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gcc build: XMScopy varargs call doesn't work as expected #171

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gcc build: XMScopy varargs call doesn't work as expected#171
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@ecm-pushbx

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@ecm-pushbx
ecm-pushbx
opened on Feb 23, 2025

This is XMScopy, a varargs function that's called with the contents for an XMS move structure:

freecom/shell/cswapc.c

Lines 97 to 125 in e24bd7e

#ifdef __GNUC__
__attribute__((noinline))
#endif
static int XMScopy(
long length, ...
/* short shandle,
long soffset,
short dhandle,
long doffset */
)
{
assert(XMSdriverAdress);
/* asm push si;
asm lea si,length
asm mov ah,0bh; */
#if defined(__TURBOC__)
_SI = (unsigned)&length;
_AH = 0xb;
XMSrequest();
/* asm pop si; */
return _AX; /* shut up warning */
#elif defined(__GNUC__)
return XMSrequest(0xb00, 0, &length);
#else
return XMSdriverAdress(0xb00, 0, &length);
#endif
}

However, in my gcc build this doesn't work. Here's an excerpt from debugging:

-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A72 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B67 NV UP EI PL NZ AC PO NC
C00F:0B67 36C706D00F0100    mov     word [ss:0FD0], 0001           SS:0FD0=0000
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A72 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B6E NV UP EI PL NZ AC PO NC
C00F:0B6E 36C606C60F02      mov     byte [ss:0FC6], 02               SS:0FC6=01
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A72 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B74 NV UP EI PL NZ AC PO NC
C00F:0B74 837EE600          cmp     word [bp-1A], +00              SS:3A76=0622
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A72 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B78 NV UP EI PL NZ NA PE NC
C00F:0B78 7421              jz      0B9B                            not jumping
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A72 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B7A NV UP EI PL NZ NA PE NC
C00F:0B7A 06                push    es
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A70 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B7B NV UP EI PL NZ NA PE NC
C00F:0B7B 06                push    es
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A6E BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B7C NV UP EI PL NZ NA PE NC
C00F:0B7C FF76EA            push    word [bp-16]                   SS:3A7A=0001
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A6C BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B7F NV UP EI PL NZ NA PE NC
C00F:0B7F FF76E4            push    word [bp-1C]                   SS:3A74=F34C
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A6A BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B82 NV UP EI PL NZ NA PE NC
C00F:0B82 06                push    es
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A68 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B83 NV UP EI PL NZ NA PE NC
C00F:0B83 06                push    es
-
AX=0001 BX=C001 CX=6220 DX=0000 SP=3A66 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B84 NV UP EI PL NZ NA PE NC
C00F:0B84 91                xchg    ax, cx
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A66 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B85 NV UP EI PL NZ NA PE NC
C00F:0B85 16                push    ss
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A64 BP=3A90 SI=C00F DI=1D60
DS=C00F ES=0000 SS=CE18 CS=C00F IP=0B86 NV UP EI PL NZ NA PE NC
C00F:0B86 1F                pop     ds
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A66 BP=3A90 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=0B87 NV UP EI PL NZ NA PE NC
C00F:0B87 E86BFE            call    09F5
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A64 BP=3A90 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09F5 NV UP EI PL NZ NA PE NC
C00F:09F5 56                push    si
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A62 BP=3A90 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09F6 NV UP EI PL NZ NA PE NC
C00F:09F6 55                push    bp
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A60 BP=3A90 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09F7 NV UP EI PL NZ NA PE NC
C00F:09F7 89E5              mov     bp, sp
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A60 BP=3A60 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09F9 NV UP EI PL NZ NA PE NC
C00F:09F9 52                push    dx
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A5E BP=3A60 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09FA NV UP EI PL NZ NA PE NC
C00F:09FA 50                push    ax
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A5C BP=3A60 SI=C00F DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09FB NV UP EI PL NZ NA PE NC
C00F:09FB 8D76FC            lea     si, [bp-04]
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A5C BP=3A60 SI=3A5C DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=09FE NV UP EI PL NZ NA PE NC
C00F:09FE 31D2              xor     dx, dx
-
AX=6220 BX=C001 CX=0001 DX=0000 SP=3A5C BP=3A60 SI=3A5C DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=0A00 NV UP EI PL ZR NA PE NC
C00F:0A00 B8000B            mov     ax, 0B00
-
AX=0B00 BX=C001 CX=0001 DX=0000 SP=3A5C BP=3A60 SI=3A5C DI=1D60
DS=CE18 ES=0000 SS=CE18 CS=C00F IP=0A03 NV UP EI PL ZR NA PE NC
C00F:0A03 2EFF1EC008        call    far [cs:08C0]             CS:08C0=F000:F685
-dw ds:si l 10
header     0    2    4    6    8    A    C    E    0123456789ABCDEF
CE18:3A50                     -          6220 0000              b..
CE18:3A60  3A90 C00F 0B8A 0000-0000 F34C           .:........L.
-

Note the lea si, [bp-04] which points not at the function parameters on the stack but rather at a local variable initialised from the length parameter.

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