#include "ModRM.h"
UINT Prefixes(VARPTR&code) {
static const BYTE plist[]={0x26,0x2E,0x36,0x3E,0x64,0x65,0x66,0x67,0xF0,0xF2,0xF3};
UINT ret=0;
for(const BYTE *p;p=(const BYTE*)memchr(plist,*code.c,sizeof plist);) {
ret|=8<<(p-plist);
code.c++;
}
return ret;
}
#define FIELDOFFSET(type,field) ((int)&(((type*)0)->field))
#define R(x) FIELDOFFSET(CONTEXT,x)
// This are the arrays for CONTEXT field offsets, in the order of the ModR/M register triplett.
static const BYTE regofs[]={R(Eax),R(Ecx),R(Edx),R(Ebx),R(Esp),R(Ebp),R(Esi),R(Edi)};
// static const BYTE r8ofs[]={R(Eax),R(Ecx),R(Edx),R(Ebx),R(Eax)+1,R(Ecx)+1,R(Edx)+1,R(Ebx)+1};
#undef R
// This is ModR/M and SIB decoding for 32-bit.
// ASSUMING mov reg1632,mem opcode (no reg8 here)
static long M32(VARPTR&code, const CONTEXT*ctx, long**dest) {
BYTE modrm=*code.b++;
#define R(x) *(long*)((char*)ctx+(regofs[(x)&7]))
if (dest) *dest=&R(modrm>>3); // the register itself
long srcaddr=R(modrm); // the register itself (assume Mod!=3 which cannot throw exception)
if ((modrm&0xC7)==0x05) {
srcaddr=*code.l++; // take disp32
}else if ((modrm&0xC0) != 0xC0) { // indirect mode
if ((modrm&7)==4) { // SIB follows, re-calculate srcaddr
BYTE sib=*code.c++;
srcaddr=R(sib);
if (!(modrm&0xC0) && (sib&7)==5) srcaddr=*code.l++; // don't take EBP here but disp32
if ((sib&38)!=0x20) srcaddr+=R(sib>>3)*(1<<(sib>>6));
}
switch (modrm>>6) {
case 1: srcaddr+=*code.c++; break; // add signed disp8
case 2: srcaddr+=*code.l++; break; // add disp32
}
}
#undef R
return srcaddr;
}
// This is for AddrSize (0x67) prefix, 16-bit (80286) mode.
// ASSUMING mov reg,mem3216 opcode (not mem8 here)
static long M16(VARPTR&code, const CONTEXT*ctx, short**dest) {
BYTE modrm=*code.b++;
#define R(x) *(short*)((char*)ctx+(regofs[(x)&7]))
if (dest) *dest=&R(modrm>>3); // the register itself
long srcaddr=LOWORD(modrm&2&&~modrm&7?ctx->Ebp:ctx->Ebx); // base is either BX or BP
if ((modrm&6)==4) srcaddr=0; // no base
if ((modrm&0xC7)==6) srcaddr=*code.s++; // take disp16
if ((modrm&7)<6) srcaddr+=LOWORD(modrm&1?ctx->Edi:ctx->Esi);// add index, either SI or DI
switch (modrm>>6) {
case 1: srcaddr+=*code.c++; break; // add disp8
case 2: srcaddr+=*code.s++; break; // add disp16
}
#undef R
return srcaddr;
}
long ModRM(UINT prefixes, VARPTR&code, const CONTEXT*ctx, VARPTR*dest) {
if (prefixes&0x400) return M16(code,ctx,&dest->s);
else return M32(code,ctx,&dest->l);
}
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