On Sat, 6 Feb 1999, CLAUDIO MASSAO KAWATA - 900293 wrote:
> Third (the first really MSX related topic): does anybody know
> how the Z80's "R" register works? It controls the "REFRESH" sign
> of the CPU, but when is it generated? When it counts "zero"? It
> wouldn't be very smart, 'cause the register can be set by soft...
> So...?
The R-register is increased after every refresh, for old memory-modules.
They had to be refreshed in parts. For the module, to see which part would
be refreshed, the code would be on the data-bus (or the lower 8 bit of the
address bus, I don't remember). This is what the Refresh-register (R for
short) is used for. I believe a refresh is done once every machine cycle,
so you can calculate the value of R. I don't see the use of setting the
R-register, except if you program a Z80 (not a MSX) without memory and you
need just one more byte. If you use it as memory, you really have to watch
your timing very much. Mostly it is used as a random generator (or as a
part of one), since in a normal program (with interrupt and so on in the
loop) it will be random what's the value in it. I use it in a game I want
to sell in Tilburg... There is one problem about it, though. The Z380,
which will most probably be the CPU in the next MSX (if there will be
any), does not change the value of R. This means that programs like my
game wouldn't work (my game would just be very boring, actually. It would
be almost impossible to loose). Of course this would not be the problem of
the program, but the problem of the computer... If it wants to call itself
MSX, it has to be compatible with the older ones. I don't think it will
be, and I don't think anybody really cares. You could put a Z80 in it if
you really want to. Those things are almost free these days (3US$ or
something).
Bye,
shevek
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