stop broadcasting it to
you whenever he sees your denial. Don't DENY a denial. If a command
is part of a series of commands to be sent (not a part of an extended
multiple command), the sender should cancel sending the remaining
queued up commands. E.g., if a game record is being sent as a series
of commands, sending it should stop as soon as a move is denied. If a
denial is given on an extended multiple command, all of its
subcommands must be undone.

       2. NEWGAME command – 1002 0000 1000 0000
Clear your board back to empty, and clear all query knowledge. The
sender guarantees he has the correct conditions. A good (but not
required) response to NEWGAME would be to use QUERY instead of OK,
to discover the playing conditions. If you are unable to query, it is
presumed the conditions are already set up correctly.  As soon as the
OK is complete (either to NEWGAME or to a series of queries
followed by the final answer) Black may send his first move.  It is
important to wait for an OK, instead of embedding the first move
command as an implicit ok, to allow both sides to complete any
querying they want to do. If a DENY is received in the query process,
the game is aborted. Whoever is BLACK should send the NEWGAME on
startup, but the recipient should not assume from the receipt of
NEWGAME that he is WHITE, but should use the query system instead
or have been set up. [If two humans play, then it doesn't matter who
initiated the NEWGAME, but if two computers are playing, you want
the game to automatically start in an orderly fashion. Therefore,
behavior on startup is that whoever is to be BLACK send the
NEWGAME, and whoever is to be WHITE sends no commands until it is
received (he may talk all he wants). ]

NEWGAME can be sent in the middle of a game, in which case it
terminates the game
and starts another (game parameters determined by sender).

       3. Query command – 1011 0sqq 1qqq qqqq
QUERY. The command asks a question, and the response comes in a
corresponding Answer command. Do not send an OK when you receive
a query. Send only an ANSWER. Otherwise your answer might get
conflicted and be lost.  When an answer is received by the querier,
he, in turn, will need to acknowledge it with an OK.  If the querier
does not like the answer he got, he should send back a DENYinstead
of an OK, which indicates the answer is unacceptable. The only query
with a fixed meaning is the "What game are you" query. The meaning
an interpretation of all other queries depends upon the game
selected. Toyogo is not a clearinghouse for the meaning of queries in
games other than Go.

Bit s is on if the question is "Do you support extended command q?"
Answer is:   15 – the answer is Yes    0 – the answer is No.

Bit s is off if the question is one of the following:

       0 = What game are you playing?
Answer 0 = unknown 1 = Go  2 = Chess  3 = Othello . If your program
only supports one kind of game, this question is unimportant to it.

       1 = How big is your modem buffer ?
Answer is multiplier of 16 bytes above the minimal 4 byte buffer.
E.g., 0 = 4 byte minumum  1= 20 bytes  2= 36 bytes

       2 = What version of the protocol do you understand?
Answer is 0, the initial version.

       3 = How many stones on your board?
Answer is 0 is if you have no answer, or else its the number of
stones on the board after you finish executing any command awaiting
OK.  Don't ask before the first voluntary move has been played;
otherwise the answer is not well defined (he may not have put
handicap stones on the board yet).

       4 = How much time has Black spent ?
Answer is 0 if you have no answer, or the number of minutes
(rounded) spent by Black. There is no requirement that the Queryer
use the answer. If he does, the recommended use is to set Black's
clock to the minimum of the answer and the Queryer's own value.
Time spent is the question, not time used, for two reasons. First, the
value field is not interpreted as a signed number and Black could
have spent his entire time allotment (be in byo-yomi or worse).
Second, programs do not necessarily support the same time limits as
interface choices. But if they support time, they know how much has
been spent. Range is up to 1023 minutes (about 17 hours), error
between programs is no more than 1 minute.

       5 = How much time has White spent?
Same concept as 4 above.

       6 = What character set do you use?
Answer: 0 = unknown  1= English (ascii)  2= Japanese.  Other codes
assigned on request.

If the querier can match the language he should do so. If he cant,
text defaults to the character sets of each machine (best of luck). If
the language is a multi-byte character language (e.g., Japanese), text
sent outside of the packets is in English. Text sent using the
extended String command is multi-byte oriented (format currently
unspecified) and represents that language.  Accidental dropping of 1
byte of a multibyte string destroys the meaning of all other bytes,
hence passing multi-byte languages via the checksummed packet is
required. The notion of character set is used in supporting talk
between two programs echoed into a talk window. Supporting talk is

       7 = What rules are you using?
Answer: 0 = unknown 1 = Japanese 2 = Ing Chinese (SST laws).

       8 = What handicap are you set for?
Answer: 0 = unknown 1 = even 2..n are handicaps.  See discussion of
handicaps under MOVE command.

       9 = What is the board size?
Answer: 0 = unknown.  N > 0 is NxN board size.

       10 = What is the time limit per player
Answer: 0 = unknown.  N>0 is time in minutes.

       11 = What color is the computer playing on your side.
Answer: 0 = unknown. 1 = White 2 = Black

        12 = who are you ?
Answer is:. 0- unknown  1- NEMESIS   2- MANY FACES OF GO  3-
POLAND.  Other ids will be assigned in future on demand. Contact
Toyogo. If a program needs to identify its version number, a new
Version # query will be created.

SETUP AGREEMENT:  The game setup parameters can be established
between the two programs by query/answer. Whoever gets the first
query command through can thereafter read off all the settings of
the other program and either verify that they  match, set himself up
accordingly, or issue a DENY.  If the querier gets back a 0 response,
either he doesn't care, or he stops proceeding into the game.  How
the initial query command occurs and by whom is not specified
(since it is not required to happen), but subject to program
implementation. Be reminded that this feature may not even be
supported by a program. In that case it is the responsibility of the
two humans to see that the programs are set up correctly.

       4. Answer command – 1100 0aaa 1aaa aaaa
THE ONLY REQUIRED ANSWER IS 0, if you haven't implemented any
other answers. Sent back in response to QUERY, instead of OK. See
Query for interpretation of a. A program is not required to QUERY,
nor to do anything with the answer.   If you receive an ANSWER, you
must acknowledge it with an OK.

       5. Move command – 1101 0pii 1iii iiii
Interpretation of the move command is game specific. For the game
of Go, top value bit (p) is 1 if player to move is white or 0 if black.
9 bit value (i)  is a board intersection value or 0 for pass.
Intersection values run 1 at A1, 2 at B1 ... with value wrapping at
end of row (varies with board size) so next row is A2.  Range for
19x19 board is thus 1...361 (A1 ...T19). A1 is in the lower left corner
and the letter I is omitted.  A program is required to
remember all the moves from the root position to the current
position. It may, but is not required to, remember moves played later
than the current position which have been taken back.  A  move
always extends the current record of moves as the next move in
sequence. A dumb program keeps no history of moves taken back, so
extending the current move sequence is a simple matter. Complex
programs may track variations or continuations, and may need to
decide whether such a sequence is a replacement of the retracted
moves, or a variant or what.  Such decisions are the domain of the
individual program. The program must only insure that the board
stays consistent between the two programs. Also, while one may
assume a program will not transmit an illegal move, the receiving
program may check for that and issue a DENY instead of an OK if such
a move is detected.

Moves transmitted are moves that players (human or computer) have
a choice about. See handicaps.

