diff options
| -rw-r--r-- | subx/014index_addressing.cc | 43 |
1 files changed, 17 insertions, 26 deletions
diff --git a/subx/014index_addressing.cc b/subx/014index_addressing.cc index 723a243a..f457c761 100644 --- a/subx/014index_addressing.cc +++ b/subx/014index_addressing.cc @@ -9,7 +9,8 @@ # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB) # SIB in binary: 00 (scale 1) 100 (no index) 000 (base EAX) +run: add EBX to r/m32 -+run: effective address is 0x60 (EAX) ++run: effective address is initially 0x60 (EAX) ++run: effective address is 0x60 +run: storing 0x00000011 :(before "End Mod 0 Special-cases") @@ -22,38 +23,27 @@ case 4: // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte uint32_t effective_address_from_sib(uint8_t mod) { 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 /*indirect+disp8*/ 000 /*EAX*/ 100 /*SIB*/ 00 /*scale*/ 100 /*no index*/ 101 /*EBP+disp8*/ 0xf0 - // - // Do the two displacements accumulate (so the instruction has *two* - // displacement fields)? - // - // Maybe they're redundant: - // "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 - // - // That's the only option that makes sense for 32-bit displacement (mod 10) - raise << "base 5 (often but not always EBP) not supported in SIB byte\n" << end(); - return 0; + uint32_t addr = 0; + if (base != EBP || mod != 0) { + addr = Reg[base].u; + trace(2, "run") << "effective address is initially 0x" << std::hex << addr << " (" << rname(base) << ")" << end(); + } + else { + // base == EBP && mod == 0 + addr = imm32(); + trace(2, "run") << "effective address is initially 0x" << std::hex << addr << " (disp32)" << end(); } uint8_t index = (sib>>3)&0x7; if (index == ESP) { // ignore index and scale - trace(2, "run") << "effective address is 0x" << std::hex << Reg[base].u << " (" << rname(base) << ")" << end(); - return Reg[base].u; + trace(2, "run") << "effective address is 0x" << std::hex << addr << end(); } else { uint8_t scale = (1 << (sib>>6)); - uint32_t addr = Reg[base].u + Reg[index].i*scale; // treat index register as signed. Maybe base as well? But we'll always ensure it's non-negative. - trace(2, "run") << "effective address is 0x" << std::hex << addr << " (" << rname(base) << " + " << rname(index) << "*" << NUM(scale) << ")" << end(); - return addr; + addr += Reg[index].i*scale; // treat index register as signed. Maybe base as well? But we'll always ensure it's non-negative. + trace(2, "run") << "effective address is 0x" << std::hex << addr << " (after adding " << rname(index) << "*" << NUM(scale) << ")" << end(); } + return addr; } :(scenario add_r32_to_mem_at_base_r32_index_r32) @@ -66,5 +56,6 @@ uint32_t effective_address_from_sib(uint8_t mod) { # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB) # SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX) +run: add EBX to r/m32 -+run: effective address is 0x60 (EAX + ECX*1) ++run: effective address is initially 0x5e (EAX) ++run: effective address is 0x60 (after adding ECX*1) +run: storing 0x00000011 |