[Coco] Implications of the Speed Up Poke

William Astle lost at l-w.ca
Sat Feb 1 13:44:49 EST 2020

On 2020-02-01 10:33 a.m., Allen Huffman wrote:
> Programs like Musica 2 on a CoCo 1/2 had the option to play music in the fast mode, losing video while the audio played at better quality. I never knew there was any RAM concern as you could play long songs with no issues.
> What is the source of the RAM refresh concern?

The source of the RAM refresh concern is the SAM data sheet which 
explicitly says that in the "Fast" mode, RAM refresh and VDG RAM access 
are both disabled because it gives the refresh/VDG part of the machine 
cycle to the CPU.

As far as things running a long time without problems under Fast mode, 
most DRAM holds its contents for a som time longer than the refresh 
interval. Refresh intervals are actually quite conservative because you 
don't want your RAM losing it's contents. For added fun, simply reading 
from memory refreshes the entire row/column (can't remember which) that 
address is in so the code running the music player in your example would 
likely be accessing the memory that implements the music player 
sufficiently often to keep the DRAM refreshed. Well implemented Music 
players would probably also be programmed to ensure that the entire RAM 
space was covered by strategic reads to prevent any of it from decaying.

More information about the Coco mailing list