RAM Mapper Back Notation

you can test it yourself on BASIC.

using BASIC's immediate mode, you can, for example:
OUT &HFE,0 and then: PRINT INP(&HFE)
You can also change &HFE (page 2) register on BASIC, and enter different programs on each Mapper Page, just don't forget to type "NEW" and hit Return key BEFORE or inserting or loading any BASIC program for the first time you use that mapper page or BASIC. So if you type:

OUT &HFE,0
NEW
10 PRINT "THIS IS THE PROGRAM ON PAGE 0"
OUT &HFE,1
NEW
10 PRINT "THIS IS THE PROGRAM ON PAGE 1"

and then you type:

OUT&HFE,0
RUN

the result will be:
THIS IS THE PROGRAM ON PAGE 0

And since it is mapper, you can have (you Memory Mapped RAM size)/8k different programs at the same time loaded on RAM. (but you can't "GOTO" from one to another, you have to OUT to the mapper control register from the immediate mode)

Remember:

• Don't mess with &HFF (page 3) register from BASIC or the computer will hang.

Or try http://www.msxarchive.nl/pub/msx/utils/misc/tstmap42.pma

Note that reading mapper register may cause hardware crash on multiple mapper environment.

"hardware crash" I mentioned doesn't mean "hang", but literally "crash".
Of course hardware conflict may cause hardware crash.
Is there any hardware never crash even if it conflicts?

One doesn’t want to, and shouldn’t, but some software does, and it does work in a lot of cases, even with multiple mappers. But in certain configurations and computers it doesn’t work, which is why it’s a nonstandard and incompatible practice.

But it’s good to have on mapper hardware nevertheless, e.g. the MAP2.COM hack uses it to run some DOS1 software in DOS2 (IMO the most practical and justifiable reason for mapper readback), as well as some software which was not programmed according to guidelines.