CODEWITHSUNDEEP
operating-systeminterruptcpu-registersinterrupt-handlingprogram-counter
charlesw
charlesw

Reputation: 91

What is the difference between Program Status Word (PSW) and Program Counter (PC)?

In an Operating Systems course, the instructor introduced PSW and PC when he talked about Interrupt Handling.

His explanation was PC holds the address of the next instruction to be fetched PSW contains execution status information

But later I searched online and found that PSW = PC + status register. This makes me quite confused.

On the one hand, I am not sure what "execution status information" refers to. On the other hand, if PSW has the functions of a PC, why do we still need it?

Appreciate any explanation.

Upvotes: 1

Views: 3312

Answers (2)

peter snape
peter snape

Reputation: 1

In general, interrupts are hardware level subroutine calls. 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 registers etc must be restored before the return from interrupt (RETI) instruction - the PC is restored by the RETI itself.

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. suffice it to say that these flags are needed in order to perform conditional jumps. And whilst they will often be ignored you can't take that chance.

You are probably correct in Your instructor using the term PSW to mean all all of the registers.

The subject of interrupts contains concepts that are common to subroutine calls in general (e.g. don't leave data that you don't want overwritten in a register before entering a subroutine). And later on in operating systems, the concept of context switches that occur during multi-tasking.

Peter

Upvotes: 0

Nate Eldredge
Nate Eldredge

Reputation: 58673

This isn't really standardized terminology. Most architectures have some register that plays the role of a status word, containing bits to indicate things like whether an add instruction caused a carry. But different architectures give it different names, and what exactly is included can vary widely. I'm not aware of any architecture that includes the program counter as part of their status word, but if they want to do that, well, who's going to stop them?

This is the kind of thing where you just have to look at the definition given by whatever book or article you are reading (or infer it from context), and realize that a different author may use the word differently.

Upvotes: 0

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