I started to adapt the fasm code for my OS, but I think I have a little problem with the reading / writing function
my adaptation of fasm compiles only the first 2 bytes of the file (it's a bit short) I think it's because the functions that I adapted do not return the correct data (in the eax registry for example)
is there somewhere a description of what the read / write / lseek functions should do? what are the changed registers?

You can find this information in the old (and never finished) guide to fasm internals. Section 2.4 covers file access functions.

it seems that it is the lseek function that I have not adapted well, I did not take care of the register al which is the choice of the origin

I guess it's the lseek function that determines the size of the file and that's what makes the compilation fail

thank you

after a little correction, I tested and it works! (at least on simple compilations, we will test other things later) it was the lseek function that was defective

thank you very very much (for help and for creating Fasm)