Handicap moves:
Handicap stones are a particularly thorny problem. The underlying
premise of the protocol is that  a pair of computers is shared
cooperatively. Either player may place any color stones at any time,
so there is no inherent program knowledge of who is Black and who
is White. Therefore it is not clear who should send handicap moves
to whom. Both programs might send the other messages about
handicap stones, either in lump, in conflict, or intermixed with each
other.  Also, programs may represent handicap moves as either a
contiguous sequence of Black moves, or a sequence of a Black move
and White passes. Also handicap stones may be represented
internally from other moves. The receiving program cannot tell what
kind of move a stone is unless it knows the handicap in advance. So,
with that can of worms in mind:

  Japanese Handicap Rules:
       Under Japanese rules, programs DO NOT transmit handicap setup
stones. The first move transmitted will be White's first move. The
placement of handicap stones for handicaps greater than 1 under
Japanese rules on a 19x19 board use these intersections in order: D4
Q16 D16 Q4 (K10 when handicap is odd)  D10  Q10 K4 K16. E.g., 2
stones at D4 & Q16, 5 stones at D4, Q16, D16, Q4, K10, 6 stones at
D4, Q16, D16, Q4, D10, Q10. On odd board sizes of 13x13 and up, the
corner handicaps are always on the 4th line and the center . On the
9x9 and 11x11, handicaps are on the 3rd line and the center. Order of
placement always mimics the 19x19 case. Handicaps on  even board
sizes, sizes under 9x9, or handicaps beyond 9 stones are not
predetermined by the standard.

Reminder, if you transmit the moves of a game record, don't
transmit the handicap stones under Japanese rules .

  Chinese Handicap Rules:
       The handicap passes by White are not transmitted.

Transmitting game records:
How a handicap game record under Japanese rules can be transmitted
depends upon the sophistication of the receiving program. To
transmit a game record, first transmit the NEWGAME command. Then,
if the receiving program is sophisticated, it can query for the
handicap and set itself up appropriately. The handcap under Japanese
rules is not transmitted. If the receiving program is dumb, it will
not know about the handicap.  So you determine if he has asked you,
and if not, you should ask him about his handicap prior to sending
your recorded moves. If his handicap is set up correctly, send the
moves w/o the handicaps. If his handicap is anything but correct,
decline to send. If you want, if his handicap is even, you could send
him all moves including the handicap, but the standard does not
require it. If you want to take the lazy approach, always transmit
the game as an even game.

       6. Take back move command – 1110 0ttt 1ttt tttt
The meaning of taking back a move is game specific. In Go, it is
taking back a move a player had a
choice about. Under Japanese rules  Black's handicap moves are fixed
and cannot be taken back.  Under Chinese rules White's passes are
required. Hence, taking back a move means removing a Black handicap
stone, and any White passes needed to accomplish this.

Value t is how many moves to retract, and ranges from 0 to 1023.
Taking back 0 has no effect. A turn may consist of multiple moves
(e.g., the SETUP command in "Standard Format" for text records. The
meaning of take back is retracting a move, not a turn.  If a program
is unable to obey the Take back move command given it,  it may
respond with a DENY instead of an OK.

       7. Extended command – 1111 0mmm 1mmm mmmm   1nnn nnnn 1sss ssss

command unless you have queried and discovered that recipient
supports it. Also be aware that receiver's buffer size may not fit
everything you want to send and you might have to break it into
smaller chunks. You should determine his buffer size with a QUERY command.

The value of the extended command is how many MORE bytes follow
after the basic four. At a minimum, the value is two.  The first
following byte has the extended command name (n). The second
following byte is a checksum (s) computed for the extended part of
the message (bytes 5, 7...4+m).  Programmers wishing an assigned
name can contact Toyogo. Commands created can be optionally
supported by more than one program. Programs are not required to
support extended commands. They may discard such a message if
they wish, but if queried about such support they
must be able to answer NO.  Current extended command names are:

       0 – String 1000 0000
The contents are to be displayed as talk output. They are analogous
to text sent outside of packets except 1) they are secure
(checksummed) and 2) they can represent multi-byte languages if

       1. Replay 1000 0001  1nnn nnnn  1nnn nnnn
If a program keeps track of moves taken back, then this command is
the inverse of TAKE BACK. The third and fourth extended bytes (n) are the
count of how many moves to replay.

       15 – Multiple 1000 1111
The contents are multiple commands (without individual header and
checksum bytes) intended to be executed as one. The purpose is to
speed up response to complex sequences of actions, and/or to insure
control is retained by sender for the duration of the sequence. OK,
DENY, QUERY, ANSWER, and the multiple extended command MAY NOT
be used within a multiple command.

CONFLICT – If both players send commands at each other at the same
time, this is detected as a conflict. Conflict is detected when you
receive a new command from him while you are awaiting his OK, and
the new command does not have your current sequence number.
Programs detecting conflict are required to retract their command
(decrement their sequence id, stop waiting for an OK on their
command, and undo any effects their command had on them.  E.g., if
they placed the move of the MOVE command, they should unplace it.
It is not essential that both programs detect the conflict, though
they will if there are no tranmission problems.

A program which is in the middle of a sequence of commands it
wishes to send, may decide to retry transmitting its conflicted
command after a random delay (so the two programs don't stay in
conflict).  If it receives an intervening foe command before it has
completed its own list, it may issue a DENY if accepting his
command would be a problem.  Issuing a DENY prohibits him from
continuing his attempt to send you his command.  If he sends you a
DENY in the middle of your sequence, you must stop sending further
commands from that sequence. DONT DENY A DENY.

on a regular basis to check that the stone count matches and keep
the player times in synch.  This can be done automatically by the
program, but such commands should be kept from interfering
(CONFLICT) with user commands without good cause.  To avoid
conflict with foe user, send an automated QUERY whenever you would
send an OK.  You are not safe from your own user unless you buffer
his command for execution for when the query/answer/ok is
complete or otherwise prevent him from executing a command until

TALK TEXT outside of packets are sent with top bit off (7bit ascii
notation). CTRL G must make sound or flash or equivalent when
printed into talk area. CTRL-H  (Ascii 8, backspace) should have the
correct effect on both machines. Text is intended to be echoed into
the talk window of the other machine. Usually you will also echo the
talk into your own window as it is typed. Supporting talk windows is
not required, and a program may just discard incoming text if it

INITIAL CONDITIONS – The protocol requirements for using the
format are as follows:
1. Sequence IDs  for you and he both start at 0.
2. Your sequence ID is incremented before creating a new non-OK
message, and is incorporated into the message. THE FIRST MESSAGE
EVER SENT WILL HAVE A START BYTE OF 0100 0001, which says foe's
last message was 0, and my new sequence value is 1.
3. Any program can spontaneously send the first command.  You
should assume a program  cannot accept any extended commands and
answers 0 to any query until you query successfully otherwise.
4. His sequence ID is taken from the incoming command after it is
successfully validated by you.
5. When you transmit a non-OK command, you are prohibited from
transmitting any NEW commands until you receive or infer an OK
from an incoming message of his.
6. After you transmit a non-OK command, if you receive no
acknowledgement from him within some time period, you should
retransmit the same message. Retransmission changes no sequence

PARSING:  Incoming characters fall into 3 classes: text, start byte,
other command bytes.  Text is any non-start byte having the top bit
off.  If you detect other command bytes and have no corresponding
start byte, just discard the extraneous command bytes.

You have just received a message. Your current sequence id is B of
{A,B}. His last sequence ID you saw was 2 of {1,2}.  What should you
do and what does it mean?

