More Assembly

Moving around

• movsx - signed mov
  • Pad with 0 for positive or F for negative
    • 0x1000 = 0x00001000
    • 0xFFFF = 0xFFFFFFFF
• movzx - ungsigned mov
  • Always pad with 0
    • 0x1000 = 0x00001000
    • 0xFFFF = 0x0000FFFF
• lea - load effective address
  • Mostly used for math or an array index
    • lea eax, [eax+eax] ; Doubles the value of eax – eax = eax * 2
    • lea edi, [esi+0Bh] ; Adds 11 to esi and stores the result
    • lea eax, [esi+ecx*4] ; Index of an array of ints. esi is pointer to the beginning of an array and ecx is the index. Index is multiplied by 4 because Integers are 4 bytes long. eax will end up storing the address of the ecx’th element of the array.
  • lea edi, [eax+eax*2] ; Triples the value of eax
  • lea edi, [eax+ebx*2] ; This indicates eax stores an array of 16 bit values and that ebx is the offset.

    Math and Logic

• add, sub
  • add eax, 3 ; adds 3 to eax
  • add ebx, eax ; adds the value of eax to ebx
  • sub ecx, 3 ; subs 3 from ecx
• inc, dec essentially increment and decrement
  • inc eax ; eax++
  • dec ecx ; ecx–
• and, or, xor, neg - logical bitwise operators
  • and eax, 7 ; eax = eax & 7 this clears all bits except last 3

  • or eax, 16 ; eax = eax

    16 This sets 5th bit to the right to 1

  • xor eax, 1 ; eax = eax ^ 1 this toggles the right most bit in eax 0=>1 or 1=>0
  • xor eax, FFFFFFFFh ; this is a bitwise negation
  • neg eax ; eax = ~eax
  • xor eax, eax ; eax = 0 this clears eax very quickly
• mul, imul, idiv, cdq
  • mul is unsigned, imul is signed
  • mul ecx ; edx:eax = eax * ecx (unsigned)
  • imul edx ; edx:eax = eax * edx (signed)
  • mul ecx, 10h ; ecx = ecx * 0x10 (unsigned)
  • imul ecx, 20h ; ecx = ecx * 0x20 (signed)
  • div does div and mod at the same time
  • div ecx ; eax = edx:eax / ecx (unsigned) ; edx = edx:eax % ecx (unsigned)
  • idiv ecx ; eax = edx:eax / ecx (signed) edx = edx:eax % ecx (signed)
  • cdq - generally used right before idiv
    • stands for convert double to quad

Common use of cdq and idiv

mov eax, 1007 ; 1007 will be divided
mov ecx, 10   ; by 10
cdq           ; extends eax into edx
idiv ecx      ; eax will be  1007/10 = 100 and edx will be 1007%10 = 7

• shl, shr, sal, sar
  • shl - shift left
  • shr - shift right
  • sal - shift arithmetic left
  • sar - shift arithmetic right
  • equivalent to « and » in C
  • Faster than mul and div involving powers of 2

Divide by 2

mov eax, 16   ;eax = 16    0000 1000
shr eax, 1    ;eax = 8     0000 0100

Multiply by 4

mov eax, 5    ;eax = 5     0000 0101
sal eax, 2    ;eax = 20    0000 1010