DOS unit tests 07 and 08 (C: copy/concatenate) are broken about x16-rom HOT 4 OPEN

The first ROM commit to have the problem seems to be b6a0f0c.
Other observations (running on that commit):
In test 7, the file contents are "FILE1" when they should be "HELLO, WORLD!".
In test 8, the file contents of "FILE4" are "FILE1".

from x16-rom.

I think I now know what's happening when doing a DOS file copy to some extent.

AFAICT, the actual file copying is done by fat32_read and fat32_write in dos/fat32/fat32.s.

It seems fat32_read uses a temporary storage buffer in RAM bank 1 where it puts the content of the file it's reading.

Furthermore it looks like fat32_write tries to output the content of the same buffer to disk, but that it fetches data from the same memory address in RAM bank 0 instead.

Even so, I have not yet a concrete bug fix. Just an idea of what might be going wrong.

from x16-rom.

functions.diff.txt

from x16-rom.

I managed to trick the X16 Kernal to do a correct file copy by the changes found in the above diff file "functions.diff.txt"
Those changes basically set the variable bank_save to zero before starting the copy procedure. But this is not the right way to really fix the problem.
@mist64, where do you think we should start looking for a solution?

from x16-rom.