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CPU Flags and Operations Guide

The flags register determines the current state of the processor by tracking the results of mathematical operations. It contains flags like the carry flag, parity flag, zero flag, and sign flag that indicate the type of result and allow conditional transfers of program control. The flags register cannot usually be directly accessed.

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115 views8 pages

CPU Flags and Operations Guide

The flags register determines the current state of the processor by tracking the results of mathematical operations. It contains flags like the carry flag, parity flag, zero flag, and sign flag that indicate the type of result and allow conditional transfers of program control. The flags register cannot usually be directly accessed.

Uploaded by

george corbilla
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© © All Rights Reserved
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Flags Register - determines the current state of the processor.

They are modified


automatically by CPU after mathematical operations and allow one to determine the
type of the result as well as determine conditions to transfer control to other parts of the
program.
Generally, you cannot access these registers directly.

1. Carry Flag (CF) - this flag is set to 1 when there is an unsigned overflow. For
example when you add bytes 255 + 1 (result is not in range 0...255). When there is no
overflow this flag is set to 0.
2. 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.
3. Auxiliary Flag (AF) - set to 1 when there is an unsigned overflow for low nibble (4
bits).
4. Zero Flag (ZF) - set to 1 when result is zero. For non-zero result this flag is set
to 0.
5. Sign Flag (SF) - set to 1 when result is negative. When result is positive it is set
to 0. (This flag takes the value of the most significant bit.)
6. Trap Flag (TF) - Used for on-chip debugging.
7. Interrupt enable Flag (IF) - when this flag is set to 1 CPU reacts to interrupts from
external devices.
8. 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.
9. Overflow Flag (OF) - set to 1 when there is a signed overflow. For example,
when you add bytes 100 + 50 (result is not in range -128...127).

Input and output


Printing hello world

Adding
Data type

MOV instructions
Moving small to larger values
Increment and decrement instructions
Add and subs instructions
Jmp instructions
Nested looping

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