At 11:42 AM 11/23/98 -0800, you wrote:
>> 3. What's the problem?
>The possibility to work with Japanese Panasonic MSX 2+ computer that
>have
>64Kb ram (and external extension ram).
^^^^^^^^^^^^^^^^^^^^^^
I guess we found the problem!
If your extension RAM is larger than 512K, MAP.COM will crash your computer.
Not everybody agrees on this, but it is said that according to the MSX
standard reading the mapper ports (#FC-#FF) is not legal. But because it
worked on almost all MSX1 and MSX2 computers, it was done very often in
older programs.
But the S1990 MSX engine, which was used in some MSX2+ and in all turbo R
computers, can only read 5 out of 8 bits of the mapper ports. The result:
if you use 512K or less, the value read is OK. If you use more memory, the
value read is wrong.
DOS2 solves this problem by introducing a mapper BIOS. If you use these
routines to read and write the mapper, everything works well on all MSXes
running DOS2. This is because the reading is not done by reading the actual
mapper port, but by remembering the last value written to that port.
The problem is that some older programs (for example MoonBlaster 1.4) write
the mapper directly. If they do so, DOS2 won't know what was written to the
mapper. So if it is asked to return the mapper status, it will return an
outdated value.
This may not seem like a problem, because older programs don't ask DOS2 for
the mapper status. But DOS2 itself does! For example, when loading from
disk, it has to access it's own data in RAM and afterwards it wants to
return the mapper to the same state it was in before the loading.
MAP.COM is a patch that will replace the DOS2 routines that read the last
known mapper value from memory with routines that read the mapper port
directly. This solves the compatibility problems on many MSXes, except the
ones with S1990 and more than 512K RAM.
To solve your problem, there are a some options:
- Expand your MSX2+'s RAM internally to either 128K, 256K or 512K. Even
expanding to just 128K will be enough to run most European productions.
- Use a smaller external expansion (512K maximum).
- Use only DOS2-compatible programs, or fix programs to make them DOS2
compatible (not an easy task).
Note that it is the size of individual mappers that matters. So, it is
possible to have both 512K internal and 512K external and run MAP.COM.
Nyyrikki, are you still working on 256K sampleRAM in MB1.4? If so, could
you also fix MB to make it DOS2 compatible?
Bye,
Maarten
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