Does the demo still work if the HEIGHT changes to 91?

[univta0001]

Hi, first of all, thanks for the great demo.

This demo seems to work for HEIGHT for both 90 and 91 and verified in AppleWin. Does HEIGHT = 91 works for real machines ?

I have also experimented the code with NTSC parameters and it works like charm

[Archange427]

I can't test it at the moment because my real A2 is in bad shape (especially the Mockingboard!). But I will test as soon as possible.

And for the NTSC, it's good to know. But this time I won't be able to test at all on a real machine because I only have a PAL version!

[univta0001]

For NTSC, the only code difference is in the main.a around line 323

               LDA VERTBLANK                       ; wait for start of VBL (IIc specific) and INT occurs
                BPL -
                LDY #70                             ; then WAIT 120 (PAL: 70+50) lines = VBL delay
                                                    ; then WAIT 70 (NTSC: 70) lines = VBL delay          
                JSR WAIT65
                LDA #00
                STA bINTIIc                         ; init to zero (IIc only)
                JMP .next
                ; ==========================================================

                ; ==========================================================
                ; CODE FOR IIe 
                ; ==========================================================
.IIe            ; wait for DISPLAY
-               LDA VERTBLANK
                BMI -        
-               LDA VERTBLANK
                BPL -                               ; wait until next DISPLAY
                ; ==========================================================

.next           ; here "beginning" of DISPLAY (no need to be more precise)
                LDY #75
                JSR WAIT65                          ; wait Y lines (first part of HGR)

                SEI

                ; set 6522-1/T2 INT delay - sync code for beginning DGR window
                LDX #$84                                                        
                LDY #$42                             ; one full PAL frame (20280-2 = 4f36)
                                                            ; one full NTSC frame (17030-2 = 4284)                                                                 
                STX $C408                           ; T2C_1-Lower                    
                STY $C409                           ; T2C_1-High  (+reset interrupt flag)

                HEIGHT = 90                         ; lines/height of DGR window
                DELAY1 = HEIGHT*65-2                ; nb of cyles for DGR window (minus 2 for INT)
                DELAY2 = (192+70-HEIGHT)*65-2    ; nb of cycles of the rest of the frame (display+vbl-window) (minus 2) 

The compiled binary is tested on AppleWin and also on my emulator at https://github.com/univta0001/emu6502. It is able to run successfully on both. Strangely my emulator do not work perfectly for PAL. It starts to work if the HEIGHT is set to 91. However using HEIGHT = 91, it does not work for the NTSC binary. AppleWin works for both HEIGHT = 90 / 91 and PAL / NTSC.

Does this demo leverages on the defer IRQ last tick of 6502 for it to work ?

Attached is the compiles binary for DD2 for NTSC

DD2_ntsc.zip

[Archange427]

I didn't know your emulator at all (shame on me). Very interesting! I added it immediately to my workspace so I could directly test my woz/dsk with it from VSC. I see that when we use HEIGHT=90 in PAL, with EMU6502 we seem to lose sync. As if the INT was not waiting for an entire frame but with a few fewer cycles (this is why the transition between modes goes back little by little.) Look around line 712 the interesting part:

            LDA #(<DELAY1-14)                   ; 14 => (LDA+SEC+ADC+STA=12 to substract) - 2 (6522/timer)
            SEC                                 ; compute new delay value (minus cycles already passed)
            ADC $C408                           ; SEC/negative value here (counter = $FFXX)
            STA $C408
            LDA #>DELAY1
            ADC $C409         
            STA $C409                           ; write T2C_1-High / reset IFR

Maybe an issue with the value returned by the 6522 counter in some cases? (DELAY1 = HEIGHT*65-2)

[univta0001]

Hi, thanks for providing the hint at looking at value return from $C408 and $C409. Indeed, it is a implementation issue when writing to $C408 (T2CL), it also writes to T2C. I have updated the implementation to only update to the T2C latch. After this fix, the demo is able to run on both PAL and NTSC with HEIGHT = 90 and 91 (same as AppleWin).

Thanks. you can close this ticket.