Neutral:   (sitting idle with no transmission,  you have no unfinished

       indicates repeated OK. Others impossible. Discard all.
       New or Old command not possible to receive.  Discard.
       Normal New command. Do it + send back OK(before or after).
       Old Command . He hasnt seen my OK yet. Resend last comand
       (implicit OK or real OK).

OKWait:    (retransmiting your  command every n seconds until
OK received or  implied)

       Not possible. Discard.
       Normal OK. Terminate OK Wait (Go to neutral state).
       New Command from him, he hasn't seen my command yet.
       This is conflict. Do not respond with an OK! Undo my command,
       revert my sequence id, don't update his id, and go to neutral. If he
       sees my command, he will do likewise.  If he doesn't see my
       command he will rebroadcast his and win the toss.
       Old Command, he hasn't seen my new command. He was unaware of
       my OK at that time. Discard. Retransmit your command ahead of
timeout rebroadcast if you want to.
       New Command from him, but he has seen my command. He must
       have sent OK and I missed it or has some reason to insure we
don't conflict. Go to neutral state and then do command+ send
       back OK (before or after).
       Not possible. Discard.

The protocol allows a program to make its own choice about when to
send a command or an OK. The most simple & inefficient way is to
send the command, wait for his OKand then execute your command. If
he sends his OK after he has completed execution, that means
delaying the time of two machines implementing the command.
However, if you do not wait for his OK before executing your
command, you must be prepared to undo the command if conflict
arises. If he can send the OK upon successful receipt of your
command, both of you can process the command in parallel.

Programs may also choose their own rebroadcast frequency. Several
seconds should be allowed for the receiving program to execute and
acknowledge the command, but there is no requirement, since
thereceiver can just discard any messages it wishes.

If both programs generate an automatic QUERY or NEWGAME on
startup the two programs may get a CONFLICT. If the programs back
off and try again later, they may still conflict.  The protocol does
not prevent an infinite loop in such a situation. Our advice is that if
you CONFLICT and you are using a loop to try again, you should select
a random delay within an ever increasing delay range.  You may need
to delay longer than the other program's rebroadcast delay to
succeed.  Alternatively after n conflicts without an accepted
command on either side, you might stop and report to the user. He
should be able to reinitiate the loop at his discretion.  We
recommend that the first command sent be a NEWGAME command by
whoever is Black. The receiver can query to establish game
parameters, and then Black can send his first move.

This is the code used by Nemesis to implement this protocol, written for
Lightspeed C on the Mac.

#define MaxBuffer (96 + 5)         /* 1 byte for length code, the rest for buffer */
/* the code allows room to listen to some extended commands, but doesnt use them */
#define Obuffer 5    /* we only write 4 byte messages (+ length) */
INT pending = 0;                   /* characters expected to receive to finish packet*/
#define CTRL_G 0x07         /* talk recognizes CTRL_G as sound */
/* 8 Commands are in COMMANDBITS */
#define COMMANDBITS 0x70           /* mask holding all commands */
#define OKMSG 0x00
#define DENY 0x10
#define RESET 0x20
#define QUERY 0x30
#define RESPONSE 0x40
#define MOVE 0x50
#define TAKEBACK 0x60
#define EXTENDED 0x70
#define STRING 0x00
#define REPLAY 0x01
/* QUERY types are */
#define FEATUREBIT 0x0400 /* asks if you support extended command named */
#define GAMEID 0                   /* what game  1 = go */
#define HOWBIG 1                   /* your buffer size in 16byte units above 4 byte minimum */
#define PROTOCOLID 2        /* Currently Protocol version is 0 */
#define BOARDVERIFY 3              /* number of stones on board after current command finished.*/
#define BLACKTIMEUSED 4            /* seconds spent by black */
#define WHITETIMEUSED 5            /* seconds spent by white */
#define LANGUAGE 6                 /* what character set */
#define WHATRULES    7                    /* what rules */
#define WHATSIZE 9
#define WHATTIME 10
#define WHATCOLOR 11
#define WHOAREYOU 12               /* what program. I am NEMESIS */
/* answers are */
#define NEMESIS 1
#define WHITEMOVE 0x0200    /* bit on move to indicate White color */
static unsigned char nconflicts = 0; /* conflicts in progress */
#define RETRYDELAY (4 SECONDS)            /* how long I wait for OK before rebroadcasting */
#define QUERYDELAY (120 SECONDS)   /* how often I poll to keep time consistent */
unsigned char outmbuffer[Obuffer],inmbuffer[MaxBuffer];
long timesent = 0;          /* to tell when to rebroadcast */
static char oursequence = 0;       /* our last sequencing bit */
static char hissequence = 0;       /* his last sequencing bit */
static long timequery = 0;         /* to tell when to reverify */
static int lastquery = -1;         /* If i have a query awaiting an answer */
static unsigned int querycnt = -1; /* for cycling thru my queries */
static unsigned int movecnt = -1;  /* to tell when to rebroadcast board verify */
INT modemmove = -1;         /* echo expected for this location, dont send it out */

VOIDFN OutModem (echo) INTM echo; {/* transfer serial output*/
       long count = (long)outmbuffer[0]; unsigned int pt,val;
       /* send new message */
       outuser(M_FIXTIME,0,0); /* update time */
       timesent = time_count; /* when we sent the message*/
       if (FSWrite(ModemOut, &count, outmbuffer+1) != noErr) return -1;}

int InModem () {/* transfer serial input into input buffer*/
       long one = 1,cnt; int type,val; unsigned char c;
       SerGetBuf(ModemIn, &cnt); /* bytes available to be read */
       while (cnt-- > 0) { /* read all available characters in buffer */
              if (FSRead(ModemIn, &one, &c) != noErr) {/* failed to read */
                     return -1;}
              if (c > 3 && c < 128) {/* talk character */
                     if (pending) pending = inmbuffer[0] = 0;

              /* start of packet recognized – will need 4 bytes */
              if (c < 4) {
                     pending = 4;
                     inmbuffer[0] = 0;}

              /* Have a command byte, discard if not expected, absorb if expected*/
              if (pending){
                     inmbuffer[++inmbuffer[0]] = c; /* move it to buffer */
                     if (inmbuffer[0] == 4) { /* have 1st 4 bytes complete */
                            if (!checksum()) { /* BAD CHECKSUM */
                                   inmbuffer[0] = 0;
                            type = gett(inmbuffer); /* lets see if it is an extended command */
                            val = getv(inmbuffer); /* and get size */
                            if (type == EXTENDED) {
                                   if ((val + 5) <= MaxBuffer) pending = val;/* accept read*/
                                   else {
                                          senddeny("extended too big");
                                          inmbuffer[0] = 0;}}}}}
       return dominput();}/* see what you have */


