I do Computer Science, so I know these things!

Essentially, nearly all computers are based on binary (base 2), which is why you see a lot of powers of two when talking about processors, memory, and that sort. However, our standard SI units use base 10. 1000 grams is a kilogram, 1000 metres is a kilometre, etc. So when creating a standard unit for memory, kilobytes, megabytes, etc. base 10 was used. KB, MB, GB, TB, etc. are all SI units. However, the IEC standard was later created that utilised base 2 instead. So KiB, MiB, GiB, TiB, etc. are all IEC units where 1024 bytes is equal to one kibibyte, "bi" for binary!

But as you said, this is probably not the issue. RAM usually uses IEC units, while storage advertises SI units. Some of your memory might be reserved for system level stuff and/or the iGPU. My laptop (running EndeavourOS, so the units properly show up as IEC units unlike on Windows), for instance, shows up as 15.2 GiB of total memory, presumably because some of it is reserved for the iGPU and other bits.

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.

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.

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.

Sorry, it's me taking up that space.

Memory makers practice bistromathics

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.