copy extended memory.
stuge-linuxbios at cdy.org
Mon Oct 28 18:28:00 CET 2002
On Sun, Oct 27, 2002 at 07:05:58PM -0500, Adam Sulmicki wrote:
> you can find nice example how to use it in linux sources
> (some file in linux/arch/i386/boot, head.S IIRC)
Using it is the easy part, I had to implement it. Fortunately that project
got dropped before I tore all my hair off. :)
> For example the description in Phoenix's user manual is just plain wrong
> :/ It will describe an single GDT entry, but it will not say how many
> entries and what is meaning of each entry.
Some resource, huh? Good thing we have RB.
> > And don't forget the 32-bit opcodes when in pmode.
> Isn't that dependent on D/B flag in appropriate GDT entry? So if I set 16
> b it D/B flag in the CS entry, I still should be able to use 16 bit code,
> even if in 32 bit mode.
Yes! This is, of course, absolutely correct. I just wasn't familiar enough
with the descriptor flags. Now I actually know enough to make my own DOS
extender. :) (Well, probably not, but a little closer.)
I haven't found a complete descriptor description so I'll post an attempt
here. It's basically just different parts of 386intel.txt put together.
(That file sure isn't very well structured. Much like the architecture
itself I guess. :)
lim_0_15 DW 0 ; limit bits (0..15)
bas_0_15 DW 0 ; base bits (0..15)
bas_16_23 DB 0 ; base bits (16..23)
access DB 0 ; access byte
; access := P DPL RES1 TYPE [A]
; P:1 Segment present (eg. for swapping to disk)
; DPL:2 Descriptor Privilege Level
; RES1:1 Means this is an application (as opposed to system) segment
; TYPE:3 Indicates what kind of segment this is and the intended use
; A:1 Accessed bit, set by CPU when the segment is used
; TYPE:4 Indicates what kind of segment this is and the intended use
gran DB 0 ; granularity byte
; gran := G (B|D|X) O AVL lim_16_19
; G:1 0=>byte granularity, 1=>page(4k) granularity
; B:1 Big segment (affects segment bounds)
; D:1 Default, determines default operand-size for code segments
#endif /* IS_DATA_SEGMENT */
; X:1 unknown, unused?
#endif /* RES1.value */
; O:1 unknown, used for protection?
; AVL:1 Available for use by systems programmers
; lim_16_19:4 limit bits (16..19)
bas_24_31 DB 0 ; base bits (24..31)
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