INTFN resetmodem(
       ) {/* clear the modem interface */

       modemwaiting = inmbuffer[0] =      oursequence = hissequence = 0;}
static VOID mylisten(text) char *text; {
       listen(text);} /* this sends text to my talk window */
static VOID myoutmodem(n)  INTM n; {
VOIDFN SendTalk(c) char c; {/* our keyboard letter we sent to us and him */
       char msg[2];
       msg[0] = c;
       msg[1] = 0;
       mylisten(msg); /* tell my talk */
       outmbuffer[0] = 1;
       outmbuffer[1] = c;
       myoutmodem(FALSE);} /* tell out modem */

static void xmit(OK) INTM OK; {/* add header into message */
       unsigned int sum;
       outmbuffer[0] = 4; /* message is this long */
       if (!OK) oursequence ^= 1; /* flip our sequence */
       outmbuffer[3] |= 0x80; /* required  bit */
       outmbuffer[4] |= 0x80; /* required bit */
       sum = outmbuffer[1] =  (hissequence << 1) | oursequence;
       sum += outmbuffer[3] + outmbuffer[4];
       outmbuffer[2] = sum | 0x80; /* put out checksum */
       myoutmodem(TRUE);} /* send the completed message */

static void sendcmd(cmd,val) INTM cmd,val; {/* send out command */
       if (cmd == QUERY) lastquery = val;
       outmbuffer[4] = val & 0x7f;
       outmbuffer[3] = (val >> 7) & 0x03;
       outmbuffer[3] |= cmd;
       modemwaiting = 2;}

VOIDFN senddeny(msg) char *msg; {
       /* msg just tells us why we are denying him */

static void sendok(type) INTM type; {/* ok his last message */
       nconflicts = 0;

       /* automatic polling, unless he is querying us */
       if (type == QUERY); /* requires RESPOND and not ack so dont */
       else if ((movecnt % 10) == 9) { /* auto poll for stone count periodically */
              sendcmd(QUERY,BOARDVERIFY);/* doublecheck the board */
       else if (G(NOWPLAYING) == S_PLAYING && (time_count – timequery) > QUERYDELAY) {/* low level status first */
              sendcmd(QUERY,(querycnt & 1) ? BLACKTIMEUSED : WHITETIMEUSED);
              timequery = time_count;} /* dont query again for a while */
       else {
              outmbuffer[3] = 0x07; /* ack command, reserved bit, and message */
              outmbuffer[4] = 0xff; /* ack message */
              xmit(TRUE);}}/* send OK */

VOIDFN SendMove (
       pt,multi) INTM pt,multi; {/* move encoded color etc */
       register INT loc = LOCATION(pt), color = WHOPLAYED(pt),where; char junk[30];
       where = loc;
       /* if move is during handicap setup, dont echo forced moves */
       /* tested before move is placed on board */
       if (G(HANDICAP) > 1 && gethandicap() < G(HANDICAP)) return TRUE; /* not enough black stones yet*/

       if (where) where = X(where) + ((Y(where) – 1) * G(BDSIZE)); /* grid ref */
       ++movecnt;           /* autopoll of stone count*/
       sendcmd(MOVE + ((color == WHITE) ? 0x04 : 0),where);}

VOIDFN SendNewGame(
       ) { /* clear */


VOIDFN SendUnmove(
       n) INTM n; {/* retract this many moves */

       ++movecnt;                  /* autopoll stone count */
       sendcmd(TAKEBACK,n);} /* only done by a user anyway */
VOIDFN rawabsorb(c) unsigned char c; { /* echo what he ships immediately*/

       char msg[2];
       if (c == CTRL_G) outuser(DOSOUND,OTHERBEEP,0); /* sound beep  */
       else  {
              msg[0] = c;
              msg[1] = 0;
              mylisten(msg);}} /* says his message */

INTFN checksum() {
       unsigned char c;
       c =  ((inmbuffer[1] + inmbuffer[3] + inmbuffer[4]) | 0X0080) & 0x00ff;
       return (c == inmbuffer[2]);}

INTFN gett(buf) unsigned char *buf; {
       return buf[3] & COMMANDBITS;}      /* 3 bit type */
INTFN getv(buf) unsigned char *buf; {
       return (buf[4] & 0x7f) | ((buf[3] & 0x07) << 7);} /* 10 bit value field */
static void undolast(deny) INTM deny; {/* take back last command */
       register INT type = gett(outmbuffer), val = getv(outmbuffer); INT tmp;
       lastquery = -1; /* there is no query pending now */

       if (type == MOVE) {
              if (deny) setflag(NOWPLAYING,S_STOPPED); /* stop trying */
       else if (type == TAKEBACK) {
              tmp = NTURN + val;
              while (NTURN < tmp) doforward(0);}

       /* nothing to do for QUERY, RESPOND, EXTENDED */
static INT bdcount() {
       register INT i,n = 0;
       SWEEP361 if (ISSTONE(i)) ++n;
       return n;}

static INT inpmove(val) INTM val; {
       register INT tmp,pt,ans;
       tmp = (val & WHITEMOVE) ? WHITE : BLACK; /* who is to play */
       pt = LOCATION(val) – 1;
       pt = MAKECOORD( (pt % G(BDSIZE)) + 1, (pt / G(BDSIZE)) + 1);
       if (!legalcheck(pt,tmp)) { /* illegal move */
              senddeny("illegal move"); /* send a denial */
              return 0;}
       if (tmp != nextplayer()) G(FLIPTURN) =  NTURN; /* switch mover */
       cmdval = pt;
       cmd2val = 0;
       modemmove = pt; /* we got this by modem – clear it when it echos*/

       ans = ClickedAtPoint(pt,0,0,0); /* see if incoming move has legal context (his move) */
       if (!ans) senddeny("not his turn"); /* send a denial */
       return ans;}

static int nemabsorb() { /* eat nemesis data */
       INT i,j,ans = 0,pt,val = getv(inmbuffer),type = gett(inmbuffer),him,us,tmp,oldquery = lastquery;
       unsigned int sum; long time;

       us = (inmbuffer[1] & 0x02) >> 1;   /* his last seen message of ours */
       him = inmbuffer[1] & 0x01;                /* his current message id */
       if (type == OKMSG) { /* OK command */
              if (us != oursequence || him != hissequence);
              else if (!modemwaiting);
              else  nconflicts = modemwaiting = 0; /* normal OK */
              goto exit;}

       /* validate command */
       if (!modemwaiting) {/* we are in neutral */
/* CASE 2: he is repeating an old command */
              if (him == hissequence) {
                     /* CASE B: he knows our last command but has missed our OK */
                     if (us == oursequence)      myoutmodem(TRUE); /* repeat OK as an echo*/
                             /* he didnt see our OK, help him */
                     /* CASE A: he doesnt know our last command – since it cleared, not possible */
                     goto exit;}
/* CASE 1: he is sending a new command */
              /* CASE A: he doesnt know our last command, but it cleared – not possible */
              else if (us != oursequence) {/* his new command does not recognize our old command*/
                     goto exit;}
              /* CASE B drops thru: he has new message and he saw our last command so he is ok.*/}
/* CASE B: he knows our last command */
       else if (us == oursequence) { /* we have a command pending, he has seen our command */
              /* CASE 2: he sends old command – can't do that and know of our command */
              if (him == hissequence) {/* he repeats his old command */
                     goto exit;} /* not possible */
              /* CASE 1: he sends new command knowing of our old one – implicit OK */
              else modemwaiting = FALSE;} /* we must have missed his OK */

