[Coco] Implications of the Speed Up Poke
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