Program status word processor status word

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They do the same thing as a subroutine call change the algorithm that the processor is executing however they do it without warning the "executing code" that they are now operating. In order to not damage the "executing code" all information that it was using must be stored.

This includes the Program Counter usually saved to the stack by the interrupt hardware in the same way that a subroutine call does and all of the registers that the interrupt function will alter- these must be saved by pushing them onto the stack. The PSW often called the flag register is a very important register and must generally be saved first.

It contains bits like Zero the last calculation resulted in a zero result Carry the last calculation resulted in a carry ie the result number is bigger than the register can hold and several other flags. I suggest that you read the data sheet of an 8 bit microcontroller for an idea of what these flags might be.

And whilst they will often be ignored you can't take that chance. The subject of interrupts contains concepts that are common to subroutine calls in general e.

And later on in operating systems, the concept of context switches that occur during multi-tasking. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

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