Re: bootsect.S: 2 questions

[Randy.Dunlap]

Hi-

(mostly "what R. Johnson said")

On Mon, 13 Jan 2003, Mad Hatter wrote:

| I was looking through the linux (2.5.56) arch/i386/boot/bootsect.S and was
| puzzled about a couple of things:
|
| 1. Near line 221 we have:
|        sread:  .word 0             # sectors read of current track
|        head:   .word 0             # current head
|        track:  .word 0             # current track
|
|    However, since a diskette can have at most 2 heads, 80 tracks and 36 sectors
|    per track, why are these not bytes instead of words especially since space is
|    at such a tight premium in this code ?

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?

| 2. Near line 272 we have "movw    $7, %bx" but the documentation I've
|     been able to find about the "int 0x10" BIOS call says that for service
|     code 0xe (write character and advance cursor), it does not take an
|     attribute byte input parameter but rather uses the existing attribute. Is
|     this movw instruction superfluous ?

Perhaps, or perhaps on some particular system it was found to be needed.
We don't have a fully indexed changelog history of each line AFAIK.
Ralf Brown's interrupt page for this function says:
  BH = page number
  BL = foreground color (graphics modes only)
(see http://www.ctyme.com/intr/rb-0106.htm)
Or it could be important to clear BH and setting BL to 7 was just safe.

-- 
~Randy