From: brain@jbrain.com   
      
   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   
      
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   Jim Brain, brain@jbrain.com   
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