In assembly there are often instructions like:
mov eax, [ebp+4]
mov eax, [ebx + ecx*4]
My question is: What can I do in this brakets? Can I add and multiply all registers and immediates or are there limits? Can I use division and subtraction too? I searched for it with google, but I couldn't find any good reference.
The general format for an address operand in 32-bit and 64-bit modes is:
[reg1 + reg2*mult + ofs]
mult must be one of 1,2,4,8. ofs is an immediate value, i.e. a constant numeric value. It can be positive or negative.
Not all three parts have to be specified, so for example [eax] or [ecx*2 + 5] are valid.
Normally, the eip/rip register cannot be used as one of the register values. But in 64-bit mode there is a special "RIP-relative addressing" mode, which can be specified as [rip + ofs].
Instruction Prefix 0 oder 1 Byte
Address-Size Prefix 0 oder 1 Byte
Operand-Size Prefix 0 oder 1 Byte
Segment Prefix 0 oder 1 Byte
Opcode 1 oder 2 Byte
Mod R/M 0 oder 1 Byte
SIB, Scale Index Base (386+) 0 oder 1 Byte
Displacement 0, 1, 2 oder 4 Byte (4 nur 386+)
Immediate 0, 1, 2 oder 4 Byte (4 nur 386+)
Format of Postbyte(Mod R/M aus Intel-Doku)
------------------------------------------
MM RRR MMM
MM - Memory addressing mode
RRR - Register operand address
MMM - Memory operand address
RRR Register Names
Filds 8bit 16bit 32bit
000 AL AX EAX
001 CL CX ECX
010 DL DX EDX
011 Bl BX EBX
100 AH SP ESP
101 CH BP EBP
110 DH SI ESI
111 BH DI EDI
---
16bit memory (No 32 bit memory address prefix)
MMM Default MM Field
Field Sreg 00 01 10 11=MMM is reg
000 DS [BX+SI] [BX+SI+o8] [BX+SI+o16]
001 DS [BX+DI] [BX+DI+o8] [BX+DI+o16]
010 SS [BP+SI] [BP+SI+o8] [BP+SI+o16]
011 SS [BP+DI] [BP+DI+o8] [BP+DI+o16]
100 DS [SI] [SI+o8] [SI+o16]
101 DS [DI] [DI+o8] [SI+o16]
110 SS [o16] [BP+o8] [BP+o16]
111 DS [BX] [BX+o8] [BX+o16]
Note: MMM=110,MM=0 Default Sreg is DS !!!!
32bit memory (Has 67h 32 bit memory address prefix)
MMM Default MM Field
Field Sreg 00 01 10 11=MMM is reg
000 DS [EAX] [EAX+o8] [EAX+o32]
001 DS [ECX] [ECX+o8] [ECX+o32]
010 DS [EDX] [EDX+o8] [EDX+o32]
011 DS [EBX] [EBX+o8] [EBX+o32]
100 SIB [SIB] [SIB+o8] [SIB+o32]
101 SS [o32] [EBP+o8] [EBP+o32]
110 DS [ESI] [ESI+o8] [ESI+o32]
111 DS [EDI] [EDI+o8] [EDI+o32]
Note: MMM=110,MM=0 Default Sreg is DS !!!!
---
SIB is (Scale/Base/Index)
SS BBB III
Note: SIB address calculated as:
<sib address>=<Base>+<Index>*(2^(Scale))
Fild Default Base
BBB Sreg Register Note
000 DS EAX
001 DS ECX
010 DS EDX
011 DS EBX
100 SS ESP
101 DS o32 if MM=00 (Postbyte)
SS EBP if MM<>00 (Postbyte)
110 SS ESI
111 DS EDI
Fild Index
III register Note
000 EAX
001 ECX
010 EDX
011 EBX
100 never Index SS can be 00
101 EBP
110 ESI
111 EDI
Fild Scale coefficient
SS =2^(SS)
00 1
01 2
10 4
11 8
