[Coco] SAM and RAM refresh.
afra at aurigae.demon.co.uk
Sat Feb 10 12:56:37 EST 2007
I've just as promised had a poke around the sam with a scope when it's
in double speed mode (poke &hffd9,1), and found the following :
The CAS and RAS signals seem to always toggle at 1.7MHz, irrespective of
the speed selected, in normal mode what happens is that the CPU and
video access the ram on alternate CAS/RAS cycles, as expected, and in
double speed the CPU gets an access every cycle and the VDG gets none.
So as documented on the SAM data sheet, the CPU literally steals all of
the VDG access, and so no refresh happens.
The interesting thing I found was that the RAS/CAS still toggle even if
the CPU is accessing ROM, I guess this is doing a dummy read and the
gating on the ram data out prevents the read data from reaching the CPU
I suppose this could be exploited with some extra circuitry, to do a
refresh when the CPU is accessing ROM, but this would of course only be
of use to BASIC or ROM based programs, and could not be used in a RAM
based os such as OS-9 or Flex.
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
"You can twist perceptions, but reality won't budge" -- Rush.
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