From: "A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk>
To: DJGPP AT SUN DOT SOE DOT CLARKSON DOT EDU
Date: Tue, 10 Jan 1995 15:30:13 GMT
Subject: Cat and mouse games

  I am developing a Gnu C++ program which used many special keystrokes:-

typedef unsigned char byte; int nextch;
/*-----*/
byte get__key(){_ax=0x700; int21(); return _ax&255;}
/* Return key typed, or 0 if the next call returns a special key */
/*-----*/
int get_key(){return get__key()?:-get__key();}
/* Return key typed (special keys as negative, e.g. alt-Q as -16) */
/*-----*/
int getkey(){int i; if(nextch) {i=nextch; nextch=0; return i;}
i=get_key(); return i;}
/* Ditto, and use `nextch' as a 1-character input buffer to hold any character
   that was read and wasn't needed until later */
/*-----*/
<<<These work OK.>>>

  I now want to bring the mouse in, putting the `read the mouse' routines in
`int getkey' in such a way that pressing the left / middle / right buttons is
returned by `int getkey' as if the three mouse buttons were extra keyboard
keys returning the characters special-253 / special-254 / special-255. So as a
part-way stage in developing it I tried this:-
/*----- is a key waiting to be read? *//* This works OK */
int typahead() {if(nextch) return 1; _ax=0xb00; int21(); return _ax&255;}
/*-----*/ /* wait n microseconds. This seems to work OK */
void wait(int n){_ax=0x8600; _cx=n>>16; _dx=n&0xffff; int15();}
/*-----*/
int getkey(){int i; if(nextch) {i=nextch; nextch=0; return i;}
Jerry.status(); /* read current mouse state */
A: Jerry.status(); if(!typahead()) goto A;
i=get_key(); return i;}
/*-----*/
  This should merely keep track of mouse movements while waiting for me to
type a key. (It doesn't yet exit the loop if I press a mouse button, I haven't
got that far.) But when I ran it I found that it made the keyboard's hearing
unreliable, i.e. sometimes it ignored and sometimes read the same key. I
thought: "This is making PC call the `read mouse state' interrupt and the `is
there a keystroke waiting?' interrupt alternately every microsecond or so, and
so there isn't time for the (keyboard interrupt that happens when I press a
key) to find enough spare time to happen.". So I tried inserting `wait(1);' or
`for(i=0;i<1000;i++) XYZ++;' after the label `A:', to leave a microsecond gap
in each loop so the keyboard interrupt has time to happen, but the PC still
kept sometimes ignoring keyboard keys in this program. What is happening?