/* CASE A: he does not know of our command yet */
       else { /* he has not seen our command */
              /* CASE 2: he sends old command – he missed OK */
              if (him == hissequence) myoutmodem(TRUE);/* repeated old cmd, our resend will fix him*/
              /* CASE 1: he sends new command */
              else { /* His Command in Conflict with ours */
                     oursequence ^= 1; /* revert to before we shipped command */
                     /* leave his sequence alone-pretend we didnt hear his command */
                     /* he will hear us and quit, or echo and win. */
                     modemwaiting = FALSE;
                     undolast(nconflicts > 4);}/* UNDO COMMAND */
              goto exit;} /* ignore his command */

       hissequence = him; /* we recognize his command */
       /* screen his command for legality with us */
       if (type == MOVE) { /* verify a move loc or pas loc */
              ans = inpmove(val); /* setup move or decline */
              if (!ans) goto exit;}
       else if (type == TAKEBACK) {
              if (NTURN < val) {
                     senddeny("takeback too big"); /* deny */
                     goto exit;}
              modemmove = 1000;} /* came by modem */
       else if (type == DENY) {
       else if (type == EXTENDED) {
              senddeny("Extended not accepted");
              goto exit;}

       /* send an acknowledge *//* the command is self-consistent */
       sendok(type); /* release him (except QUERY)... we expect to do his command */
       if (type == TAKEBACK) {
              tmp = NTURN – val; /* go back to this turn */
              while (NTURN > tmp && NTURN > 0) dobackup(TRUE);}
       else if (type == QUERY) {/* give him an answer */
              tmp = 0;
              /* If his query freed us from OK-wait, this will put us back again in it */
              if (val & FEATUREBIT) tmp = 0; /* we dont do windows etc */
              else if (val == GAMEID) tmp = 1; /* go */
              else if (val == HOWBIG) tmp = (MaxBuffer – 5) / 16;
              else if (val == PROTOCOLID) tmp = NEMESIS;
              else if (val == BOARDVERIFY) tmp = bdcount();
              else if (val == BLACKTIMEUSED) tmp = (INT) (bticks/60) ;
              else if (val == WHITETIMEUSED) tmp = (INT) (wticks/60);
              else if (val == LANGUAGE) tmp = 1; /* english ascii*/
              else if (val == WHATRULES) tmp = G(CHINESERULES) + 1;
       else if (val == WHATHANDICAP) {
                     tmp = G(HANDICAP);
                     if (tmp < 2) tmp = 1;}
              else if (val == WHATTIME) tmp = G(TIMELIMIT);
              else if (val == WHATSIZE) tmp = G(BDSIZE);
              else if (val == WHATCOLOR) { /* what computer color */
                     if (bcomputer() && !wcomputer()) tmp = 2;
                     else if (wcomputer() && !bcomputer()) tmp = 1;}
              else if (val == WHOAREYOU) tmp = NEMESIS; /* react negatively to questions */

       else if (type == RESPONSE) {/* answer to my question */
              if (oldquery == BOARDVERIFY) {
                     if (val != 0 && val != bdcount()) mylisten(myouttext);}
              else if (oldquery == BLACKTIMEUSED && val != 0) {/* settle on min time */
                     time = val * 60;
                     if (bticks > time) bticks = time;}
              else if (oldquery == WHITETIMEUSED && val != 0) {/* settle on min time */
                     time = val * 60;
                     if (wticks > time) wticks = time;}
              lastquery = -1;} /* I will not recognize future response w/o query */
       else if (type == RESET) {/* refresh */
              modemwaiting = 0;}

       /* eat off message accepted */
exit: inmbuffer[0] = 0; /* we only allow 1 message in buffer */
       return ans;}
INTFN dominput() {
       if (inmbuffer[0] == 0 || pending) {/* no message completed yet */
              if (modemwaiting) {/* we are expected a message */
                     if ((time_count – timesent) >= RETRYDELAY) myoutmodem(2);}} /* repeat the message as an echo*/
       else if (G(MODEM_STATE) == 2 && !inside) return nemabsorb(); /* we have a command to eat */
       return 0;}

This is the code used by Many Faces of Go to implement this protocol, written
in Microsoft C 6.0 for the IBM-PC.  It includes a serial COM port interrupt

# include 
# include "g2hd.h"
# include "keys.h"

/* Copyright 1991 david fotland.  Permission granted to use this
 * code for any commercial or noncommercial purposes as long as this
 * copyright notice is not removed.  This code was written for
 * Microsoft C 6.0

#ifdef __STDC__

static void putcommand(char *com);
void putamove(int ptr);
int domodem();
static void fillsendpacket();
static void sendthepacket();
void displaychar(char c, int side);
static void domodemcommand();
static void takeback();
void puttakeback(int n);
static void putquery(int query);


static void putcommand();
void putamove();
int domodem();
static void fillsendpacket();
static void sendthepacket();
void displaychar();
static void domodemcommand();
static void takeback();
void puttakeback();
static void putquery();


/* modem interface routines.  putmodem() puts a single character out through
 * the modem.  getmodem() gets a single character from the modem or returns
 * FALSE if one is not available.  The host interface is through
 * putmodem() for typed characters for the talk window, and
 * putamove(), puttakeback(), and putreset() for commands.
 * The host should call domodem() frequently.
 * displaychar() is called to display input characters in the talk window.
 * domodemcommand() is called to parse the modem command string.
 * Modem format is:
 * Bytes outside of packet – 0CCCCCCC.
 *  C is character to echo to the talk window.  All characters typed during
 *  a game are echoed in the local talk window and sent over the modem.
 *  The ^G character should ring the bell, not be echoed into the window.
 * Byte 1 – 000000AS  (Start packet)
 *  A is Ack bit – same as seq from previous received command
 *  S is Seq bit – toggle for each sent command except OK
 * Byte 2 – 1KKKKKKK
 *  K is checksum – sum of bytes 1,3,4.  Received packets with checksum
 *   errors are ignored.
 * Byte 3,4 – 1CCCRVVV  1VVVVVVV
 *  C is command, R is reserved, V is value.
 *   Commands:
 *   0 – OK.  Value is 0.  Send in response to received command other than
 *      OK.  Used to detect simultaneous actions and conflicts.  Do
 *      not accept new move or retract from user until see OK for previous
 *      command.  If no OK for two seconds, send command again (with same
 *      Seq).
 *   1 – Denial.  Deny receipt of last command.
 *   2 – Reset.  Clear board and start new game – does not reset sequence
 *       numbers
 *   3 – Query.  Value is:
 *       1EEEEEEE – Do you support extended command E?  // Correct value is 1EEEEEEEEE according to Joe Author
 *           Respond with 0x0f: Yes, 0: No
 *       Others described in code below.
 *   4 – Response.  Value as described above.  Sent in response to a query
 *       command.
 *   5 – Move. MSB of value is 0: Black, 1: White.  Rest of value is square
 *      number.  0: pass, 1: lower left corner, boardsize: lower right corner.
 *   6 – Take back.  Value is number of moves to take back (0-1023)
 *   7 – Extended command.  Value is number of additional bytes (1-1024)
 * Additional byte 1 – 1EEEEEEE
 *   E is extended command
 * Additional byte 2 – 1KKKKKKK
 *   K is checksum: Sum of all additional bytes except this one.  Received
 *   packets with checksum errors are ignored.

