Instruction Set

Data Transfer Instructions Overview

General Purpose Data Transfer

MOV

Copy data from source to destination.

Usage:

MOV D, S

The following situations are not allowed to use MOV:

• Memory → Memory
• Immediate → Segment Register
• Segment Register → Segment Register

PUSH

Usage:

PUSH SRC

Process:

1. SP <- SP - 2
2. (SP + 1, SP) <- SRC

POP

Usage:

POP DEST

Process:

1. DST <- (SP + 1, SP)
2. SP <- SP + 2

It loads $16$ bits content from SS:SP to DST.

XCHG

Usage:

XCHG OPR1, OPR2

It exchanges values between two operands.

XCHG is not allowed:

• Memory ↔ Memory
• Segment ↔ Registers

XLAT

Usage:

XLAT

Process:

AL <- DS:[BX + AL]

Address Object Transfer

LEA

Load effective address.

Usage:

LEA REG, SRC

Process:

REG <- SRC

LDS

Load pointer into DS.

Usage:

LDS REG, SRC

Process:

1. REG <- SRC
2. DS <- SRC + 2

LES

Load pointer into ES.

Usage:

LES REG, SRC

Process:

1. REG <- SRC
2. ES <- SRC + 2

Flag Transfer

IN/OUT Instructions

Usage:

IN REG, PORT
OUT PORT, REG

The PORT ranges from $0$ to $\text{FFH}$.

Arithmetic Instructions

Addition

Subtraction

Multiplication

Division

CBW

Usage:

CBW

Process:

AL -> AX.

If MSB of AL is $1$, then AH = 255. Otherwise, AH = 0.

CWD

Usage:

CWD

Process:

AX -> (DX, AX)

If MSB of AX is $1$, then DX = 65535. Otherwise, DX = 0.

Adjusting Instruction of BCD Code

DAA and DAS

These two instructions are used in packed BCD.

• DAA: Decimal Adjust after Addition
• DAS: Decimal Adjust after Subtraction

If $\text{AL} \; \& \; \text{0FH} > 9$ or AF = 1, then $\text{AL} \; \pm= \text{06H}$ and AF = 1.

If $\text{AL} \; \& \; \text{F0H} > \text{90H}$ or CF = 1, then $\text{AL} \; \pm= \text{60F}$ and CF = 1.

AAA and AAS

These two instructions are used in unpacked BCD.

• AAA: ASCII Adjust after Addition
• AAS: ASCII Adjust after Subtraction

If $\text{AL} \; \& \; \text{0FH} > 9$ or AF = 1, then the adjustment happens, and AF = CF = 1.