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Assembly Language :CSC 225 (Lec#4: Flag Register and Conditional Statements) By Dr. Syed Noman.

Presentation on theme: "Assembly Language :CSC 225 (Lec#4: Flag Register and Conditional Statements) By Dr. Syed Noman."— Presentation transcript:

Assembly Language :CSC 225 (Lec#4: Flag Register and Conditional Statements)
By Dr. Syed Noman

Flag Register 6 Status Flags: C, A, S, Z, P, O
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

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 D7h + CAh D7h + CAh = A1h
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)

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 is 254, but using 8-bit arithmetics the result would be binary, which is -2 in two's complement, and thus negative.

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

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

Jumps Based on Signed Comparisons

Difference In Interpretation Of Signed And Unsigned Numbers
.MODEL SMALL .STACK 100H .DATA MSG1 DB 13,10,"YES JUMP HAPPENS IN SING\$" MSG2 DB 13,10,"JUMP HAPPENED FOR UNSIGNED\$" .CODE START: MOV MOV DS,AX MOV BH, B ;CMP BH, B CMP BH, B JG SIGNM JA UNSIGN SIGNM: MOV AH,9 LEA DX,MSG1 INT 21H JMP TERM UNSIGN: LEA DX,MSG2 TERM: MOV AX,4C00H END START END

Conditional Jump Instructions

CMP Instruction (1 of 3) Example: destination < source mov al,5
cmp al,5 ; Zero flag set Example: destination < source mov al,4 cmp al,5 ; Carry flag set

CMP Instruction (2 of 3) Example: destination > source
mov al,6 cmp al,5 ; ZF = 0, CF = 0 (both the Zero and Carry flags are clear)

CMP Instruction (3 of 3) Example: destination > source
The comparisons shown here are performed with signed integers. Example: destination > source mov al,5 cmp al,-2 ; Sign flag == Overflow flag Example: destination < source mov al,-1 cmp al,5 ; Sign flag != Overflow flag

Example-1 Example : Using the jz instruction.
mov ax, 2 ; ax = 2 sub ax, bx ; ax = 2 - bx jz nextl ; jump if (ax-bx) == 0 inc ax ; ax = ax + 1 nextl: inc bx The above is equivalent to: ax = 2; if ( ax != bx ) { ax = ax + 1 ; } bx = bx + 1 ;

Example-2 C version if ( i == 10 ) { i = i + 5 ; j = j + 5 ; }
/* Rest of program */ Assembly version cmp i, 10 jne rest ; if i != 10 goto rest add i, 5 ; otherwise do action part add j, 5 rest: ; rest of program

Practice Program in class
Write a program that inputs a character and prints whether it is uppercase or lowercase English alphabet.

Assignment 4a Write a program that inputs a character and prints it is a digit, or uppercase/lowercase English alphabet or some other character.

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