On 11/13/2021 10:03 PM, George wrote:
> I was going over some of my old CBM files from back in the day, and ran
> across an error I had found in the Plus 4 kernel rom.  I don't think I ever
> found a way to report it to anyone, so I thought I would see if anything
> has changed.
>
> The error is in the 6551 ACIA servicing routine where a byte is read in
> from the ACIA:
>
> LDA $FD00
> BEQ EAC2
> STA $07D5
>
> Incoming bytes are first stored at $0FD5, and later moved from there into
> the input buffer.  But as the rom is written, any null byte (00) received
> would be later stored as whatever the most recent non-null byte was.  And
> it's impossible to receive a null byte.  The solution is to reverse the
> second and third instructions:
>
> LDA $FD00
> STA $07D5
> BEQ EAC2
>
> Or you could duplicate the beginning of the IRQ servicing up to this point
> in your code, with the correction, then jump back into the rom.
>
> Of course, this being the Plus 4, it may be that nobody would ever care
> about this, but it would be nice to make a record in case anyone ever
> wanted to make other revisions to the ROM, or actually make use of the UART
> capability of the Plus 4.
>
> George Hug
>

I forwarded it the cbm-hackers mailing list, where a bunch of the
technical gurus hang out.

Jim

--
Jim Brain, brain@jbrain.com
RETRO Innovations: Contemporary Gear for Classic Systems
www.go4retro.com

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