Processor Status Register

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The Processor Status Register (P) is a 1 byte register with each bit representing a flag. These flags are used to indicate whether something has or has not occurred based on the result of an instruction.

As instructions are executed, they have the possibility of altering flags based on the opcode. These alterations can either Change, Ignore, Set, or Clear a flag.

  • Change - This flag will either be set or cleared based on the operand.
  • Ignore - This flag will not change.
  • Set - This flag will always be set to on, that is, have a value of 1.
  • Clear - This flag will always be set to off, that is, have a value of 0.
Bit # Flag
7 N - Negative Flag
6 V - Overflow Flag
5 1 - Always 1 Flag
4 B - Break Flag
3 D - Decimal Flag
2 I - Interrupt Flag
1 Z - Zero Flag
0 C - Carry Flag