int13h Read and write don't check for errors

[joshudson]

I was looking into adding LBA bios extensions (I have plans...) and I found this issue.

extended_read_disk always writes its return value to AL (not AX), but the invocation does:

	mov	ah, 0
	cpu	186
	shl	ax, 9
	extended_read_disk
	shr	ax, 9
	cpu	8086
	mov	ah, 0x02	; Put read code back

	cmp	al, 0
	je	rd_error

throwing AL away after extended_read_disk. Here I found disk read errors are thrown away. I was scratching my head on how this worked because extended_read_disk clearly sets AL to 0 on a successful read, but I finally figured out that AL is thrown away by the SHR instruction; thus the error code is never checked.

It's almost like the code wants the caller to do a ~read like the ~lseek, but even so that does nothing until this code is fixed. The following assembly change should fix the errors not being checked but will immediately break until the C code is fixed:

	sub  al, 1
	adc  ah, 0        ; Preserve CF through the following SHR
	shr	ax, 9
	cpu	8086
	mov	ah, 0x02	; Put read code back
	
	jc	rd_error

Checking lseek is useless anyway: lseek will happily seek out of bounds. Only checking read or write matters. Thus, the error code returned is always wrong. So rd_error and wr_error need work.

[joshudson]

I do have this fixed on my home PC; not pushing anything until I get int 13h ah=42h debugged.