1 //: operating on memory at the address provided by some register plus optional scale and offset
  2 
  3 :(scenario add_r32_to_mem_at_r32_with_sib)
  4 % Reg[EBX].i = 0x10;
  5 % Reg[EAX].i = 0x2000;
  6 == 0x1  # code segment
  7 # op  ModR/M  SIB   displacement  immediate
  8   01  1c      20                             # add EBX to *EAX
  9 # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
 10 # SIB in binary: 00 (scale 1) 100 (no index) 000 (base EAX)
 11 == 0x2000  # data segment
 12 01 00 00 00  # 1
 13 +run: add EBX to r/m32
 14 +run: effective address is initially 0x00002000 (EAX)
 15 +run: effective address is 0x00002000
 16 +run: storing 0x00000011
 17 
 18 :(before "End Mod 0 Special-cases(addr)")
 19 case 4:  // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte
 20   addr = effective_address_from_sib(mod);
 21   break;
 22 :(code)
 23 uint32_t effective_address_from_sib(uint8_t mod) {
 24   const uint8_t sib = next();
 25   const uint8_t base = sib&0x7;
 26   uint32_t addr = 0;
 27   if (base != EBP || mod != 0) {
 28     addr = Reg[base].u;
 29     trace(90, "run") << "effective address is initially 0x" << HEXWORD << addr << " (" << rname(base) << ")" << end();
 30   }
 31   else {
 32     // base == EBP && mod == 0
 33     addr = next32();  // ignore base
 34     trace(90, "run") << "effective address is initially 0x" << HEXWORD << addr << " (disp32)" << end();
 35   }
 36   const uint8_t index = (sib>>3)&0x7;
 37   if (index == ESP) {
 38     // ignore index and scale
 39     trace(90, "run") << "effective address is 0x" << HEXWORD << addr << end();
 40   }
 41   else {
 42     const uint8_t scale = (1 << (sib>>6));
 43     addr += Reg[index].i*scale;  // treat index register as signed. Maybe base as well? But we'll always ensure it's non-negative.
 44     trace(90, "run") << "effective address is 0x" << HEXWORD << addr << " (after adding " << rname(index) << "*" << NUM(scale) << ")" << end();
 45   }
 46   return addr;
 47 }
 48 
 49 :(scenario add_r32_to_mem_at_base_r32_index_r32)
 50 % Reg[EBX].i = 0x10;  // source
 51 % Reg[EAX].i = 0x1ffe;  // dest base
 52 % Reg[ECX].i = 0x2;  // dest index
 53 == 0x1  # code segment
 54 # op  ModR/M  SIB   displacement  immediate
 55   01  1c      08                             # add EBX to *(EAX+ECX)
 56 # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
 57 # SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
 58 == 0x2000  # data segment
 59 01 00 00 00  # 1
 60 +run: add EBX to r/m32
 61 +run: effective address is initially 0x00001ffe (EAX)
 62 +run: effective address is 0x00002000 (after adding ECX*1)
 63 +run: storing 0x00000011
 64 
 65 :(scenario add_r32_to_mem_at_displacement_using_sib)
 66 % Reg[EBX].i = 0x10;  // source
 67 == 0x1  # code segment
 68 # op  ModR/M  SIB   displacement  immediate
 69   01  1c      25    00 20 00 00              # add EBX to *0x2000
 70 # ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
 71 # SIB in binary: 00 (scale 1) 100 (no index) 101 (not EBP but disp32)
 72 == 0x2000  # data segment
 73 01 00 00 00  # 1
 74 +run: add EBX to r/m32
 75 +run: effective address is initially 0x00002000 (disp32)
 76 +run: effective address is 0x00002000
 77 +run: storing 0x00000011
 78 
 79 //:
 80 
 81 :(scenario add_r32_to_mem_at_base_r32_index_r32_plus_disp8)
 82 % Reg[EBX].i = 0x10;  // source
 83 % Reg[EAX].i = 0x1ff9;  // dest base
 84 % Reg[ECX].i = 0x5;  // dest index
 85 == 0x1  # code segment
 86 # op  ModR/M  SIB   displacement  immediate
 87   01  5c      08    02                       # add EBX to *(EAX+ECX+2)
 88 # ModR/M in binary: 01 (indirect+disp8 mode) 011 (src EBX) 100 (dest in SIB)
 89 # SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
 90 == 0x2000  # data segment
 91 01 00 00 00  # 1
 92 +run: add EBX to r/m32
 93 +run: effective address is initially 0x00001ff9 (EAX)
 94 +run: effective address is 0x00001ffe (after adding ECX*1)
 95 +run: effective address is 0x00002000 (after adding disp8)
 96 +run: storing 0x00000011
 97 
 98 :(before "End Mod 1 Special-cases(addr)")
 99 case 4:  // exception: mod 0b01 rm 0b100 => incoming SIB (scale-index-base) byte
100   addr = effective_address_from_sib(mod);
101   break;
102 
103 //:
104 
105 :(scenario add_r32_to_mem_at_base_r32_index_r32_plus_disp32)
106 % Reg[EBX].i = 0x10;  // source
107 % Reg[EAX].i = 0x1ff9;  // dest base
108 % Reg[ECX].i = 0x5;  // dest index
109 == 0x1  # code segment
110 # op  ModR/M  SIB   displacement  immediate
111   01  9c      08    02 00 00 00              # add EBX to *(EAX+ECX+2)
112 # ModR/M in binary: 10 (indirect+disp32 mode) 011 (src EBX) 100 (dest in SIB)
113 # SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
114 == 0x2000  # data segment
115 01 00 00 00  # 1
116 +run: add EBX to r/m32
117 +run: effective address is initially 0x00001ff9 (EAX)
118 +run: effective address is 0x00001ffe (after adding ECX*1)
119 +run: effective address is 0x00002000 (after adding disp32)
120 +run: storing 0x00000011
121 
122 :(before "End Mod 2 Special-cases(addr)")
123 case 4:  // exception: mod 0b10 rm 0b100 => incoming SIB (scale-index-base) byte
124   addr = effective_address_from_sib(mod);
125   break;