microprocessor presentation 8086 flags

Lab-06
Introduction to 8086 flags and addressing modes, and
control transfer instructions

2
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
OF DF IF TF SF ZF AF PF CF
Carry Flag
This flag is set, when there is a carry out
of MSB in case of addition or a borrow in
case of subtraction.
Parity Flag
This is even parity flag. When result has even
number of 1, it will be set to 1, otherwise 0 for
odd number of 1s
Auxiliary Carry Flag
This flag is set, when there is a carry out of low-
nibble in case of addition or a borrow in case of
subtraction
Zero Flag
This flag is set, if the result of the
computation or comparison performed by
an instruction is zero
Sign Flag
After any operation if the MSB is 1, then
it indicates that the number is negative.
And this flag is set to 1
Trap Flag
If this flag is set, the processor enters the
single step execution mode by generating
internal interrupts after the execution of
each instruction
Interrupt Flag
Causes the 8086 to recognize external mask
interrupts; clearing IF disables these interrupts.
Direction Flag
This is used by string manipulation instructions. If this flag bit is ‘0’, the string is
processed beginning from the lowest address to the highest address, i.e., auto
incrementing mode. Otherwise, the string is processed from the highest address
towards the lowest address, i.e., auto decrementing mode.
Over flow Flag
The overflow flag is set to 1 when the result of a signed operation is too large to fit.
Flag Register

3
Addressing Modes
8086 Microprocessor
Every instruction of a program has to operate on a data.
The different ways in which a source operand is denoted in an instruction are
known as addressing modes.
1. Register Addressing
2. Immediate Addressing
3. Direct Addressing
4. Register Indirect Addressing
5. Based Addressing
6. Indexed Addressing
7. Based Index Addressing
8. String Addressing
9. Direct I/O port Addressing
10. Indirect I/O port Addressing
11. Relative Addressing
12. Implied Addressing

4
6. Control Transfer Instructions
Instruction Set
8086 Microprocessor
Mnemonics Explanation
JC disp8 Jump if CF = 1
JNC disp8 Jump if CF = 0
JP disp8 Jump if PF = 1
JNP disp8 Jump if PF = 0
JO disp8 Jump if OF = 1
JNO disp8 Jump if OF = 0
JS disp8 Jump if SF = 1
JNS disp8 Jump if SF = 0
JZ disp8 Jump if result is zero, i.e, Z = 1
JNZ disp8 Jump if result is not zero, i.e, Z = 1
 8086 conditional branch instructions affecting individual flags

5
Checking Different Registers & Flags
Mnemonics Machine
Codes
AX BX CX DX Set Flags
MOV AX, 805EH B8, 5E, 80
MOV DX, 0540H BA, 40, 05
MOV CL, 02H B1, 02
MOV CH, 91H B5, 91
ADD AX, DX 03, C2
MOV BX, 0050H BB, 50, 00
SUB AX, BX 2B, C3
INC DL FE, C2
DEC BX 4B
XCHG CX, BX 87, CB
HLT F4

6
Understanding Overflow and Carry Flags
Mnemonics Machine Code OF CF Comment
MOV AX, 805EH B8, 5E, 80 -ve number
MOV DX, 8040H BA, 40, 80 -ve number
ADD AX, DX 03, C2 +ve number
MOV AX, 505EH B8, 5E, 50 +ve number
MOV DX, 5040H BA, 40, 50 +ve number
ADD AX, DX 03, C2 -ve number
MOV AX, 805EH B8, 5E, 80 -ve number
MOV DX, 5040H BA, 40, 50 +ve number
ADD AX, DX 03, C2 -ve number

7
Understanding Addressing Modes
Mnemonics Machine
Code
Check Contents Comments on
Addressing
Mode
MOV AX, 0001H B8, 01, 00
MOV DS, AX 8E, D8
MOV BX, 2000H BB, 00, 20
MOV AX, [1FFEH] A1, FE, 1F AX = ?
MOV CH, [BX] 8A, 2F CH=?
MOV CL, [BX+01H] 8A, 4F, 01 CL=?
MOV SI, BX 8B, F3
MOV CX, [SI+02H] 8B, 4C, 02 CX=?
MOV DX, [BX+SI+08H] 8B, 50, 08 DX=? CF=?
UP: CMC F5 CF=?
JC UP 72, FD
HLT F4
Memory Data
0001:1FFE 01
1FFF 02
2000 03
2001 04
2002 05
2003 06
0001:4008 07
0001: 4009 08

MOV AX, 0001H
MOV DX, 0000H
MOV CX, 0008H
NEXT: MUL CX
LOOP NEXT
HLT
TEST: Write a program to calculate 8!.

microprocessor presentation 8086 flags

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microprocessor presentation 8086 flags