That's an amusing coincidence for an operand, in light of the issue being examined.

Mike B.

Hah, I didn't see it. Thanks for lightening my day.

After reading parts of WDC's '265 datasheet and tracing into the '265 Monitor ROM listing (available here) I'm now pretty sure the '265 ACIA is unlike the 'c51, and there's no reason to suppose it has the same bug. For example it appears the ROM uses an interrupt-powered queue to output characters to the PC host -- and interrupt-driven output is where the 'c51 fails. More detail later... this is just an update.

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Man, how did you read all that? The typesetting on the ROM listing is atrocious!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Ok yah I was wondering if the output there was using interrupts. And also feeling foolish for using the UART there would use the same bits. I'll read the docs for the chip on the UART section and see if I can change my code.

It would be a relief. I am hoping to make a few PCBs to stack/connect to it; a RAM expander, but also a MIDI one, which will use the UARTs. Just wish WDC had published the measurements of their PCB and all the components in it. Better yet, Eagle CAD schematics + PCB layouts for the board would be amazing, for those wanting to use this chip in their own designs.