Re: Reproducing the 6522 shift register bug

Thank you Bill. I'll check that out.

----- Original message -----
From: Bill Degnan <billdegnan_at_gmail.com>
To: cbm-hackers_at_musoftware.de
Subject: Re: Reproducing the 6522 shift register bug
Date: Sunday, December 24, 2023 11:42 AM

Byte Dec 1980 page 24 I think touches upon this.
"Multimachine Games"
It has been a while but I'd check a scan I made to see if they address this issue, with the PET which has the same chip, right?
https://vintagecomputer.net/cisc367/ByteDec1980MultimachineGames.pdf

I think there is code you can reference. I did not check to verify 

Bill

On Sat, Dec 23, 2023, 6:13 PM Dave McMurtrie <cbm-hackers_at_commodore.international> wrote:
> Hi cbm-hackers,
> 
> I'm trying to reproduce the 6522 VIA shift register bug for demonstration purposes. I'm doing so with two Commodore VIC-20 computers connected at the user port via CB1, CB2 and PB0. CB1 and CB2 are used for data and clock, and PB0 is used as a busy indicator by the receiver to tell the sender when it's ready to receive another byte.
> 
> I wrote simple sender and receiver code. The sender just sends the ASCII bytes A through Z. The receiver receives those bytes and outputs them to the screen. The sender is using mode 110 (shift out at system clock rate). The receiver is using mode 011 (shift in under control of external input pulses) which is supposed to be the buggy mode.
> 
> The good news is that the data that is output by the receiver isn't always correct. So either I am successfully demonstrating the 6522 shift register bug or I'm just misunderstanding how to use the 6522 properly and my code is wrong.
> 
> The sender is:
> 
> START          LDY #$41 ; Char to send. start with ascii 'A'
>                LDX #$7F
>                STX IER2 ; disable IRQ
>                LDX #$00
>                STX PCR1
>                STX ACR1
>                STX DDRB1 ; PB0 is an input
> LOOP           LDX #%00011000 ; SR mode 110 (bits 2-4) shift out under control
>                               ; of system clock
>                STX ACR1
> BUSY           LDA PB
>                AND #%00000001 ; if PB0 is high, the receiver is busy
>                BNE BUSY
>                STY SR1  ; store the char to send in the shift register
>                INY
>                CPY #$5B ; if we're beyond ascii Z, reset back to A
>                BNE WAIT
>                LDY #$41
>                BRK ; break instead of looping to only send A-Z once
> WAIT           SEI
>                LDA IFR1 ; load the Interrupt Flag Register
>                AND #$04 ; check to see if the SR bit is set
>                BEQ WAIT ; if not, wait.
>                LDX #$00
>                STX IFR1 ; clear the interrupt flag
>                CLI
>                JMP LOOP 
> 
> The receiving code is:
> 
> START          LDX #$7F
>                STX IER2 ; disable IRQ
>                LDX #$00
>                STX PCR1
>                LDX #%00000001 ; set PB0 as an output
>                STX DDRB1
> LOOP           LDX #%00001110 ; SR mode 011 (shift in under control of
>                               ;              external clock pulses)
>                               ;              (bits 2-4)
>                               ; latching on PB is enabled via bit 1
>                STX ACR1
> WAIT           SEI
>                LDX #$00
>                STX PB ; pull PB0 low to indicate ready
>                LDA IFR1 ; load interrupt flag register
>                AND #%00010000 ; check if CB1 interrupt bit is set
>                BEQ WAIT ; if not, WAIT
>                LDX #$01
>                STX PB  ; release PB0 high to indicate busy
>                LDA SR1
>                CLI
>                JSR $FFD2
>                LDX #$00
>                STA IFR1
>                JMP LOOP
> 
> The only documentation I found on how this should work was the MOS6522 data sheet and that was a bit like trying to read the blueprints for a minotaur's home. Let me know if I'm doing something completely wrong here.
> 
> Thanks!
> 
> Dave
>