CEN 226: Computer Organization & Assembly Language :CSC 225(Lec#5)

ByDr. Syed Noman

2

Flag Register

•Flag is a bit of special information usually implemented with flip flop

• Total 9 Flags ▫6 Status Flags: C, A, S, Z, P, O ▫3 Control flags: I, T, D

8086 Flags - Bit Positions and Names

Bit Name0 CF Carry12 PF Parity34 AF Auxiliary Carry5 6 ZF Zero7 SF Sign8 TF Trap9 IF Interrupt Enable

10 DF Direction11 OF Overflow12 131415

Debug Flag Mnemonics F=0

F=1

NV UP DI PL NZ NA PE NCOV DN EI NG ZR AC PO CY

Overflow Direction InterruptEnable

CarryCarry(Negative)

Sign Zero Auxiliary Parity

Debug Flag Mnemonics      

Status Flags

• Sign(SF) – set when the most significant bit is a one.

• Zero(ZF) – set when the result of an arithmetic or logical operation is zero.

• Carry (CF) – set when the result of an unsigned arithmetic operation produces a carryout.

• Overflow(OF) – set when the result of a signed arithmetic operation is in error.

• Auxilary (AF) – set when carry is generated from bit 3 to 4 in addition or borrow is taken during subtraction from bit 4 to 3.

• Parity (PF) – set when number of 1’s are odd in the answer.

Problem•What are the flag settings of the result of

the following 8-bit HEX addition?▫D7h + CAh▫D7h + CAh = A1h ▫Zero (0) ▫Negative(1)▫Carryout (1) ▫Overflow (0)▫Auxiliary Carry (1)▫Parity (1)

Problem•What are the flag settings of the result of

the following 8-bit HEX addition?▫38h + C8h▫38h + C8h = 00h▫Zero (1) ▫Negative(0)▫Carryout (1) ▫Overflow (0)▫Auxiliary Carry (1)▫Parity (0)

8

Overflow flag

•A negative result out of positive operands (or vice versa) is an overflow

•if we add 127 and 127 using 8-bit registers. 127+127 is 254, but using 8-bit arithmetics the result would be 1111 1110 binary, which is -2 in two's complement, and thus negative.

9

Program Control Instructions

•Instructions that direct the flow of a program and allow the flow to change.

•Unconditional Jump (jmp)•Conditional Jumps

Jumps Based on Specific Flags

Jumps Based on Equality

Jumps Based on Unsigned Comparisons

Jumps Based on Signed Comparisons

Conditional Jump Instructions

15

The Compare Command

•Compares the destination operand to the source operand▫Nondestructive subtraction of source

from destination (destination operand is not changed)

•Syntax: CMP destination, source•Example: destination == source

CMP Instruction (1 of 3)

mov al,5cmp al,5 ; Zero flag set

• Example: destination < source

mov al,4cmp al,5 ; Carry flag set

CMP Instruction (2 of 3)

• Example: destination > source

mov al,6cmp al,5 ; ZF = 0, CF = 0

(both the Zero and Carry flags are clear)

CMP Instruction (3 of 3)

• Example: destination > source

mov al,5cmp al,-2 ; Sign flag == Overflow flag

The comparisons shown here are performed with signed integers.

• Example: destination < source

mov al,-1cmp al,5 ; Sign flag != Overflow flag

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Difference In Interpretation Of Signed And Unsigned Numbers

.MODEL SMALL

.STACK 100H

.DATAMSG1 DB 13,10,"YES JUMP HAPPENS

IN SING$"MSG2 DB 13,10,"JUMP HAPPENED

FOR UNSIGNED$".CODESTART:MOV AX,@DATAMOV DS,AX

MOV BH,10000000B;CMP BH,11111111BCMP BH,01111111BJG SIGNMJA UNSIGN

SIGNM:MOV AH,9LEA DX,MSG1INT 21HJMP TERM

UNSIGN:MOV AH,9LEA DX,MSG2INT 21H

TERM:MOV AX,4C00HINT 21HEND STARTEND

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Example-1• Example : Using the jz instruction.

mov ax, 2 ; ax = 2sub ax, bx ; ax = 2 - bxjz nextl ; jump if (ax-bx) == 0inc ax ; ax = ax + 1nextl:inc bx

• The above is equivalent to:ax = 2;if ( ax != bx ){ax = ax + 1 ;}bx = bx + 1 ;

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Example-2

•C versionif ( i == 10 ){i = i + 5 ;j = j + 5 ;}/* Rest of program */• Assembly versioncmp i, 10jne rest ; if i != 10 goto restadd i, 5 ; otherwise do action partadd j, 5rest: ; rest of program

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Practice Program in class

•Write a program that inputs a character and prints whether it is uppercase or lowercase English alphabet.

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Practice Program solution.model small

.stack 100h

.datamsg0 db 13,10, "Enter a character:: $"msg1 db 13,10,"it is an uppercase letter.$"msg2 db 13,10,"it is a lowercase letter.$"msg3 db 13,10,"it is NOT a letter!$"

.codestart:mov ax, @datamov ds, ax

mov ah,9lea dx,msg0int 21h

mov ah,1int 21hmov bh, al

cmp bh,'A'jb not_a_lettercmp bh,'Z'jbe uppercase

cmp bh,'a'

jb not_a_letter; means between 91 - 96 included

cmp bh,'z'

jbe lowercase

not_a_letter:

mov ah,9

lea dx,msg3

int 21h

jmp finish

uppercase:

mov ah,9

lea dx, msg1

int 21h

jmp finish

lowercase:

mov ah,9

lea dx, msg2

int 21h

finish:

mov ax,4c00h

int 21h

end start

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Assignment 2a

•Write a program that inputs a character and prints it is a digit, or uppercase/lowercase English alphabet or some other character.