Background change at Raster Location

[tlr]

I just loved this into vice and it didn't do anything. I'm not sure what happened. I tried PAL and NTSC

It’s currently assembled for PAL with 8KB expansion or more. Maybe you tried to start it on an unexpanded configuration?

[srowe]

2. At that point timer 2 will fire. I don't understand the numbers stored in VIA2T2CH. My little understanding tells me that this fires timer 2 after counting down the number in VIA2T2CL.

Timer 2 is a 16 bit counter, but because the only one register at a time can be set the VIA requires you to first write the LSB then the MSB. The write to VIA2TCH starts the timer running.

4. I intuit that EABF is just doing the normal IRQ. What does EB18 do? Rate Collin West used EB15. What is the difference?

These are entrypoints back into the KERNAL IRQ routine

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10560     LAB_EB12
10561         JSR     FSCNKEY                 ; scan keyboard
10562         BIT     VIA2T1CL                ; test VIA 2 T1C_l, clear the timer interrupt flag
10563         PLA                             ; pull .Y
10564         TAY                             ; restore .Y
10565         PLA                             ; pull .X
10566         TAX                             ; restore .X
10567         PLA                             ; restore .A
10568         RTI

so the difference is in whether the IFR flag for the timer is cleared.

[tlr]

Best home work is to read the 6522 datasheet. There are details in there such as the order of writing to the 16 bit timer registers and interrupt enabling that just aren't obvious from looking at code.

In short you load the timer register with a value and when it crosses from 0>FFFF it flags an event. If that event is defined to cause a CPU interrupt then one is raised. Depending on which VIA it is this is either an IRQ or an NMI.

The tricky bit with raster interrupts is that the cycles to set up the timer and those needed to reach the interrupt handler code are significant.

I just realized these data sheets never surfaced in this thread, so hear goes:

A "recent" one from Rockwell, should be mostly correct: rockwell_r6522_via.pdf
Synerteks version from 1978 seems quite elaborate: synertek_sy6522_via_1978_jan.pdf
Some details might be buried in the preliminary data sheet from MOS: mos_6522_preliminary_nov_1977.pdf

[beamrider]

deleted

[tlr]

@tlr

I just realised there's a typo in your listing.

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time_list:
	SPLIT	76*LINETIME,$1b

should be

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	SPLIT	7*6*LINETIME,$1b

76 is intentional. PAL only. Admittedly only tested using the latest vice trunk build.

[beamrider]

My bad...

I had 12*8 further down...