#2055 CIA1 timer A IRQ triggers faster on VICE than on real C64

[Dr. Stephan Pabst]

(Using “x64sc.exe” from GTK3VICE-3.8-win64-r45202 under Windows 11)
(Possibly related to ticket #2502, and thus helpful for analyzing)

When triggering an CIA timer interrupt as fast as possible by setting the start value of timer A to $0000 and starting the timer, VICE will execute only one more opcode before jumping to the interrupt handler, while a real C64 (and "xscpu64.exe") executes two more opcodes.
This behaviour is reproducible by running the following routine with “x64sc.exe”:

          LDX #$FF   ; avoid VIC badlines
@A: CPX $D012
         BNE @A
         INX        ; X=$00
         STX $DC0E  ; halt timer A of CIA 1
         STX $DC05  ; set timer A start value to $0000
         STX $DC04
         LDX #$0E   ; let IRQ vector point to RTI opcode in Kernal ROM
         STX $0314
         LDX #$F0
         STX $0315
         LDA #$5B   ; character code 91 (cross)
         LDX #$19   ; start timer A of CIA 1
         STX $DC0E
         STA $0400  ; write character code 91 to first location of video RAM
         STA $0401  ; ... and to second location
         STA $0402  ; ... and to third location
         RTS        ; return to calling function

You can easily try this out by running the following BASIC program both with VICE and with the real hardware:

10 FOR N=0 TO 43:READ D:POKE 49152+N,D:NEXT
20 DATA162,255,236,18,208,208,251,232,142,14,220,142,5,220,142,4,220,162,14,142
30 DATA20,3,162,240,142,21,3,169,91,162,25,142,14,220
40 DATA141,0,4,141,1,4,141,2,4,96
40 SYS49152

On a real C64, two crosses (character code 91) will show up in the upper left corner of the screen.
With VICE, only one cross appears (see attached screenshot).

[Ingo Korb]

Did you verify that VICE is configured to emulate the same CIA model that your real C64 has? IIRC they differ slightly in their interrupt timing.

[gpz]

are you sure this isn't a difference caused by old vs new CIA?

[gpz]

yup:

x64sc -model c64 bug2055.prg ->two crosses
x64sc -model c64c bug2055.prg -> one cross

(c64 implies old cia here, c64c new cia)

my c64c (with new cia) -> one cross

so its all fine :)

[Dr. Stephan Pabst]

... and my real C64 is the "classic" model - thus the corresponding VICE emulation behaves correctly.

[Uffe Jakobsen]

Strange - pasting the basic program into x64sc gives me multiple crosses no matter what model I specify... I guess I must have missed some important detail here :-)

[gpz]

not using the original kernal perhaps? shrug

[Uffe Jakobsen]

not using the original kernal perhaps?

No - I was using the wrong basic test program from bug #2052 :-D

[gpz]

OOOPS :D

ok closing here :)