diff options
Diffstat (limited to 'subx/014index_addressing.cc')
| -rw-r--r-- | subx/014index_addressing.cc | 14 |
1 files changed, 13 insertions, 1 deletions
diff --git a/subx/014index_addressing.cc b/subx/014index_addressing.cc index e489dba1..389808d1 100644 --- a/subx/014index_addressing.cc +++ b/subx/014index_addressing.cc @@ -17,6 +17,18 @@ case 4: // exception: SIB addressing uint8_t sib = next(); uint8_t base = sib&0x7; + if (base == EBP) { + // This gets complicated. In the example below, do the two disp8's accumulate? multiply? cancel out?! + // 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 + // + // 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 @@ -25,7 +37,7 @@ case 4: } else { uint8_t scale = (1 << (sib>>6)); - uint32_t addr = Reg[base].u + Reg[index].u*scale; + 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)); } |