1 //: operating on memory at the address provided by some register
 2 
 3 :(scenario add_r32_to_mem_at_r32)
 4 % Reg[3].i = 0x10;
 5 % Reg[0].i = 0x60;
 6 # word in addresses 0x60-0x63 has value 1
 7 % Mem.at(0x60) = 1;
 8 # op  ModR/M  SIB   displacement  immediate
 9   01  18                                     # add EBX (reg 3) to *EAX (reg 0)
10 +run: add reg 3 to effective address
11 +run: effective address is mem at address 0x60 (reg 0)
12 +run: storing 0x00000011
13 
14 :(before "End Mod Special-cases")
15 case 0:
16   // mod 0 is usually indirect addressing
17   switch (rm) {
18   default:
19   ¦ trace(2, "run") << "effective address is mem at address 0x" << std::hex << Reg[rm].u << " (reg " << NUM(rm) << ")" << end();
20   ¦ assert(Reg[rm].u + sizeof(int32_t) <= Mem.size());
21   ¦ result = reinterpret_cast<int32_t*>(&Mem.at(Reg[rm].u));  // rely on the host itself being in little-endian order
22   ¦ break;
23   // End Mod 0 Special-cases
24   }
25   break;
26 
27 //:
28 
29 :(scenario add_mem_at_r32_to_r32)
30 % Reg[0].i = 0x60;
31 % Reg[3].i = 0x10;
32 % Mem.at(0x60) = 1;
33 # op  ModR/M  SIB   displacement  immediate
34   03  18                                      # add *EAX (reg 0) to EBX (reg 3)
35 +run: add effective address to reg 3
36 +run: effective address is mem at address 0x60 (reg 0)
37 +run: storing 0x00000011
38 
39 :(before "End Single-Byte Opcodes")
40 case 0x03: {  // add r/m32 to r32
41   uint8_t modrm = next();
42   uint8_t arg1 = (modrm>>3)&0x7;
43   trace(2, "run") << "add effective address to reg " << NUM(arg1) << end();
44   const int32_t* arg2 = effective_address(modrm);
45   BINARY_ARITHMETIC_OP(+, Reg[arg1].i, *arg2);
46   break;
47 }
48 
49 //:: subtract
50 
51 :(scenario sub_r32_from_mem_at_r32)
52 % Reg[0].i = 0x60;
53 % Mem.at(0x60) = 10;
54 % Reg[3].i = 1;
55 # op  ModRM   SIB   displacement  immediate
56   29  18                                      # subtract EBX (reg 3) from *EAX (reg 0)
57 +run: subtract reg 3 from effective address
58 +run: effective address is mem at address 0x60 (reg 0)
59 +run: storing 0x00000009
60 
61 //:
62 
63 :(scenario sub_mem_at_r32_from_r32)
64 % Reg[0].i = 0x60;
65 % Mem.at(0x60) = 1;
66 % Reg[3].i = 10;
67 # op  ModRM   SIB   displacement  immediate
68   2b  18                                      # subtract *EAX (reg 0) from EBX (reg 3)
69 +run: subtract effective address from reg 3
70 +run: effective address is mem at address 0x60 (reg 0)
71 +run: storing 0x00000009
72 
73 :(before "End Single-Byte Opcodes")
74 case 0x2b: {  // subtract r/m32 from r32
75   uint8_t modrm = next();
76   uint8_t arg1 = (modrm>>3)&0x7;
77   trace(2, "run") << "subtract effective address from reg " << NUM(arg1) << end();
78   const int32_t* arg2 = effective_address(modrm);
79   BINARY_ARITHMETIC_OP(-, Reg[arg1].i, *arg2);
80   break;
81 }