At 22:28 25/05/98 +0100, you wrote:
>> >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.
By the way I've upgraded internaly my Turbo R GT and some ST and GT of some
friends and I can say that it's possible to uprade the internal mapper up
to 1MB without problem. I did try up to 4MB but it did not work even when
I've tryed to use the different "BANK" of the R800
The R800 do have CAS pin for different BANK of DRAM if I remember well, but
only two of them seem to be activated... Since I don't have the R800 and
S1990 datasheet I'm not able to check if 4MB internal will work one day.
For 1MB you'll also need to add some 74LSxx or 74HCxx chips for the
handling of the 'missing' bit of the mapper registers of the S1990.
>Does anybody know what sort of process the S1990 is made with?
>(cmos, nmos or pmos)
It should and must be CMOS and maybe it is HCMOS
Greetings,
Martial.
alias: SLAYERMAN
function: coder
group: ABYSS Software
real name:
Martial BENOIT
36, av Maurice Gallais
93150 LE BLANC MESNIL
FRANCE
TEL: 01 48 65 22 79
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