flags registers in assembly language

1. TOPIC
FLAG USAGE
Presented by :
Saman iftikhar
Hira Nasir
Shahwana Fida
Presented to:
Dr. Fouzia jabeen

2. TABLE OF CONTENT
• Define flag
• Kind of flag
• Status Flag
• Control flag
• Discuss 16bits of flag
• Examples
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3. FLAG REGISTER
• Flag register contains 16 bits that show the status of some activities
• Instructions that involve comparison and arithmetic will change the flag status

4. KINDS OF FLAGS
• The status flags are set/reset depending on the results of some arithmetic or logical
operations during program execution
• The control flags to control the operation of CPU
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5. FLAGS
X X X X OF DF IF TF SF ZF X AF X PF X CF
Bits marks X is
undefined
direction Trap Zero
Auxiliary
parity Carry
interrupt
Sign
overflow
 6 are status flags
 3 are control flags
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6. • Status flags
CF
AF
SF
ZF
PF
OF
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7. control flags
DF TF
IF
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8. CARRY FLAG
• When there is last carry out : 1
• When there is not last carry out: 0
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9. ZERO FLAG (ZF)
• Set to 1 when result is 0.
• For non zero result this flag is set to 0.
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10. SIGN FLAG(SF)
• Set to 1 when result is negative.
• When result is positive it is set to 0.
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11. OVERFLOW FLAG(OF)
• When result is to big to fit in the destination: 1
• When result is not to big to fit in the destination: 0
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12. PARITY FLAG(PF)
• This flag is set to 1 when there is even number of one bits in result
• And to 0 when there is odd number of one bits.
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13. AUXILIARY FLAG(AF)
• Every third bit will handle by Auxiliary flag
• When 3rd bit carry exits: 1
• When 3rd bit carry does not exits: 0
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14. INTERRUPT FLAG(IF)
When interrupt is called : 1
When interrupt is not called : 0
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15. DIRECTION FLAG(DF)
• This flag is used by some instructions to process data chains:
• when this flag is set to 0, the processing is done forward,
• when this flag is set to 1 the processing is done backward.
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16. TRAP FLAG:
• System use it when debugging is required;
• When single step mode( debugging) is needed: 1
• When single step mode( debugging) is not needed: 0
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17. HOW INSTRUCTION EFFECT THE
FLAG
Instructions
MOV/ XCHG
ADD/SUB
INC/DEC
Affect flag
None
All
All except CF
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18. •Example of Zero Flag
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22. Example of Sign Flag
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26. •Example of Parity Flag
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30. •Example of Carry flag
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35. •Example of overflow
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39. SUMMARY
• We have discussed flag register and kinds of flag
• Flag register contains bits that show the status
• 9 of its 16bits indicate the current status of the computer and the
result of processing

40. ANY QUESTION?

41. REFERENCES
• https://www.quora.com/What-is-the-difference-between-the-general-
purpose-and-flag-register-in-the-8085-Why-does-the-value-of-the-flag-
register-change-periodically
• https://www.youtube.com/watch?v=4mm_ba3KvvI
• https://www.youtube.com/watch?v=zhO8CLaQrr8&t=1s
• http://www-
id.imag.fr/~briat/perso.html/NACHOS/NACHOS_DOC/CA225b.html
• http://www.apirobuzz.com/2014/08/8086-emulator-for-windows-78-64-
bit.html
• https://emu8086-microprocessor-emulator.en.softonic.com/

42. THANK YOU