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| author | Kartik K. Agaram <vc@akkartik.com> | 2018-01-24 19:27:13 -0800 |
|---|---|---|
| committer | Kartik K. Agaram <vc@akkartik.com> | 2018-01-24 19:27:13 -0800 |
| commit | 070b4642fc7330745bb5f023be948e3ad479069c (patch) | |
| tree | 97b42ce7b364f09adc624e767a761ea8122843fc | |
| parent | 4e8041619808c860d41b3de135db7de90e153493 (diff) | |
| download | mu-070b4642fc7330745bb5f023be948e3ad479069c.tar.gz | |
4187
| -rw-r--r-- | subx/014index_addressing.cc | 16 |
1 files changed, 9 insertions, 7 deletions
diff --git a/subx/014index_addressing.cc b/subx/014index_addressing.cc index 4deff72a..826a488f 100644 --- a/subx/014index_addressing.cc +++ b/subx/014index_addressing.cc @@ -6,7 +6,7 @@ % 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) +# 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 effective address +run: effective address is mem at address 0x60 (EAX) @@ -21,16 +21,18 @@ case 4: // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte // 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 + // 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 disp8's accumulate (so the instruction has *two* disp8's)? - // multiply? cancel out?! + // Do the two displacements accumulate (so the instruction has *two* + // displacement fields)? // - // Maybe this is the answer: + // 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(); break; } @@ -55,7 +57,7 @@ case 4: // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte % 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) +# 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 effective address +run: effective address is mem at address 0x60 (EAX + ECX*1) |