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Assembly Language :CSC 225
(Lec#4: Flag Register and Conditional Statements)
By
Dr. 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
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Name
CF
Carry
PF
Parity
AF
Auxiliary Carry
ZF
SF
TF
IF
DF
OF
Zero
Sign
Trap
Interrupt Enable
Direction
Overflow
Debug Flag Mnemonics
Debug Flag Mnemonics
F=0
F=1
NV
UP
OV
Overflow
DI
DN
Direction
PL
EI
Interrupt
Enable
NZ
NG
Sign
(Negative)
NA
ZR
Zero
PE
AC
Auxiliary
Carry
NC
PO
Parity
CY
Carry
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
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
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 AX,@DATA
MOV DS,AX
MOV BH,10000000B
;CMP BH,11111111B
CMP BH,01111111B
JG SIGNM
JA UNSIGN
SIGNM:
MOV AH,9
LEA DX,MSG1
INT 21H
JMP TERM
UNSIGN:
MOV AH,9
LEA DX,MSG2
INT 21H
TERM:
MOV AX,4C00H
INT 21H
END START
END
Conditional Jump Instructions
CMP Instruction
(1 of 3)
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)
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.