|システムコア||低||解決済み||修正済み||2013-02-09 00:24||2021-05-21 00:25|
|詳 細||This bug can be observed by typing the following after starting up the emulator in BASIC:|
POKE &HFF23,&H38:POKE &HFF22,&HF0:POKE &HFF23,&H3C:SOUND 1,100
This code initializes the Port B control and data direction registers of the 2nd 6821 PIA chip (labeled IC4 / 68B21 on the Coco3 schematic). It shouldn't do anything at all; it just initializes them to their defaults. The sound statement is just there to insert a delay before the coco BASIC resets the screen.
So the bug is that during the sound delay, the screen goes into some coco 2-style graphics mode while running this in MESS. It does not do this on my real coco 3; it stays in the 32-column text mode during the delay. Removing the second POKE statement ($FF22) will result in the expected behavior, so this appears to be the source of the problem. Could it be that this memory address which is reserved for the PIA is also being routed to the VDG? I have not tested the coco1/2 system under MESS to see if the problem also appears there.