Assembler coding help needed.

[Marcin Dalecki]

Martin Nilsson wrote:
> I'm trying to find out why I can't boot 5.2 from USB CDROM on Supermicro 
>  motherboards. (I have an old Gateway P3 that can!).
> 
> I've found out that that only 0x20 of 0x4c sectors of the loader are 
> read in and it therfor traps when executed. (read is only called once).
> 
> My last attempt at programming x86 assembler was ~15years ago so I'm a 
> bit rusty :-)
> 
> The below loop from cdboot.s is what I'm having problem understanding, 
> how can this fail on one box but not on another?
> 
> #
> # Load the binary into the buffer.  Due to real mode addressing limitations
> # we have to read it in in 64k chunks.
> #
>         mov DIR_SIZE(%bx),%eax        # Read file length
>         add $SECTOR_SIZE-1,%eax        # Convert length to sectors
>         shr $11,%eax
> 
> %eax is 0x4c here on both machines!
> 
>         cmp $BUFFER_LEN,%eax
>         jbe load_sizeok
>         mov $msg_load2big,%si        # Error message
>         call error
> load_sizeok:    movzbw %al,%cx            # Num sectors to read
>         mov DIR_EXTENT(%bx),%eax    # Load extent
>         xor %edx,%edx
>         mov DIR_EA_LEN(%bx),%dl
>         add %edx,%eax            # Skip extended
>         mov $MEM_READ_BUFFER,%ebx    # Read into the buffer
> load_loop:    mov %cl,%dh
>         cmp $MAX_READ_SEC,%cl        # Truncate to max read size
>         jbe load_notrunc
>         mov $MAX_READ_SEC,%dh
> load_notrunc:    sub %dh,%cl            # Update count
>         push %eax            # Save
>         call read            # Read it in

The fun will be ^^^^ here. The rest is self contained and
doesn't depend on CPU variant or periphery.


>         pop %eax            # Restore
>         add $MAX_READ_SEC,%eax        # Update LBA
>         add $MAX_READ,%ebx        # Update dest addr
>         jcxz load_done            # Done?
>         jmp load_loop            # Keep going
> load_done:
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