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Load Effective Address LEA

Syntax:

l dest, [expr]
  • lea calculates expr and stores the result inside dest.
  • It doesn't actually access any memory. It only calculates the resulting address.
  • Can be used to calculate addresses.
  • Doesn't change the flags register.
  • dest has to be a register.

Examples​

; will add 1 to eax and store the result in eax
lea  eax, [eax+1]

lea esi, [eax + 2 * edx]

; di is 16 bits (2 bytes), we might need to
; wrap around if expression so (eax + 2 * edx + 5) is larger than 2 bytes 
; ==> (eax + 2 * edx + 5) % 2^16
lea di, [eax + 2 * edx + 5]

Calculate Addresses​

section '.data' data readable writeable
 nums      dd 100h dup (12345678h)
 snums     dw 100h dup (0ababh)
 
section '.text' code readable executable
start:
 mov       esi,nums
 mov       edi,snums
 call      read_eax
 
 ; Get address of dword number eax
 lea       edx, [esi + 4*eax]
 
 ; Get dword number eax
 mov       edx, [esi + 4*eax]
 
 ; Get address of word number eax
 lea       ebx, [edi + 2*eax]
 
 ; Get word number eax
 mov       ebx, [esi + 2*eax]

Adding Numbers from 0 to 100 using lea and test​

 mov       ecx, 100
 xor       ecx, ecx
 
add_num
 add       edx, edx
 lea       ecx, [ecx - 1]
 test      ecx, ecx
 jnz       add_num

Combining Instructions​

mov      esi, ecx
shl      edx, 2
add      esi, edx
add      esi, 5

Can be converted to:

lea      esi, [ecx + edx * 4 + 5)