_C PROGRAMMING COLUMN_
by Al Stevens


[LISTING ONE]

/* --------- tsr.c --------- */

/*
 * A Terminate and Stay Resident (TSR) engine
 */

#include <dos.h>
#include <stdlib.h>
#include <stdio.h>
#include "mouse.h"
#include "keys.h"

void tsr_program(void);

#define KEYMASK  8
#define SCANCODE 52

extern unsigned _stklen = 1024;
extern unsigned _heaplen = 8192;

/* ------- the interrupt function registers -------- */
typedef struct {
    int bp,di,si,ds,es,dx,cx,bx,ax,ip,cs,fl;
} IREGS;

/* --- vectors ---- */
#define DISK    0x13
#define CTRLBRK 0x1b
#define INT28   0x28
#define CRIT    0x24
#define CTRLC   0x23
#define TIMER   8
#define KYBRD   9
#define DOS     0x21

unsigned highmemory;

/* ------ interrupt vector chains ------ */
static void (interrupt *oldtimer)(void);
static void (interrupt *old28)(void);
static void (interrupt *oldkb)(void);
static void (interrupt *olddisk)(void);

/* ------ ISRs for the TSR ------- */
static void interrupt newtimer(void);
static void interrupt new28(void);
static void interrupt newdisk(IREGS);
static void interrupt newkb(void);
static void interrupt newcrit(IREGS);
static void interrupt newbreak(void);

static unsigned sizeprogram; /* TSR's program size     */
unsigned dossegmnt;          /* DOS segment address    */
unsigned dosbusy;            /* offset to InDOS flag   */
static int diskflag;         /* Disk BIOS busy flag    */
unsigned mcbseg;             /* address of 1st DOS mcb */
static char far *mydta;      /* TSR's DTA              */

int hotkeyhit = FALSE;
int tsrss;          /* TSR's stack segment */
int tsrsp;          /* TSR's stack pointer */

/* -------------- context for the popup ---------------- */
unsigned intpsp;            /* Interrupted PSP address   */
int running;                /* TSR running indicator     */
char far *intdta;           /* interrupted DTA           */
unsigned intsp;             /*     "       stack pointer */
unsigned intss;             /*     "       stack segment */
unsigned ctrl_break;        /* Ctrl-Break setting        */
void (interrupt *oldcrit)(void);
void (interrupt *oldbreak)(void);
void (interrupt *oldctrlc)(void);

/* ------- local prototypes -------- */
static void resident_psp(void);
static void interrupted_psp(void);
static void popup(void);

void main(void)
{
    unsigned es, bx;
    /* ---------- compute memory parameters ------------ */
    highmemory = _SS + ((_SP + 256) / 16);
    /* ------ get address of DOS busy flag ---- */
    _AH = 0x34;
    geninterrupt(DOS);
    dossegmnt = _ES;
    dosbusy = _BX;
    /* ---- get the seg addr of 1st DOS MCB ---- */
    _AH = 0x52;
    geninterrupt(DOS);
    es = _ES;
    bx = _BX;
    mcbseg = peek(es, bx-2);
    /* ----- get address of resident program's dta ----- */
    mydta = getdta();
    /* ------------ prepare for residence ------------ */        
    tsrss = _SS;
    tsrsp = _SP;
    oldtimer = getvect(TIMER);
    old28 = getvect(INT28);
    oldkb = getvect(KYBRD);
    olddisk = getvect(DISK);

    /* ----- attach vectors to resident program ----- */
    setvect(KYBRD, newkb);
    setvect(INT28, new28);
    setvect(DISK, newdisk);
    setvect(TIMER, newtimer);
    /* ------ compute program size ------- */
    sizeprogram = highmemory - _psp + 1;
    /* ----- terminate and stay resident ------- */
    _DX = sizeprogram;
    _AX = 0x3100;
    geninterrupt(DOS);
}

/* ---------- break handler ------------ */
static void interrupt newbreak(void)
{
    return;
}

/* -------- critical error ISR ---------- */
static void interrupt newcrit(IREGS ir)
{
    ir.ax = 0;            /* ignore critical errors */
}

/* ------ BIOS disk functions ISR ------- */
static void interrupt newdisk(IREGS ir)
{
    diskflag++;
    (*olddisk)();
    ir.ax = _AX;        /* for the register returns */
    ir.cx = _CX;
    ir.dx = _DX;
    ir.es = _ES;
    ir.di = _DI;
    ir.fl = _FLAGS;
    --diskflag;
}

/* ----- keyboard ISR ------ */
static void interrupt newkb(void)
{
    static unsigned char kbval;

    kbval = inportb(0x60);
    if (!hotkeyhit && !running)
        if ((peekb(0, 0x417) & 0xf) == KEYMASK)
            if (SCANCODE == kbval)    {
                hotkeyhit = TRUE;
                /* --- reset the keyboard ---- */
                kbval = inportb(0x61);
                outportb(0x61, kbval | 0x80);
                outportb(0x61, kbval);
                outportb(0x20, 0x20);
                return;
            }
    (*oldkb)();
}

/* ----- timer ISR ------- */
static void interrupt newtimer(void)
{
    (*oldtimer)();
    if (hotkeyhit && (peekb(dossegmnt, dosbusy) == 0) &&
            !diskflag)
        popup();
}

/* ----- 0x28 ISR -------- */
static void interrupt new28(void)
{
    (*old28)();
    if (hotkeyhit)
        popup();
}

/* ------ switch psp context from interrupted to TSR ----- */
static void resident_psp(void)
{
    intpsp = getpsp();
    _AH = 0x50;
    _BX = _psp;
    geninterrupt(DOS);
}

/* ---- switch psp context from TSR to interrupted ---- */
static void interrupted_psp(void)
{
     _BX = intpsp;
     _AH = 0x50;
    geninterrupt(DOS);
}

/* ------ execute the resident program ------- */
static void popup(void)
{
    running = TRUE;
    hotkeyhit = FALSE;
    intsp = _SP;
    intss = _SS;
    _SP = tsrsp;
    _SS = tsrss;
    oldcrit = getvect(CRIT);    /* redirect critical err  */
    oldbreak = getvect(CTRLBRK);
    oldctrlc = getvect(CTRLC);
    setvect(CRIT, newcrit);
    setvect(CTRLBRK, newbreak);
    setvect(CTRLC, newbreak);
    ctrl_break = getcbrk();     /* get ctrl break setting */
    setcbrk(0);                 /* turn off ctrl break    */
    intdta = getdta();          /* get interrupted dta    */
    setdta(mydta);              /* set resident dta       */
    resident_psp();             /* swap psps              */
    intercept_mouse();          /* intercept the mouse    */
    /* ------ save the video cursor configuration ------- */
    savecursor();
    normalcursor();
    unhidecursor();
    enable();
    tsr_program();              /* call the TSR C program */
    disable();
    /* ----- restore the video cursor configuration ----- */
    restorecursor();
    restore_mouse();            /* restore the mouse      */
    interrupted_psp();          /* reset interrupted psp  */
    setdta(intdta);             /* reset interrupted dta  */
    setvect(CRIT, oldcrit);     /* reset critical error   */
    setvect(CTRLBRK, oldbreak);
    setvect(CTRLC, oldctrlc);
    setcbrk(ctrl_break);        /* reset ctrl break       */
    disable();
    _SP = intsp;                /* reset interrupted stack*/
    _SS = intss;
    running = FALSE;            /* reset semaphore        */
}