Chapter 4 - Data Transfers, Addressing, and Arithmetic

Before discussing different modes of addressing, let's lay out the possible types of operands that can exist.

Operand  Description
-------  -----------
r8       8-bit general purpose register [GPR] (AH, AL, ... DH, DL)
r16      16-bit GPR (AX, BX, CX, DX, SI, DI, SP, BP)
r32      32-bit GPR (EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP)
reg      any GPR
sreg     16-bit seg reg (CS, DS, SS, ES, FS, GS)
imm      any immediate value of 8, 16, or 32-bits
imm8
imm16
imm32
r/m8     8-bit register or memory byte
r/m16
r/m32
mem      a memory operand of size 8, 16, or 32-bits

Memory Addressing

Remember, the IA-32 Processor can address 1MB of memory in Real Mode and 4 GB in Protected Mode. We will assume Real Mode unless otherwise stated.

Direct Memory Addressing

Consider:

.data
val1 BYTE 5h

If val1 was located at address 1400h, then an assembled instruction would look something like:

mov al,[1400]

(i1) MOV dest, src

MOV places the contents of the src operand into the destination location.

Operands must match in size
Both operands cannot be memory
The destination operand cannot be CS, EIP, or IP
One cannot move an immediate value into a segment register

When looking at possible MOV instructions, the following are possible legal instructions:


MOV reg, reg
MOV mem, reg
MOV reg, mem
MOV mem, imm
MOV reg, imm
MOV r/m16, sreg       ; Real Mode Only
MOV sreg, r/m16       ; Real Mode Only

P#1: Since we cannot do a mem to mem MOV instruction, write an assembly language program segment that has both a data and code segment. Place two variables in the data segment and move the value from the first variable to the second.

(i2) MOVZX dest, src

MOVZX r32, r/m8
MOVZX r32, r/m16 
MOVZX r16, r/m8

The MOVZX zero extends the src value and places the result into the destination operand.

Q#1: What does it mean to zero extend a value?

Q#2: Why would we use an instruction that zero extends a value? HINT: Think about unsigned vs 2's complement numbers?

(i3) MOVSX dest, src

MOVSX r32,r/m8
MOVSX r32,r/m16 
MOVSX r16,r/m8

The MOVSX sign extends the src value and places the result into the destination operand.

Q#3: What does it mean to sign extend a value?

(i4) LAHF ; load the low byte of the flags register into AH
(i5) SAHF ; store the value of AH into low byte of the flags register

(i6) XCHG op1, op2

XCHG reg,reg 
XCHG reg,mem 
XCHG mem,reg

The XCHG instruction interchanges the values in op1 and op2.