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sys.c: in _dos_findfirst set DTA rather than copying from PSP:80h, and work around Xi8088 ROM-BIOS bug (13.41 ds=40h) #226

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sys.c: in _dos_findfirst set DTA rather than copying from PSP:80h, and work around Xi8088 ROM-BIOS bug (13.41 ds=40h) #226

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a1a65bf
sys.c: in _dos_findfirst set DTA rather than copying from PSP:80h
ecm-pushbx Oct 10, 2025
d655a34
sys.c: work around Xi8088 ROM-BIOS bug for int 13.41 [#156]
ecm-pushbx Oct 10, 2025
a57aae8
sys.c: set ds = 40h for 13.41 on Watcom build too
ecm-pushbx Oct 10, 2025
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50 changes: 40 additions & 10 deletions sys/sys.c
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Original file line number Diff line number Diff line change
Expand Up @@ -105,24 +105,34 @@ struct _diskfree_t {
unsigned short avail_clusters, sectors_per_cluster, bytes_per_sector;
};

int int86(int ivec, union REGS *in, union REGS *out)
int int86x(int ivec, union REGS *in, union REGS *out, struct SREGS *s)
{
/* must save sp for int25/26 */
asm("mov %5, (1f+1); jmp 0f; 0:push %%ds; mov %%di, %%dx; mov %%sp, %%di;"
asm("mov %7, %%cs:(1f+1); jmp 0f; 0:"
"mov %%di, %%dx; mov %%sp, %%di;"
"push %%di; push %%di;"
/* push twice to work both for int 25h/26h and int 21h */
"1:int $0x00; pop %%di; pop %%di;"
/* second pop always reads the correct SP value.
the first pop may read the FL left on stack. */
"mov %%di, %%sp; pop %%ds; sbb %0, %0" :
"mov %%di, %%sp; sbb %0, %0" :
"=r"(out->x.cflag),
"=a"(out->x.ax), "=b"(out->x.bx), "=c"(out->x.cx), "=d"(out->x.dx) :
"=a"(out->x.ax), "=b"(out->x.bx), "=c"(out->x.cx), "=d"(out->x.dx),
"=e"(s->es), "=Rds"(s->ds) :
"q"((unsigned char)ivec), "a"(in->x.ax), "b"(in->x.bx),
"c"(in->x.cx), "D"(in->x.dx), "S"(in->x.si) :
"c"(in->x.cx), "D"(in->x.dx), "S"(in->x.si), "e"(s->es),
"Rds"(s->ds)
:
"cc", "memory");
return out->x.ax;
}

int int86(int ivec, union REGS *in, union REGS *out)
{
struct SREGS sr = {0,};
return int86x(ivec, in, out, &sr);
}

int intdos(union REGS *in, union REGS *out)
{
return int86(0x21, in, out);
Expand Down Expand Up @@ -197,13 +207,20 @@ int _dos_findfirst(const char *file_name, unsigned int attr,
struct find_t *find_tbuf)
{
union REGS in, out;
struct SREGS sr;
in.h.ah = 0x1A; /* set DTA */
in.x.dx = FP_OFF(find_tbuf);
sr.ds = FP_SEG(find_tbuf);
intdosx(&in, &out, &sr);
in.h.ah = 0x4e;
in.x.dx = FP_OFF(file_name);
in.x.cx = attr;
intdos(&in, &out);
sr.ds = FP_SEG(file_name);
intdosx(&in, &out, &sr);
if (out.x.cflag)
return out.x.ax;
memcpy(find_tbuf, (void *)0x80, sizeof(*find_tbuf));
/* memcpy(find_tbuf, (void *)0x80, sizeof(*find_tbuf)); */
/* did set DTA to find_tbuf prior */
return 0;
}
#else
Expand Down Expand Up @@ -1134,12 +1151,14 @@ int generic_block_ioctl(unsigned drive, unsigned cx, unsigned char *par);
int int2526readwrite(int DosDrive, void *diskReadPacket, unsigned intno)
{
union REGS regs;
struct SREGS sregs;

regs.h.al = (BYTE) DosDrive;
regs.x.bx = (short)diskReadPacket;
regs.x.cx = 0xffff;
sregs.ds = FP_SEG(diskReadPacket);

int86(intno, &regs, &regs);
int86x(intno, &regs, &regs, &sregs);

return regs.x.cflag;
}
Expand All @@ -1155,13 +1174,15 @@ int2526readwrite(DosDrive, diskReadPacket, 0x26)
int fat32readwrite(int DosDrive, void *diskReadPacket, unsigned intno)
{
union REGS regs;
struct SREGS sregs;

regs.x.ax = 0x7305;
regs.h.dl = DosDrive;
regs.x.bx = (short)diskReadPacket;
regs.x.cx = 0xffff;
regs.x.si = intno;
intdos(&regs, &regs);
sregs.ds = FP_SEG(diskReadPacket);
int86x(0x21, &regs, &regs, &sregs);

return regs.x.cflag;
} /* fat32readwrite */
Expand Down Expand Up @@ -1272,7 +1293,11 @@ BOOL haveLBA(void); /* return TRUE if we have LBA BIOS, FALSE otherwise */
"mov ax, 0x4100" /* IBM/MS Int 13h Extensions - installation check */ \
"mov bx, 0x55AA" \
"mov dl, 0x80" \
"push ds" \
"mov cx, 0x40" \
"mov ds, cx" \
"int 0x13" \
"pop ds" \
"xor ax, ax" \
"cmp bx, 0xAA55" \
"jne quit" \
Expand All @@ -1286,10 +1311,15 @@ BOOL haveLBA(void); /* return TRUE if we have LBA BIOS, FALSE otherwise */
BOOL haveLBA(void)
{
union REGS r;
struct SREGS sr;
r.x.ax = 0x4100;
r.x.bx = 0x55AA;
r.h.dl = 0x80;
int86(0x13, &r, &r);
sr.ds = 0x40;
/* ds = 40h is to work around a Xi8088 ROM-BIOS bug,
refer to https://github.com/FDOS/kernel/issues/156
and https://www.bttr-software.de/forum/forum_entry.php?id=21275 */
int86x(0x13, &r, &r, &sr);
return r.x.bx == 0xAA55 && r.x.cx & 1;
}
#endif
Expand Down