Re: bootsect.S: 2 questions

[Mad Hatter]

Thanks for the pointers to Ralf Brown's interrupt page.

--- "Richard B. Johnson" <[e-mail]> wrote:

>FYI, the startup functions are used once-per-boot. Any "improvements"
>other than those necessary to "fix" something are useless in the
>overall scheme of things.

Sure. I'm a novice at both the x86 instruction set and the GNU assembler
and I'm just trying to understand the reason the code is written the way
it is.

--- "Randy.Dunlap" <[e-mail]> wrote:
 
> If you change those to bytes to save 3 bytes of space, and then you
> change all of the instructions that load or store those values to 16-bit
> registers, do you have a net saving of space?
> Or you load/store them to 8-bit registers and rewrite the code, what
> happens?

I looked at one place where these words are used: the read_track routine.
Part of that code looks like this:
-------------------------------
	movw	4(%si), %dx		# 4(%si) = track
	movw	(%si), %cx		# (%si)  = sread
	incw	%cx
	movb	%dl, %ch
	movw	2(%si), %dx		# 2(%si) = head
	movb	%dl, %dh
	andw	$0x0100, %dx
	movb	$2, %ah
---------------------------------

If this code is rewritten to use byte instructions (and sread, head, track
are changed to .byte instead of .word) like so (the last two instructions
are unchanged):
-----------------------------------------------
	movb	(%si), %cl		# (%si)  = sread
	incb	%cl
	movb	2(%si), %ch		# 2(%si) = track
	movb	1(%si), %dh		# 1(%si) = head
	andw	$0x0100, %dx
	movb	$2, %ah
------------------------------------------------

there is a net saving of 6 bytes. (I compiled this with "gcc -c", linked
with "ld --oformat binary -Ttext 0x0" and compared the size of the results).

I'm not proposing this as a fix BTW; just an academic exercise, really.

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