# define WHITEUNDO 1024
# define TAKEBACK 2048
# define PROGRAM_ID 2
# define EXTRABUFSIZE 16
# define STARTMASK 0xfc
# define STARTVAL 0

/* commands */

# define HLACMD 0
# define DENIALCMD 1
# define RESETCMD 2
# define QUERYCMD 3
# define RESPONDCMD 4
# define MOVECMD 5
# define TAKEBACKCMD 6
# define EXTENDEDCMD 7
# define STRINGCMD 0
# define MULTICMD 15

/* queries */

# define QUERYGAME 0
# define QUERYBUF 1
# define QUERYSTONES 3
# define QUERYBTIME 4
# define QUERYWTIME 5
# define QUERYRULES 7
# define QUERYCOLOR 11
# define QUERYWHO 12

# define QUERYSTRING 0x400  // The correct value is 0x200, according to Joe Author
# define QUERYMULTI 0x40f

/* responses to QUERYWHO */

# define NEMESIS 1
# define MFGO 2
# define SMARTGO 3
# define GOLIATH 4
# define GOINT 5
# define STARPOL 6

extern int modemconnected;     /* modem connection exists */
extern char colordisplayed[];  /* color displayed on board per point */
extern list_t highlighted;     /* points that are highlighted */
extern int compmoveinprogress; /* computer is thinking */

list_t undocommands = EOL;  /* commands for undo */
unsigned char hostdata[4];  /* command ready to send */
unsigned char sentdata[4];  /* command most recently sent */
unsigned char recdata[4];   /* data most recently received */

char recextra[EXTRABUFSIZE];  /* buffer for extra received data */
char sendextra[EXTRABUFSIZE];  /* buffer to accumulate output multiple command */
int hisbuffersize = 0;  /* his extra buffer size */
int hesupportsmulti = FALSE;
int hesupportsstring = FALSE;
int whoishe = 0;
int hishandicap = 0;
int hisboardsize = 0;
int hisrules = 0;

int querylist[] = { QUERYWHO, QUERYRULES, QUERYHANDICAP, QUERYBOARDSIZE, -1};  /* queries to send at startup */
int nextquery = 0;      /* next query in querylist to send */
int lastquerysent = 0;  /* which query does response match? */

int mylastseq = 0;  /* my last sequence number */
int hislastseq = 0;  /* his last sequence number */

int hisprotocolversion = PROTOCOL_VERSION;
int hisprogramid = PROGRAM_ID;  /* assume talking to myself */
char waitinghighack = FALSE;  /* waiting for acknowledgement */
int querysent;   /* which query did I send */
int denycount = 0;  /* how many consecutive denials */

long sendtime,firstsendtime;  /* when did I send last */
int randtime = 0;  /* random timeout to resend after conflict or deny */

/* set up to query again when start new game */

       if(querylist[nextquery] == -1)nextquery = 1;

/* calculate the checksum of packet p */

static unsigned char checksum(p)
unsigned char p[4];{
       unsigned char sum;
       int i;
       sum = p[0] + p[2] + p[3];
       sum |= 0x80;  /* set sign bit */

/* put the two byte command in com into hostdata and make it ready to send */

static void putcommand(com)
char com[2]; {
       hostdata[0] = 0x0;  /* make first byte */
       hostdata[2] = com[0] | 0x80;
       hostdata[3] = com[1] | 0x80;
       mylastseq = sentdata[0] & 1;
       sendtime = time10();
       firstsendtime = sendtime;

/* send him an OK */

static void putack(){
       unsigned char sendc[2];
       if(querylist[nextquery] != -1){  /* can send next query instead of HLA */
       sendc[0] = (HLACMD << 4) | 7;
       sendc[1] = 0xff;

/* deny his command – force him to undo it – send take back 0. */

static void putdenial(){
       unsigned char sendc[2];
       waitinghighack = TRUE;
       sendc[0] = DENIALCMD << 4;
       sendc[1] = 0x0;

/* send a new game command */

void putreset(){
       unsigned char sendc[2];
       waitinghighack = TRUE;
       sendc[0] = RESETCMD << 4;
       sendc[1] = 0x0;

/* send a move over the modem.  mvs[ptr] contains the location of
 * the move (0-360, or PASS).  mvcolor[ptr] is the color of the move
 * (0-black, 1-white).

void putamove(ptr)
int ptr; {
       unsigned char sendc[2];
       int val,x,y;
              outerr("Internal error – not ready for command!\n");
       waitinghighack = TRUE;

       sendc[0] = MOVECMD << 4;
       if(mvcolor[ptr] == WHITECOLOR)sendc[0] |= 1 << 2;
       if(mvs[ptr] == PASS)
              val = 0;
       else {
              x = xval[mvs[ptr]];
              y = yval[mvs[ptr]];
              val = x+1+boardsize*(boardsize-y-1);
       sendc[1] = val;
       sendc[0] |= val >> 7;
       adflist(TAKEBACK,&undocommands);  /* put take back on undo list */

/* take back n moves.  must be called while moves are still in mvs[] */

void puttakeback(n)
int n; {
       unsigned char sendc[2];
       int i;
              outerr("Internal error – not ready for command!\n");
       waitinghighack = TRUE;
       sendc[0] = TAKEBACKCMD << 4;
       sendc[0] |= n >> 7;
       sendc[1] = n & 0x7f;
       for(i = msptr; i > msptr-n && i >= 0; --i)
              adflist((WHITEUNDO*(mvcolor[i] == WHITECOLOR))+mvs[i],&undocommands);

/* query the other program.  */

static void putquery(query)
int query; {
       unsigned char sendc[2];
              outerr("Internal error – not ready for command!\n");
       lastquerysent = query;
       waitinghighack = TRUE;
       sendc[0] = QUERYCMD << 4;
       sendc[0] |= (query >> 7) & 7;
       sendc[1] = query;

/* respond to a query */

static void putresponse(){
       int query,n,i;
       unsigned char sendc[2];
       waitinghighack = TRUE;
       query = (recdata[3] & 0x7f) | (recdata[2] << 7);
       query &= 0x3ff;
       sendc[0] = RESPONDCMD << 4;
       sendc[1] = 0;
             case QUERYGAME:
              sendc[1] = 1;  /* GO */
             case QUERYWHO:
              sendc[1] = PROGRAM_ID;
             case QUERYBUF:
              sendc[1] = 4 + EXTRABUFSIZE/16;
             case QUERYPROTOCOL:
              sendc[1] = PROTOCOL_VERSION;
             case QUERYSTONES:
              n = 0;
              for(i = 0; i < boardsquare; ++i)
                     if(colordisplayed[i] != NOCOLOR)++n;
              sendc[1] = n;
             case QUERYCHARSET:
              sendc[1] = 1;  /* ascii */
             case QUERYRULES:
              sendc[1] = chineseflag + 1;
              case QUERYHANDICAP:
              sendc[1] = handicap;
              if(sendc[1] = 0)sendc[1] = 1;
              case QUERYBOARDSIZE:
              sendc[1] = boardsize;
             case QUERYCOLOR:
              if(cplay[WHITECOLOR])sendc[1] = 1;
              else if(cplay[BLACKCOLOR])sendc[1] = 2;
             case QUERYSTRING:
              sendc[1] = 0;  /* no string support yet */
             case QUERYMULTI:
              sendc[1] = 0;  /* no multisupport yet */

/* domodem handles the modem interaction.  It
 * reads an acknowledges any packet available from the
 * input port and executes the command.
 * it returns TRUE if a command was executed, or a move was taken back.

