On the CoCo mailing list, Ciaran (author of the Xroar emulator), said this:

FWIW this is the bodge I have saved out for doing PCLEAR0 without such
ROM patching:

POKE183,PEEK(183)-6:POKE188,PEEK(188)-6:PCLEAR1:POKE183,PEEK(183)+6:POKE188,PEEK(188)+6

Annoyingly verbose, but should account for DOS, and works on the
Dragon too.

..ciaran

– https://pairlist5.pair.net/mailman/listinfo/coco

This topic came up because of Juan Castro‘s experiments with updating HDB-DOS to add new functionality on a CoCo 1 and 2 (but that is a discussion for a dedicated blog post sometime). Juan had recently “fixed” Extended Color BASIC to allow using “PCLEAR 0” to remove all graphics memory and give more RAM to BASIC. I have discussed PCLEAR 0 in the past…

This mysterious line performs a PCLEAR 0 without needing to load and run program of assembly code!

POKE183,PEEK(183)-6:POKE188,PEEK(188)-6:PCLEAR1:POKE183,PEEK(183)+6:POKE188,PEEK(188)+6

And it works!

But … how does it work!?!

Ciaran, you’ve got some ‘splainin’ to do…

Until then…