Interrupt Enable Register ($911e)

[chysn]

Based on my reading of Mapping the VIC, bit 7 of $911e specifies whether you're enabling or disabling the interrupt types.

For example,

Code: Select all

lda #%00000010 ; Disables RESTORE
sta $911e
...
lda #%10000010; Enables it again
sta $911e

So based on this, I would expect

Code: Select all

lda #%01000000
sta $911e

to disable the Timer 1 interrupt. I would expect the cursor to stop flashing with this interrupt disabled.

But it persists in flashing. Why? And what is the proper way to signal that I want to ignore that interrupt?

[Mike]

The KERNAL IRQ (jiffy clock/cursor/keyboard) 'sits' on the VIA #2 Timer 1, so LDA #%01000000:STA $912E does what you want.

[chysn]

The KERNAL IRQ (jiffy clock/cursor/keyboard) 'sits' on the VIA #2 Timer 1, so LDA #%01000000:STA $912E does what you want.

Yes it does, wrong VIA. Thanks!

[chysn]

Disabling the VIA#2 Timer 1 interrupt stopped my data reception from working, so I had to delve into why.

Interrupts coming into VIA#2 use the IRQ vector at $0314/5, while interrupts coming into VIA#1 use the NMI vector at $0318/9.

So my code was only working "accidentally" until now. It also explains the speed problems I had. I only gained speed when I bypassed the BIT $9124 at the end of the service routine.

This is all falling into place. I have to go back to the lab, but I think I understand what I need to do...