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Blues76

6502 processor status register

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Hi, 

 

I hope this was not asked already. I didn’t find anything 

 

Why does the status register reserve bit 5 (unused)? what was the thinking ? maybe is internally used ? 

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My initial post wrong - the Atari OS does in fact have BRK as the last checked for IRQ type.

If the status copy on the stack doesn't indicate an instructional push, then it just returns from the IRQ handler (something that should never happen on stock hardware)

Edited by Rybags

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On 11/27/2020 at 5:21 AM, RevEng said:

Yup, It's used internally.

In the article referenced, the use of bit 4 as a pseudo-register bit to indicate an interrupt when the P register is pushed to the stack is explained, but bit 5, being pushed as '1' under all circumstances, still appears redundant....

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1 hour ago, zbyti said:

I found this very interesting

Yes, I was surprised I watched it till the end 😀

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