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

2. 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

3. 8086 Flags - Bit Positions and Names

4. Debug Flag Mnemonics      

5. 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.

6. 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)

7. 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 is 254, but using 8-bit arithmetics the result would be 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

10. Jumps Based on Specific Flags

11. Jumps Based on Equality

12. Jumps Based on Unsigned Comparisons

13. 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

14. Jumps Based on Signed Comparisons

15. 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

16. Conditional Jump Instructions

17. 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

18. 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)

19. 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

20. 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 ;

21. 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

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

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