Hi Bo, The CA1 line generates IRQ only. The NMI is not actually connected in the PET. You may be able to use IRQ but remember that multiple things generate IRQ's, such as the CRTC vertical retrace, and cassette data. Steve On Wednesday, May 3, 2023 at 07:39:28 p.m. EDT, Bo Zimmerman <bo_at_zimmers.net> wrote: My original program is indeed assembly, but much more complex. I thought a nice short BASIC program might reveal my problem more clearly. Here's the identical program in assembly -- with the exact same results. I have a multimeter sitting on the CA1 line, so I can see it go high->low, and also see the lack of activity. *=$1000 lda #<NMIH sta $94 lda #>NMIH sta $95 lda #0 sta $e84d lda #$82 sta $e84e NOSP jsr $ffe1 bne NOSP rts NMIH inc $8000 rti Sorry, I'm typo-ing all over my emails today. The $ea4e should be $e84e. I had it right in the code I'm running though, so again-- I don't understand why my screen character does not change. Thanks again, - BoReceived on 2023-05-04 04:00:03
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