Alex Wulms wrote:
>
> ] On Tue, 20 Jun 2000, David Heremans wrote:
> ]
> ] > Is there some page which could tell me how all the MSX version react on
> ] > an in a,(#FF). I know it shouldn't be done but I'm trying to change a
> ] > program...
> ]
> ] In MSX turbo R and some MSX2+ machines, the upper 3 bits will always be 1.
> ]
> ] The value read will be the selected segment in the largest mapper in the
> ] system.
> Unfortunately this is not the case on the MSX turbo R when you insert an
> external mapper with a size larger then 512kB. The MSX turbo R has
> low-impedance pull-up registers for the 3-upper bits. They pull the line up
> stronger then the average external mapper can pull the line down. Hence, the
> 3-upper bits will always be 1. Even if the external mapper tries to pull one
> of those bits down to 0.
>
> Kind regards,
> Alex Wulms
Some time ago I noticed that Turbo-R machines does that with all MSX
system
reserved I/O port ... Then if you try to read any I/O port that has a
internal
pehiperal installed the S1990 chip engine will block output from the
cartridge.
I guess it's made this way to avoid "totem pole" (bit conflicts) of
S1990
internal buffer with the internal machine devices such as PPI, VDP,
PSG , Mapper, etc ....
Cya MSXers...
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