Software that reads the mapper registers to find out the amount of RAM (which is WRONG) will only read back 512kB on turboR anyways, because only the first 5 bits are connected, which means you can only discover 32 (amount of 5 bits)*16kB (block size) = 512kB...
I'd say: no problem at all to connect 4MB. Software with bad RAM detection won't fail (but will only find 512kB) and good software will find all 4MB.
I think the RAM detection is rarely the problem... The biggest problem is really that the read back does not work correctly. (Can some hardware expert set this straight to me? Is it 5bits or 6bits? It seems there is plenty of different information around)
This makes it impossible to run for example any mapper using DOS1 software (demos games) on DOS2 (ie. from CF or IDE) And yes... we can talk about "it is not standard to read back from the mapper registers" issue forever, but it does not make the games work any better...
On the other hand, you can also say that this makes only one program not to work: "MAP2" How ever because this is the program that makes all those other programs to run, I don't think it is that simple.
My 512k turbo-r st should be arriving some time this week if all is well, and I will shortly be ordering Padial's 4MB cart, so I can assist with testing once my stuff arrives!
Can you please tell, what you get if you write in BASIC:
?inp(-1)
It says '192'
@manuel: You are wrong, 6 bits is possible! You can upgrade MSX tR to 1MB without any problems.
if you want 1Mo on GT or ST you added a little board with 74LS670 and decode chip for read bit 6
original Turbo-R have 5 bits
and for holland software or french megarom required Mapper.com
Ah, so I think the complete answer would be:
MSX tR (A1ST) motherboard is ready for 512KB update (5bits) but S1990 (A1ST/A1GT) can handle also 1MB (6bits) if you add mapper logic. For 4MB (8bits) there is no known solution to get mapper readback working correctly.
inp(-1)? Not peek(-1)?
No, but that command is usefull also... It can be used for checking RAM subslot. Now we were how ever talking about memory mapper size and support logic.
Problem is when you execute something like OUT-1,INP(-1) on computer that has more RAM than the S1990 can return (2MB or 4MB) the computer will crash. On standard MSX tR: business as usual... This may not sound very reasonable thing to do, but this kind of tricks are needed for example to run DOS1 programs under DOS2. (MAP2)
RAM detection is also problematic, if it has been done in "quick and ugly" way like for example: PRINT "RAM:"; (256-INP(-1))*16 ;"KB"