int domodem(){
       long t;
       unsigned char seq,ack,hla;
       int command,retval = FALSE;
       while(getpacket()){  /* handle input packet */
              seq = recdata[0] & 1;
              ack = (recdata[0] & 2) >> 1;
              command = (recdata[2] >> 4) & 0x7;

                     if(command == HLACMD)continue;
                     else if(ack != mylastseq)continue;
                     else if(seq == hislastseq)
                            putack();  /* he missed HLA, resend */
                     else {
                            hislastseq = seq;
                            retval = TRUE;
              else {  /* waiting for OK */
                     if(command == HLACMD){
                            if(ack != mylastseq || seq != hislastseq)
                                   continue;  /* sequence error */
                            waitinghighack = FALSE;
                            denycount = 0;
                            gamestatus();  /* tell user ready for command */
                     else if(seq == hislastseq)continue;
                     else if(ack == mylastseq){
                            waitinghighack = FALSE;
                            gamestatus();  /* tell user ready for command */
                            hislastseq = seq;
                            retval = TRUE;
                     else {
                            randtime = rand()%400+200;  /* back off for random time 2-6 seconds */
                            outerr("Conflict with opponent");
                            takeback();  /* conflict */
                            mylastseq = 1-mylastseq;
                            retval = TRUE;
                            waitinghighack = FALSE;

       t = time10();

       /* timeout – send packet again or give up */

       if(waitinghighack && t-sendtime > 200){  /* 2 seconds */
              if(t-firstsendtime > 6000){  /* 60 seconds */
                     outerr("Missing command acknowlege, continuing");
                     waitinghighack = FALSE;
              else {
                     sendtime = t;

/* fill the send packet */

static void fillsendpacket(){
       int i,command;
       char *dummy = NULL;
       for(i = 0; i < 4; ++i)sentdata[i] = hostdata[i];
       command = (sentdata[2] >> 4) & 0x7;
       if(command == HLACMD)
              sentdata[0] |= mylastseq;  /* set up sequence number */
              sentdata[0] |= 1-mylastseq;  /* set up sequence number */
       sentdata[0] |= hislastseq << 1;  /* ack his last message */
       sentdata[1] = checksum(sentdata);

/* sendthepacket sends a packet through the modem */

static void sendthepacket() {
       int i,command;
       char buf[100];
       for(i = 0; i < 4; ++i)
#ifndef PCMIN
       if(debug != 0){
              command = (sentdata[2] >> 4) & 0x7;
              sprintf(buf,"sent cmd %d, ack %d, seq %d\n",command,(sentdata[0]&2)>>1,sentdata[0] & 1);

/* get packet assembles a packet from the modem input.  It returns TRUE
 * if a packet has been assembled in receivepacket, and FALSE otherwise.
 * if it returns FALSE, there are no more characters in the modem input buffer.
 * it handles characters outside of packets.

static char nextpacketbyte;
static int nextextrabyte,numextrabytes;

static char receivestate = 0;
/* 0 – waiting for start of packet
 * 1 – reading packet
 * 2 – reading extra data

# define COUNTLIMIT 1000

int getpacket(){
       unsigned char c;
       int command,count = 0,flag;
       char buf[100];
       while((flag = getmodem(&c)) || receivestate && count++ < COUNTLIMIT){
                            /* get a character from the modem if available */
                            /* if get first character of a packet, spin wait for rest */
                    case 0:  /* idle, looking for start of packet */
#ifndef PCMIN
                     if(debug != 0 && (c&STARTMASK) != STARTVAL){
                     if((c&STARTMASK) == STARTVAL){  /* start ofpacket */
                            receivestate = 1;
                            recdata[0] = c;
                            nextpacketbyte = 1;
                     else if(c&0x80)break;  /* error */
                     else displaychar(c,1);  /* character outside of packet, display it */
                    case 1:  /* reading packet */
                     if((c&0x80) == 0){ /* error */
                            if((c & STARTMASK) == STARTVAL){
                                   receivestate = 1;
                                   recdata[0] = c;
                                   nextpacketbyte = 1;
                            else {
                                   receivestate = 0;
                     recdata[nextpacketbyte++] = c;
                     if(nextpacketbyte == 4){  /* check for extra bytes */
                            command = (recdata[2] >> 4) & 0x7;
                            if(command == EXTENDEDCMD){
                                   receivestate = 2;
                                   nextextrabyte = 0;
                                   numextrabytes = recdata[3] & 0x7f;
                                   numextrabytes |= ((int)recdata[2] & 7) << 7;
                            else { /* done, no extra bytes */
                                   receivestate = 0;
#ifndef PCMIN
                                   if(debug != 0){
                                          command = (recdata[2] >> 4) & 7;
                                          sprintf(buf,"getpacket: cmd %d ack %d seq %d\n",command,(recdata[0] & 2)>>1, recdata[0] & 1);
                                          if(checksum(recdata) != recdata[1])
                                                 outerr("checksum error");
                                   return(checksum(recdata) == recdata[1]);
                                   /* good packet if checksum matches */
                    case 2:
                     if((c&0x80) == 0){ /* error */
                            if((c & STARTMASK) == STARTVAL){
                                   receivestate = 1;
                                   recdata[0] = c;
                                   nextpacketbyte = 1;
                            else {
                                   receivestate = 0;
                     if(nextextrabyte < EXTRABUFSIZE)  /* dump extras */
                            recextra[nextextrabyte] = c;
                     if(nextextrabyte == numextrabytes){  /* done */
                            receivestate = 0;
                            return(numextrabytes <= EXTRABUFSIZE && checksum(recdata) == recdata[1]);

/* output a local message and send it over the modem */

char *s; {
       while(*s != 0){

/* execute commands received over the modem.  If waiting for high level
 * ack, ignore command and undo save commands

static void domodemcommand(){
       int s,num,i,command,x,y;
       char buf[100];

       command = (recdata[2] >> 4) & 0x7;

#ifndef PCMIN
       if(debug != 0){
              sprintf(buf,"got cmd %d\n",command);

       if(recdata[2] & 8){  /* reserved bit – don't know what this means */

             case  DENIALCMD:
              if(denycount > 3){
                     outerr("Opponent denying moves.  Stopping computer play");
             case QUERYCMD:
             case RESETCMD:
              if(compmoveinprogress){  /* can't take back moves while computer is thinking */
                     putmsg("\n\nComputer thinking, command denied.\n");
              nextquery = 0;
#ifdef PC
              mouseShow(HIDECURSOR);  /* hide the mouse */
              newgame();  /* initialize for new game */
#ifdef PC

              case TAKEBACKCMD:  /* take back */
              if(compmoveinprogress){  /* can't take back moves while computer is thinking */
                     putmsg("\n\nComputer thinking, command denied.\n");
              num = recdata[3] & 0x7f;
              num |= (recdata[2] & 7) << 7;

              if(num > msptr-handicap){
                     putmsg("\n\nCan't take back handicap stones.\n");

