6502 interrupt question

[DEBRO]

Does the 6502 restore the status flags after it returns from an interrupt?

 

Lets say you're in a loop...

.doSomething

 lda (somePageValue),y

 sta (newPageValue),y

 dey

 bpl .doSomething

 

and the interrupt happes right after the dey. When you come back if the status flags have changed you could get strange results.

 

2600 programmers don't have to worry too much about this since there are no interrupts (neglecting BRK and location $285). But what of programmers that utilize a true 6502 CPU?

 

Just curious

[Cybergoth]

Hi Dennis!

 

Does the 6502 restore the status flags after it returns from an interrupt?

 

Quick answer: Yes

 

Longer Answer: When an interrupt occurs, this happens:

 

- The MSB of the PC is pushed onto the stack

- The LSB of the PC is pushed onto the stack

- The status flags are pushed onto the stack

- The interrupt flag is set

- The interrupt vector is loaded into the PC

- The interrupt code is executed

 

Ending the interrupt code properly with RTI will pull the three values from the stack and restore everything exactly as it was when the interrupt occured.

 

Greetings,

Manuel

[Nukey Shay]

When you RTI the processor status is automatically fetched from the stack.

[Wrathchild]

Yep, the 6502 handles this, an interrupt causes the status register to be pushed to the stack and then when an RTI is executed, this is popped.

 

On the 4/6/8/XL/XE family, if you use the OS hooks for VBI's (vertical blank interrupts) then the saving of A,X and Y are done for you, wheras you must always restore them in a DLI (display list interrupt).

 

Rgds,

 

Mark

[DEBRO]

Thanks for the quick response guys.