It turned out that this loader was severely broken for many years,
at least since 2012 (the git repo's creation). This variable was
read as a word to set cx for a loop, but only the low byte had
been written. The high byte happened to be 0EEh so cn_loop would
always zero out the FAT sector to read.
This _happened_ to work if the first root directory cluster and
all the kernel file clusters were all referenced in the first
sector of the FAT. Otherwise it would blow up.
This should be Enhanced DR-DOS compatible.
oemboot.asm contains an optimisation to save 2 bytes,
by using mov instead of a lea with bp evaluating to
a constant value. This was needed to fit the addition
for 256 spc support. It is otherwise unrelated.
The LBA detection patch offsets in SYS have also been
updated to match the changed boot.asm offsets.
Prior to this commit the first cluster specified from the last
compared directory entry would be used if the file is not found.
Fix the wrong (boot.asm) or missing (oemboot.asm) jump to the
error handler so that on file not found an error is displayed.
Reference: https://sourceforge.net/p/freedos/bugs/97/
"Bugs in FAT12/16 boot sector loaders if file not found"
This commit fixes the most crucial bug of this 2012-09-27 report.
Reference: https://hg.ulukai.org/ecm/instsect/file/33218c729b43/instsect.asm#l1257
lDOS's instsect recently started to default to checking
the filesystem ID string in order to validate that the
boot sector loader to write matches the detected FS.
Using the /S=filename option or building instsect to
include the FreeDOS kernel's loaders would require use
of the /SN switch without this commit.
In addition, boot.asm and oemboot.asm are made to check
that exactly one of the ISFAT12 and ISFAT16 defs is
defined so as to select the FS. Prior to this commit
using both or neither def would silently result in a
broken loader.
Reference: https://www.bttr-software.de/forum/forum_entry.php?id=16934
The SBC188 video ROM-BIOS handling returns with al=0Ah (LF)
upon interrupt 10h function 0Eh calls that scroll the screen.
This made the FAT12 boot loader crash when booting FreeDOS
off a 1440 KiB diskette. While a fix is expected from their
side too, hardening the loader too is the right thing to do.
With the way this fix is implemented by this commit, it only
needs one additional byte for the loader. The error message
changes from "Error!." to just "Error!", other than
that the behaviour is the same.
The FAT32 loaders already used zero-terminated strings and
thus were not affected by the dependence.
