19. More about LEA Instruction

• LEA does loading of memory offset value into a register.

• Suppose we want to load EBX with the offset value of table1.

• We could write

          mov    ebx, OFFSET table1
  

• OFFSET operator resolves offset at the assembly time.

• Another way of loading memory offset is by using the LEA instruction:

• LEA resolves offset at run time:

          lea    ebx, [table1]     ;  load effective address of table1 into ebx
  

• The format of LEA instruction is

  • LEA register, source

• We must to use LEA when the needed offset is available at run time only.

• Consider array index, passed as a parameter to a procedure in register ESI, would require

• To load EBX with the address of the element of table1, where element index is in ESI,

          lea    ebx, [ table1 + esi ]
          lea    ebx, [ table1 ] + esi   ;  does the same thing
          lea    ebx, table1[ esi ]      ;  does the same thing
  

  NOTE: We cannot use the MOV instruction to do this !