int13h Read and write don't check for errors

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.

ecm-pushbx / 8086tiny

int13h Read and write don't check for errors #6