by White Flame
This sets off a single-shot NMI to interrupt immediately before an INC statement. The older 6526 triggers one cycle later, so it will run the INC while the newer one won't.
Make sure the screen & sprites are off first.
oldCia should be in zeropage, and will have a 0 or 1 after this routine.
testCIAVersion: ; Set NMI vector lda #<continue sta $fffa lda #>continue sta $fffb lda #$81 ;also don't forget to set mask. Setting $01 to an appropriate value will also help :-) (Bitbreaker/Oxyron) sta $dd0d ; Set timer to 5 cycles lda #4 sta $dd04 lda #0 sta $dd05 ; Clear the detection flag sta oldCIA ; Fire a 1-shot timer lda #%10011001 sta $dd0e ; This should be interrupted before the INC ; only if it's a newer chip. lda $dd0d lda $dd0d inc oldCIA jmp * ; just in case continue: lda $dd0d pla pla pla
Note by Karoshier: I had problems making this work once adapted to use CIA1 instead of CIA2, as the interrupt wouldn't fire at all and the CPU remained locked in the JMP *. The cause of this was the two consecutive LDA $DC0D before the INC oldCIA instruction. It happened by chance that one of them had been cycle-exact synchronized with the CIA and cleared the interrupt flag in the very same cycle as the one in which the interrupt was asserted. The interrupt pulse didn't last enough and the CPU didn't see it (tried on both VICE 2.4 and on the real hardware). I have solved the problem by replacing those LDA $DC0D with NOPs and readjusting the number of clock cycles the timer A is set to timeout at.
Modified to the following to allow detection of both CIAs and should account for the comment above from Bitbreaker & Karoshier. Note that the routine can be slimmed down considerably if you run this in a interrupt controlled environment:
// Detects both CIAs and returns to BASIC after being run // Based on White Flame's routine + kickass format // By TWW/Creators :BasicUpstart2(start) start: sei lda #$35 sta $01 // Disable IRQs and bank out BASIC and KERNAL lda #<continue1 sta $fffe lda #>continue1 sta $ffff lda #<continue2 sta $fffa lda #>continue2 sta $fffb // Set IRQ/NMI vectors lda #$7f sta $dc0d sta $dd0d // Mask out all IRQ/NMI sources lda #$82 sta $dc0d lda #$81 sta $dd0d // Mask in Interrupt Sources from CIA #1 Timer B and CIA #2 Timer A lda $dc0d lda $dd0d // Ack any pending interrupts tsx // Preserve STACK Pointer lda #0 sta $dc06 sta $dc07 sta $dd04 sta $dd05 // Set timers to 1 cycle cli // Allow IRQs to trigger lda #%00011001 sta $dc0f // Fire a 1-shot timer bit $00 // IRQ happens here if it's the new CIA type inc oldCIA1 // IRQ happens here if it's the old CIA type continue1: sei lda #%10011001 sta $dd0e // Fire a 1-shot timer bit $00 // NMI happens here if it's the new CIA type inc oldCIA2 // NMI happens here if it's the old CIA type continue2: txs // Restore STACK Pointer lda #$7f sta $dc0d sta $dd0d lda #$81 sta $dc0d // Mask out IRQs and re-enable CIA #1 Timer A to allow BASIC to run. lda $dc0d // ack the IRQ lda $dd0d // ack the NMI lda #$37 sta $01 cli // enable IRQs and Bank back BASIC and KERNAL lda oldCIA1 sta $0400 lda oldCIA2 sta $0402 // Show result rts oldCIA1: .byte $00 oldCIA2: .byte $00