No Stupid Questions

RAM and cpus use the proper 1024 B to kB to MB to GB. Unlike hdd makers that paid ISO to retroactively make it 1000. Or worse internet providers that use bits (b) instead of bytes (B) (reasoning that signal transmission uses bits).

It's definitely the kernel and bios reserving pieces. They need it to run the computer, after all. Afaik even other hardware can reserve memory for itself, like gpus or network devices.

OK, slow down there, you might go into the BIOS and check to see if you have a hardware reserve for internal graphics. Sometimes it'll default to something small like 64MB, by you can configure it up to 2048MB on some Intel motherboards iirc, maybe AMD is higher because it's an "APU" which will reserve some in the same way.

When the computer boots, it puts part of the bios in RAM and locks this area. Some hardware drivers do the same.

Maybe it's not counting memory that can't be tested because it's needed for the kernel and device drivers. For example according to its man page free reports total memory "minus a few reserved bits and kernel binary code" and on my desktop here that's about 15.2 out of 16GB.

I don't know though, most of a gigabyte does seem like a suspiciously large amount of memory for that, never mind 2.4 gigs. I guess it's i/o buffers, page tables, and things like that.

Unlike hard drives and SSDs which as you have observed are incessantly manufactured in powers-of-ten mega/giga/terabytes but marketed as if they were powers-of-two mebi/gibi/tebibytes, a RAM chip's capacity is absolutely, definitely, 100% down to the individual bit precisely the capacity at which it is rated in powers-of-two megabytes. Due to the way that memory is accessed there is no other way and it cannot be fudged. (The exception is ECC RAM which typically has an extra bit per byte to hold the parity data, but this is not accessible to the user so that's moot.)

There is a small bite of your memory space taken out for Memtestx86 to reside in, which is necessary in order for it to run. Your BIOS probably has some portion of memory reserved as well, either for peripheral memory mapping or for use as video memory, or similar.

There is probably also some rounding going on in the total capacity that Memtestx86 reports.

Memory makers practice bistromathics

Sorry, it's me taking up that space.

I don't know the answer but I think "The memory on a modern computer is not necessarily mapped in a linear way" is glossing over a lot of complexity.

Screenshots of MemTest86+ (on their own site and elsewhere) all seem to include an oddball number for the "memory" figure. Your situation is normal.