Assembly Language

Lecture 3

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Lecture Outline

•Named Constants• Registers in 8086• Instruction Types• MOV, XCHG, LEA, ADD, SUB, INC, DEC, NEG

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Named Constants - EQU (Equates)

• To assign a name to a constant, we can use the EQU pseudo-op.

• Syntax: name EQU constant

• Examples: LF EQU 0AH MOV DL,0AH = MOV DL,LF

PROMPT EQU 'Any Thing' MSG DB 'Any Thing' = MSG DB PROMPT

• Note: no memory is allocated for EQU names. 3

Registers

• Information inside the microprocessor is stored in registers.

• The registers are classified according to the functions they perform In general there are fourteen 16-bit registers:

• Data registers:• There are four general data registers. • They hold data for an operation.

• Address registers: • They are divided into segment, pointer, and index registers.• They hold the address of an instruction or data.

• Status register: • It is called the FLAGS register.• It keeps the current status of the processor.

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RegistersAH AL

BH BL

CH CL

DH DL

AX

BXCXDX

CS

DSSSES

SIDISPBP

IP

Data Registers

Segment Registers

Pointer and Index Registers

FLAGS Register

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Data Registers: AX, BX, CX, DX

• These four registers, in addition to being general-purpose registers, also perform special functions.•The high and low bytes of these registers can be accessed separately.

• Ex. The high byte of AX is called AH, and the low byte is called AL.

• This arrangement gives us more registers to use when dealing with byte-size data.

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Data Registers: AX, BX, CX, DX

• AX (Accumulator Register) is the preferred register to use in arithmetic, logic, and data transfer instructions• BX (Base Register) also serves as an address register.• CX (Count Register) Program loop constructions are facilitated by the use of CX, which serves as a loop counter.•DX (Data Register) is used in multiplication and division.

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Address Registers - Segment Registers CS, DS, SS, ES

• Address registers store addresses of instructions and data in memory.

• These values are used by the processor to access memory locations.

• In the 8086 processor (16-bit processor):• Memory is a collection of bytes, each memory byte has an

address, starting with 0.• The processor assigns a 20-bit physical address to its memory locations thus it is possible to address 2 = 1,048,576 bytes (one megabyte) of memory.• The bytes in memory have addresses 00000h to FFFFFh.

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Address Registers - Segment Registers CS, DS, SS, ES

• To keep track of the various program segments, the 8086 is equipped with four segments registers to hold segment numbers:

• CS (Code Segment): contains the code segment number.• DS (Data segment): contains the data segment number.• SS (Stack Segment): contains the stack segment number.• ES (Extra Segment): is used if a program needs to access a second data segment.

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Instruction Types

• Data transfer instructions:• Between registers.• Between registers and memory.• Between registers and I/O devices.• Examples: move, exchange, push, pop, input, output.

• Data manipulation instructions:• Arithmetic operations: add, subtract.• Logical operations: and, or.• Shift/Rotate: shift right, shift left.

• Program control instructions:• Examples: jump, call, loop, test, compare.

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Address Registers - Pointer and Index Registers: SP, BP, SI, DI

• SP (Stack Pointer) register is used in conjunction with SS for accessing the stack segment.

• BP (Base Pointer) register is used primarily to access data on the stack. However, unlike SP, BP can be used to access data in the other segments.

• SI (Source Index) register is used to point to memory locations in the data segment addressed by DS. By incrementing the contents of SI, we can easily access consecutive memory locations.

• DI (Destination Index) register performs the same functions as SI. There is a class of instructions, called string operations, that use DI to access memory locations addressed by ES.

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Address Registers - Instruction Pointer (IP)

• IP is updated each time an instruction is executed so that it will point to the next instruction.

• Unlike other registers, the IP cannot be directly manipulated by an instruction (i.e. The instruction cannot contain IP as its operand).

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MOV Instruction

• The MOV (move) instruction is used to:• Transfer data between Registers.• Transfer data between registers and memory locations.• Move a number directly into a register or memory location.

• Syntax: MOV destination, source

• Example: MOV AX, WORD1 MOV AX, BX MOV AX, 'A'

Before After 0006 0008

AX AX

0008 0008

WORD1 WORD1

Note: any register can be used except CS & IP

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Legal Combinations of operands for MOV

Destination Operand General Segment Memory

Source operand register register location Constant General register yes yes yes noSegment register yes no yes noMemory location yes yes no noConstant yes no yes no

• Illegal: MOV WORD1, WORD2 • Legal: MOV AX, WORD2 MOV WORD1, AX

• Illegal: MOV DS, CS • Legal: MOV AX, CS MOV DS, AX

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XCHG Instruction

• The XCHG (exchange) operation is used to exchange the contents of:

• Two registers.• A register and a memory location.

• Syntax: XCHG destination, source

• Example: XCHG AH, BL XCHG AX,WORD1

Before After

AH AL AH AL

BH BL BH BL

1A 00 05 00

00 05 00 1A

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Legal Combinations of operands for XCHG

Destination Operand General Memory

Source Operand register location General register yes yesMemory location yes no

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Data Definition + Basic Instructions 17

LEA Instruction

• LEA (Load Effective Address) puts a copy of the source offset address into the destination.

• Syntax: LEA destination, source Where destination is a general register and source is a memory location

• Example: MSG DB 41H, 42H, 43H LEA DX, MSG puts the offset address of the variable MSG into DX.

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ADD and SUB Instructions

• The ADD (add) and SUB (subtract) instructions are used to:• Add/subtract the contents of:

• Two registers.• A register and a memory location.

• Add/subtract a number to/from a register or memory location.

• Syntax: ADD destination, source SUB destination, source

• Examples: ADD WORD1, AX SUB AX, DX

Before After 01BC 01BC

AX AX 0523 06DF

WORD1 WORD1

Before After 0000 FFFF

AX AX 0001 0001

DX DX18

Legal Combinations of operands for ADD & SUB

• Illegal: ADD BYTE1, BYTE2• Legal: MOV AL, BYTE2 ADD BYTE1, AL

Destination Operand General Memory

Source Operand register location General register yes yesMemory location yes noConstant yes yes

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INC and DEC Instructions

• INC (increment) is used to add 1 to the contents of a register or memory location.

• DEC (decrement) is used to subtract 1 from a register or memory location.

• Syntax: INC destination DEC destination

• Examples: INC WORD1 DEC BYTE1

Before After

WORD1 WORD1 0002 0003

Before After

BYTE1 BYTE1 FFFE FFFD

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NEG Instruction

• NEG is used to negate the contents of the destination.

• It does this by replacing the contents by its two’s complement.

• Syntax: NEG destination

• Examples: NEG BX

Before After

BX BX 0002 FFFE

Negative integers are stored using 2’s complement in the computer

Example:

Want to store -5 in memory

1’s complement= 11111010

+1

11111011

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Type Agreement of Operands

• The operands of any two-operand instruction must be of the same type (i.e. Both bytes or words).

• Illegal: MOV AX, BYTE1

• However, the assembler will accept both of the following:• MOV AH, 'A' moves 41H into AH• MOV AX, 'A' moves 0041H into AX

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