Lecture 4Chapter 6 –Symbolic Instruction and Addressing

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Chapter Outline

Data Transfer Instruction

Basic Arithmetic Instruction

The INT Instruction

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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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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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Data Transfer Instruction

MOVMOVSX-MOVZXXCHGLEA

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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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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MOVE-and-Fill Instruction (MOVSX-MOVZX)

• The MOVSX (move) instruction is used to:• Move a byte or word source to a word or doubleword destination.•Use with signed arithmetic values

• Syntax: MOVSX destination, source

• Example: MOVSX CX,10110000B . CX= 11111111 10110000

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MOVE-and-Fill Instruction (MOVSX-MOVZX)

• The MOVZX (move) instruction is used to:• Move a byte or word source to a word or doubleword destination.•Use with unsigned arithmetic values

• Syntax: MOVZX destination, source

• Example: MOVZX CX,10110000B . CX= 00000000 10110000

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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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Basic Arithmetic Instruction

ADDSUBINCDEC

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 DX21

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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INT Instruction

• To invoke a DOS or BIOS routine, the INT (interrupt) instruction is used.

• Format: INT interrupt_number

where interrupt_number is a number that specifies a routine.

INT 21h

• INT 21h may be used to invoke a large number of DOS functions.

• A particular function is requested by placing a function number in the AH register and invoking INT 21h.

• Some of the functions are:

• INT21h functions expect input values to be in certain registers and return output values in other registers.

Function number Routine1 single-key input2 single-character output9 character string output

INT 21h

• To invoke the routine, the following instructions should be executed: MOV AH,1 ; input key function INT 21H ; ASCII code in AL

Function 1: Single-Key InputInput: AH = 1Output: AL = ASCII code if character key is pressed = 0 if non-character key is pressed

INT 21h

• To invoke the routine, the following instructions should be executed: MOV AH, 2 ; display character function MOV DL, '?' ; character is '?' (or any other character) INT 21H ; display character

Function 2: Display a character or execute a control functionInput: AH = 2

DL = ASCII code of the characterOutput AL = ASCII code of the character

INT 21h

• Function 2 may be used to perform control functions.

• If DL contains the ASCII code of a control character, INT 21h causes the control function to be performed.

• The principal control characters are :

ASCII code (Hex) Symbol Function07H BEL beep (sounds a tone)08H BS backspace09H HT tab0AH LF line feed (new line)0DH CR carriage return (start of current line)

INT 21h

• To invoke the routine, the following instructions should be executed: MOV AX, @DATA MOV DS, AX MOV AH, 9 ; display string function LEA DX, MSG ; get message (Load Effective Address) INT 21H ; display string

Function 9: Display a stringInput: AH = 9 DX = offset address of string.

The string must end with a '$' character

A program containing a data segment should begins with these two instructions

INT 21h

• To invoke the routine, the following instructions should be executed: MOV AH, 4CH ; DOS exit function INT 21H ; exit to DOS

Function 4CH: Returning control to DOSInput: AH = 4CH