| <<< JMP Unconditional Jump | Index | LOOP Loop until CX/ECX = 0 >>> |
O D I T S Z A P C OF: Overflow flag TF: Trap flag AF: Aux carry
F F F F F F F F F DF: Direction flag SF: Sign flag PF: Parity flag
<none> IF: Interrupt flag ZF: Zero flag CF: Carry flag
LEA EBX,[EAX+EDX*4+128] ;Loads calculated address into EBX
LEA BP,MyWordVar ;Loads offset of MyWordVar to BP
LEA derives the offset of the source operand from the start of its segment and loads that offset into the destination operand. The destination operand must be a register and cannot be memory. The source operand must be a memory operand, but it can be any size. The address stored in the destination operand is the address of the first byte of the source in memory, and the size of the source in memory is unimportant.
This is a good, clean way to place the address of a variable into a register prior to a procedure or interrupt call.
LEA can also be used to perform register math, since the address specified in the second operand is calculated but not accessed. The address can thus be an address for which your program does not have permission to access. Any math that can be expressed as a valid address calculation may be done with LEA.
This is one of the few places where NASM does not require a size specifier before an operand that gives a memory address, again, because LEA calculates the address but moves no data to or from that address.
r8 = AL AH BL BH CL CH DL DH r16 = AX BX CX DX BP SP SI DI
sr = CS DS SS ES FS GS r32 = EAX EBX ECX EDX EBP ESP ESI EDI
m8 = 8-bit memory data m16 = 16-bit memory data
m32 = 32-bit memory data i8 = 8-bit immediate data
i16 = 16-bit immediate data i32 = 32-bit immediate data
d8 = 8-bit signed displacement d16 = 16-bit signed displacement
d32 = 32-bit unsigned displacement
| <<< JMP Unconditional Jump | Index | LOOP Loop until CX/ECX = 0 >>> |