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//: operating on memory at the address provided by some register plus optional scale and offset
:(scenario add_r32_to_mem_at_r32_with_sib)
% Reg[3].i = 0x10;
% Reg[0].i = 0x60;
% SET_WORD_IN_MEM(0x60, 1);
# op ModR/M SIB displacement immediate
01 1c 20 # add EBX to *EAX
# ModR/M in binary: 00 (indirect mode) 011 (src EBX) 000 (dest EAX)
# SIB in binary: 00 (scale 1) 100 (no index) 000 (base EAX)
+run: add EBX to effective address
+run: effective address is mem at address 0x60 (EAX)
+run: storing 0x00000011
:(before "End Mod 0 Special-cases")
case 4: // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte
uint8_t sib = next();
uint8_t base = sib&0x7;
if (base == EBP) {
// Need to sometimes use a displacement either in addition to or in place
// of EBP. This gets complicated, and I don't understand interactions with
// displacement mode in Mod/RM. For example:
//
// op (hex) ModR/M (binary) SIB (binary) displacement (hex)
// 0x01 01 100 /*SIB+disp8*/ 000 /*EAX*/ 00 /*scale*/ 100 /*no index*/ 101 /*EBP+disp8*/ 0xf0
//
// Do the two disp8's accumulate (so the instruction has *two* disp8's)?
// multiply? cancel out?!
//
// Maybe this is the answer:
// "When the ModR/M or SIB tables state that a disp value is required..
// then the displacement bytes are required."
// -- https://wiki.osdev.org/X86-64_Instruction_Encoding#Displacement
raise << "base 5 (often but not always EBP) not supported in SIB byte\n" << end();
break;
}
uint8_t index = (sib>>3)&0x7;
if (index == ESP) {
// ignore index and scale
trace(2, "run") << "effective address is mem at address 0x" << std::hex << Reg[base].u << " (" << rname(base) << ")" << end();
result = reinterpret_cast<int32_t*>(&Mem.at(Reg[base].u));
}
else {
uint8_t scale = (1 << (sib>>6));
uint32_t addr = Reg[base].u + Reg[index].u*scale; // TODO: should the index register be treated as a signed int?
trace(2, "run") << "effective address is mem at address 0x" << std::hex << addr << " (" << rname(base) << " + " << rname(index) << "*" << NUM(scale) << ")" << end();
result = reinterpret_cast<int32_t*>(&Mem.at(addr));
}
break;
:(scenario add_r32_to_mem_at_base_r32_index_r32)
% Reg[3].i = 0x10; // source
% Reg[0].i = 0x5e; // dest base
% Reg[1].i = 0x2; // dest index
% SET_WORD_IN_MEM(0x60, 1);
# op ModR/M SIB displacement immediate
01 1c 08 # add EBX to *(EAX+ECX)
# ModR/M in binary: 00 (indirect mode) 011 (src EBX) 000 (dest EAX)
# SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
+run: add EBX to effective address
+run: effective address is mem at address 0x60 (EAX + ECX*1)
+run: storing 0x00000011
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