Contest! Detecting the memory mapper size, closes 08/31/2022.

That reminds me something I had to do for SofaRunIt (an MSX tool that simulates a floppy drive using hard disk images).

The "fake" MSX-DOS driver of SofaRunIt is located in higher memory mapper pages. When invoked, it has to install itself at 4000-7FFF, perform the disk operations, and then restore the "game" memory mapper segment at 4000-7FFF. But reading memory mapper register is indeed not allowed and not working on some MSX !

I was desperate at the time because SofaRunIt was not working for games using memory mapper on MSX with no mapper "readback" functionnalities.

I finally figured out a workaround by first writing a "signature" at 4000 in the existing game segment (something like "SofaRunItSignature") and backuping the existing game data at that location. When the disk operation is finished, I scan all segments at 4000-7FFF for that signature, which implies doing up to 255 OUTs to 0xFD (writing to the mapper segments is kind of allowed, at least it works on all MSX). When the magic signature is found, I restore the game data and return. That approach is kind of slow, but that time is very small anyway compared to the disk operation itself.

As Eugeny and sdsnatcher73 pointed out, your contest goes on bad bases.

It is impossible to know the values of the memory mapper registers directly except in two cases.

1/ when the MSX has an internal mapper and if it has no other added mapper of different size.
2/ Just after starting the MSX.

Indirectly, perhaps it is possible by writing values in the RAM and changing segment as you go in the deduction, and also taking care to restore the contents of the RAM just after.

You can see my routines used in BMMS and DBMM if you want.
https://github.com/gdx-msx/BMMS
https://github.com/gdx-msx/DBMM

They consider that the FCH-FFH registers have the initial values to work.

Gave it some more thought after reading replies. By accessing RAM you can only find out the size of RAM in the mapper. You can not know size of mapper without writing 0 to mapper register and then reading value back seeing how many 0s you actually read. But in this case you obtain mapper size with maximal capacity, there's no guarantee that there're no more mappers of smaller sizes, and that smaller mapper is not accidentally located in the slot you are looking in.

Question: what is the practical application of the resulting code? Why do you need to know size of mapper (rather than size of available RAM in the mapper)?

See reply above, because it would normally reflect the actual size of the available RAM in the slot containing the mapper. So in the end, to determine the available RAM in slot, whether this RAM is mapped or not.

And that is highly likely why the MSX2 standard made these ports write-only, so that 'duplicated' mapper hardware (example, both internal and external through a cartridge slot) will not cause bus congestion when the ports are read. Multiple mapper register implementations ofcourse may all respond to the same write.
As an idea, I'm tempted to make a 4MB cartridge that would be just like any other cartridge. But, that's a project for when I retire, still some 20 years from now on lol. The CPLD housing the logic would decode the full address bus and have it's own mapper write-only ports.

The internal mapper I made however, does offer read functionality, but since the values are the same, there is no buscongestion; Though for the upper bits, it may possibly short the MSX engine bits, but I remember having tested in the past for these data pins to be 3-state; undriven. The computer still works haha. In retrospect, this means that if one goes by reading the mapper registers to find out the size, one gets the largest size available. Regardless, reading the IO registers had been forbidden as a requirement and the size detecting code should work only on a per-slot basis where one has to manage slots oneself. This contest is about determining the size within a given slot without further assumption about other existing mappers.

It doesn't always work. MSXs that don't have a problem with this probably have circuitry designed to prevent shorting, but that's not always the case. Especially those who have been upgraded. I've heard the story before of people fried the circuit of the MSX and/or another external mapper. Those who make mapper cartridges should not make register reading possible, or only optional and with a warning.