> >Are you saying that I can expand my computer to 512 kb without
> >problems? (internally or externally?)
>
> Yes. In the S1990 the upper 3 bits are fixed to '1'. So the lower 5 bits
> work. 5 bits gives you 2^5 = 32 pages, 32*16K = 512K.
I know the problem of the fixed '1's in the S1990. It really is a
design flaw and I cannot imagine why they made it this way. It takes
more transistors to drive the upper 3 bits to one than to leave them
floating. But even when it would take less transistors it would not
make sense as the exact number of transistors does not
matter for the costs of a chip, not even at that time.
A possible explanation may be that a full 8-bit mapper is implemented
in the chip. On the chip are two pins to select between a 64K, 128K,
256K and 512K mapper. In the chip, there may be an additional
signal to select between 1024K, 2048K and 4096K. However, as this
signal is not lead to any pin, it is impossible to use this part of
the mapper.
> Note that if you use an external mapper of a different size that the
> internal mapper, you will get the problem of two mappers returning
> different sizes when read. Although this probably will work correctly (it
> did in my NMS8250), it might damage your MSX in the long run (although I'm
> not sure, I'm not really good at hardware). But this problem is not related
> to the S1990 problem, it will happen in any MSX.
I think this is right. Two mappers in one computer may work, but
it is possible that one or both get damaged, even when they are of
the same size. In general, a databus should not be driven from more
than one source.
Greetings,
Jerome Borsboom
Does anybody know what sort of process the S1990 is made with?
(cmos, nmos or pmos)
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