Assembler coding help needed.

[Martin Nilsson]

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
		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: