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A status register (also: flag register or condition code register (CCR)) is a collection of flag bits for a processor. A popular example of a status register is the FLAGS register of x86 architecture based microprocessors.
[edit] Common status register flags
| Flag |
Name |
Description |
| Z |
Zero flag |
Indicates that the result of a mathematical or logical operation was zero. |
| C |
Carry flag |
Indicates that the result of an operation produced an answer greater than the number of available bits. (This flag may also be set before a mathematical operation as an extra operand to certain instructions, e.g. "add with carry".) |
| X |
Extend flag |
(?) |
| N / S |
Negative / Sign flag |
Indicates that the result of a mathematical operation is negative. In some processors[1], the N and S flags have different meanings: the S flag indicates whether a subtraction or addition has taken place, whereas the N flag indicates whether the last operation result is positive or negative. |
| V / O / W |
Overflow flag |
Indicates that the result of an operation has overflowed according to the CPU's word representation, similar to the carry flag but for signed operations. |
| I / E |
Interrupt enable flag |
Interrupts can be enabled or disabled by respectively setting or clearing this flag. Modifying this flag may be restricted to programs executing in supervisor mode. |
| P |
Parity flag |
Indicate that the number of bits of the result is odd or even. |
| D / T |
Debug / Trap flag |
Specifies that a special tracing interrupt is generated immediately after each single instruction is executed. This is used to implement program code debuggers. |
[edit] See also
[edit] References
- ^ Toshiba 900 Operation Manual, chap. 3
