MSX1 Memory Mapper

Mapper memory was designed to be allowed up to 4 on a computer, from the beginning.
Limitation of "4" was determined by fan-out capability of MSX slot bus.
So, every software using memory mapper MUST NOT read mapper register.
Also mapper hardware SHOULD NOT allow to read register.
However, as you know, as actually some system chip did, but even, it will not cause any problem as far as the software complies with the spec.

This limitation is just hardware. The registers reading function has not been implemented by mistake in conception of MSX.

As explained above yes but at first this was planned otherwise. There were not only good ideas in developing the MSX. The management of slots for example.

Me too and there are a lot of amazing things in the MSX.

No, this conclusion is come after I have gathered much information to write a document on the MSX (Not in English). I note only the info I found in this PDF. Many information come from Japanese documentions.

Right! Because this limitation is a minimal for a standard MSX but manufacturers may make better if they want

That number of 9 mappers I mentioned is based on 2 slotexpanders with 4 mappers in each, plus the built-in mapper. The maximum number of mappers is restricted by the number of slots, so that’s why I don’t understand why there would be a minimal of 4 in the design or the standard, doesn’t make sense to me.

(And, this too is a statement of fact without providing a source.)

@madscient Ah so you mean that DOS2 supports up to 8 mappers. Makes sense, to limit the amount of DOS2 system memory used for mapper support data structures.