DOS unit tests 07 and 08 (C: copy/concatenate) are broken

[mist64]

This breaks in tests 07 and 08 (C: COPY FILE/C: CONCATENATE FILES):
unzip -o sdcard.img.zip
./x16emu -sdcard sdcard.img -warp -bas ../x16-rom/test/dos.bas

It reproduces also when disabling all other tests.

This has been broken since at least r39. Bisecting older ROMs needs more work because it needs an older emulator.

[Jaxartes]

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".

[stefan-b-jakobsson]

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.

[stefan-b-jakobsson]

functions.diff.txt

[stefan-b-jakobsson]

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?