#ifdef PC
              if(num == 0)break;
              outerr("Retracting move\n");
              for(i = 0; i < num; ++i){
                     if(msptr > 0)
                            retractmove(FALSE);  /* retract but don't send */
#ifdef PC
             case MOVECMD:
              if(compmoveinprogress){  /* can't make moves while computer is thinking */
                     putmsg("\n\nComputer thinking, command denied.\n");
              mvcolor[msptr] = (recdata[2] & 4) >> 2;
              s = ((int)recdata[2] << 7) | (recdata[3] & 0x7f);
              s &= 0x1ff;
              if(s == 0)
                     mvs[msptr] = PASS;
              else {
                     x = s%boardsize;
                     y = s/boardsize;
                     mvs[msptr] = boardsize*(boardsize-y-1)+x;
              if(mvs[msptr] != PASS && (mvs[msptr] >= boardsquare ||
                     mvs[msptr] < 0 || S_COLOR(mvs[msptr]) != NOCOLOR)){  /* got illegal move */
                     putmsg("\n\nIllegal move: denied.\n");
#ifdef PC
              check(FALSE);  /* make the move */
#ifdef PC
             case RESPONDCMD:
              num = recdata[3] & 0x7f;
              num |= (recdata[2] & 7) << 7;
              nextquery++;  /* prepare to send next query */
                    case QUERYBUF:
                     hisbuffersize = num*16 + 4;
                    case QUERYSTRING:
                     hesupportsstring = num;
                    case QUERYMULTI:
                     hesupportsmulti = num;
                    case QUERYHANDICAP:
                     hishandicap = num;
                     if(hishandicap == 1 && handicap != 0 || hishandicap > 1 && hishandicap != handicap){
                            if(hishandicap == 1)hishandicap = 0;
                            sprintf(buf,"His handicap (%d) and your handicap (%d) are different!",hishandicap,handicap);
                     case QUERYBOARDSIZE:
                     hisboardsize = num;
                     if(hisboardsize != 0 && hisboardsize != boardsize){
                            sprintf(buf,"His board size (%d) and your board size (%d) are different!",hisboardsize,boardsize);
                     case QUERYRULES:
                     hisrules = num;
                     if(hisrules != 0 && hisrules-1 != chineseflag)
                            outerr("He is playing different rules than you");
                    case QUERYWHO:
                     whoishe = num;
                     outerr("Connected to ");
                           case NEMESIS:
                           case MFGO:
                            outerr("Many Faces of Go");
                           case SMARTGO:
                            outerr("Smart Go Board");
                           case GOLIATH:
                           case GOINT:
                            outerr("Go Intellect");
                           case STARPOL:
                            outerr("Star of Poland");
              putack();  /* might send next query */
              putdenial();  /* tell him I don't understand his command */

/* take back the last command */

static void takeback(){
       list_t ptr;
#ifdef PC
       for(ptr = undocommands; ptr != EOL; ptr = link[ptr])
              if(list[ptr] == TAKEBACK)
                     if(msptr > 0){
                            retractmove(FALSE);  /* retract, but don't send */
              else {
                     mvcolor[msptr] = (list[ptr] & WHITEUNDO) == WHITEUNDO;
                     mvs[msptr] = list[ptr] & 511;
#ifdef PC

 *   Serial com port for IBM-PC

# define RS232 0x14 /* serial communication BIOS interrrupt */
# define COMBUFSIZE 68

char icombuf[COMBUFSIZE];

int icbfront = 0, icbback = 0;
int vector = 0;  /* interrupt vector substituted */
unsigned int portaddr;  /* com port base address */
unsigned int portnum;  /* com port number */
int mask;  /* interrupt enable mask for 8259 */

void modemintoff(){
       int tmp;
       tmp = inp(0x21);
       tmp &= ~mask;          /* turn off com port interrupt mask */

void modeminton(){
       int tmp;
       tmp = inp(0x21);
       tmp |= mask;          /* turn off com port interrupt mask */

static void (_interrupt _far *old)();  /* old handler address */

/* handle COM port interrupt for "Data received".
 * IMPORTANT NOTE:  This interrupt handler assumes a vaid stack exists.
 * It will cause the PC to crash if called when there is not enough
 * stack space (about 20 or 30 bytes).  In particular if it is called
 * while the microsoft overlay manager is executing I have seen it cause
 * a crash.  I recommend disabling the com port interrupts before loading
 * an overlay.  Alternatively, you can rewrite it to use a local stack
 * which requires assembler.

void _interrupt _far comhandler(){
       int intid,intr,lsr,msr,count = 0;;
       _enable();   /* enable higher priority interrupts */

       while(TRUE){  /* process all pending interrupts */
              intid = inp(portaddr+2);    /* get interrupt type */
              lsr = inp(portaddr+3);  /* clear any overrun condition */
              msr = inp(portaddr+6);  /* clear any modem signal condition */

              if((intid & 1) == 1){              /* no interrupts */
                     outp(0x20,0x20);       /* reenable interrupts */

              if(intid >= 4){  /* reenable interrupt if needed */
                     intr = inp(portaddr+1);
                     if((intr & 2) == 0)outp(portaddr+1,intr | 2);

              if(intid == 4){  /* receive data available */
                     icombuf[icbfront] = inp(portaddr);  /* stick char in front of queue */
                     if(icbfront == COMBUFSIZE)
                            icbfront = 0;
                     if(icbfront == icbback){  /* overrun – throw away old char */
                            if(icbfront == -1)icbfront = COMBUFSIZE-1;


/* initialize modem.  comport contains the com port number (1-2) */
/* baudrate is 0-300, 1-1200, 2-2400, 3-4800, 4-9600 */
/* return FALSE if no com port */

int comport,baudrate;
char *modemstring; {
       union REGS regs;
       char buf[80],dummy,tmp;
       unsigned int init = _COM_CHR8 | _COM_STOP1 | _COM_NOPARITY;  /* no parity, 1 stop bit, 8 bits */
              case 0:
                     init |= _COM_300;
              case 1:
                     init |= _COM_1200;
              case 2:
                     init |= _COM_2400;
              case 3:
                     init |= _COM_4800;
              case 4:
                     init |= _COM_9600;
       if(comport == 1){
              portnum = 0;
              portaddr = *((short far *)0x400000L);
              vector = 0xc;
              mask = 0x10;
       else {
              portnum = 1;
              portaddr = *((short far *)0x400002L);
              vector = 0xb;
              mask = 0x8;
       if(portaddr == 0){
              outerr("Serial port not found");

       old = _dos_getvect(vector);  /* save old vector */
       _dos_setvect(vector,comhandler); /* install my vector */
       _bios_serialcom(_COM_INIT,portnum,init);  /* initialize uart */

       outp(portaddr+4,11);    /* initialize modem control register */
                                                 /* bit 3 enables ints */
                                                 /* bit 0,1 DTR and RTS */
       outp(portaddr+1,1);         /* enable input interrupts */
       tmp = inp(0x21);            /* enable interrupt through 8259 */
       tmp &= ~mask;                      /* clear interrupt mask bit */

       modemconnected = TRUE;
       while(*modemstring != 0)putmodem(*modemstring++);
              /* put the call or answer string */
       while(getmodem(&dummy));  /* eat the chars echoed from the modem */

/* turn off the modem and remove the interrupt handler */

       unsigned char tmp;
       if(vector != 0){
              outp(portaddr+1,0);  /* disable all serial interrupts */
              outp(portaddr+4,0);  /* turn off modem controls */
              tmp = inp(0x21);
              tmp |= mask;          /* turn off com port interrupt mask */
              _dos_setvect(vector,old);  /* restore interrupt vector */
              vector = 0;
              outerr("Modem disabled");
       if(modemconnected && hayesflag){
              hayesflag = FALSE;
       modemconnected = FALSE;
       waitinghighack = FALSE;

/* get a charcter from the serial port input buffer.  Return FALSE if
 * the buffer is empty

char *c; {
       if(icbback == icbfront)return(FALSE);  /* empty buffer */
       *c = icombuf[icbback];
       if(icbback == COMBUFSIZE)icbback = 0;

/* send char c to output buffer */

unsigned char c; {
       int status;
       do {
              status = inp(portaddr+5);
              } while(!(status & 0x20));  /* wait for transmit register empty */

The material on this page was supplied by David Fotland.