Unreal Mode: Difference between revisions
[unchecked revision] | [unchecked revision] |
buggy granularity bit - the limit was 1M |
Syntax higlighting |
||
Line 17: | Line 17: | ||
Finally, note that IP is unaffected by all this, so the code itself is still limited to 64k. |
Finally, note that IP is unaffected by all this, so the code itself is still limited to 64k. |
||
<source lang="asm"> |
|||
<pre> |
|||
; Assembly example |
|||
AsmExample: |
|||
;========================================== |
|||
; nasmw boot.asm -o boot.bin |
; nasmw boot.asm -o boot.bin |
||
; partcopy boot.bin 0 200 -f0 |
; partcopy boot.bin 0 200 -f0 |
||
Line 66: | Line 65: | ||
db 0x55 ; req'd by some BIOSes |
db 0x55 ; req'd by some BIOSes |
||
db 0xAA |
db 0xAA |
||
</source> |
|||
;========================================== |
|||
</pre> |
|||
[[Category:X86 CPU]] |
[[Category:X86 CPU]] |
Revision as of 19:23, 3 June 2009
Unreal mode consist of breaking the '64Kb' limit of real mode segments, but still keeping 16 bits instruction and segment*16+offset address formation by tweaking the descriptor caches.
Usage
Unreal mode is recommended in the two following cases :
- you're trying to extend a legacy 16-bits DOS program so that it can deal with larger datas and neither vm86, nor xms is suitable for your needs
- you're trying to load something that will run in 32 bits mode and which is larger than 640Kb (so you cannot load it in conventionnal memory) and you don't want to bother with a disk driver called from pmode yet, and you do not wish to switch between real and protected mode for copying chunks from the conventional memory buffer to the high memory areas ...
Of course, unreal mode is kindof useless if you do not have have the A20 Line enabled.
Implementation
The reason for doing this is to enable 32-bit offsets in real mode. However, you won't be able to go past 1 meg quite yet.
In protected mode, the bits 3-15 in the segment register are an index into the descriptor table. That's why in this code 0x08 = 1000b gets you the 1 entry.
When this register given a "selector", a "segment descriptor cache register" is filled with the descriptor values, including the size (or limit). After the switch back to real mode, these values are not modified, regardless of what value is in the 16-bit segment register. So the 64k limit is no longer valid and 32-bit offsets can be used with the real-mode addressing rules (i.e. shift segment 4 bits, then add offset).
Finally, note that IP is unaffected by all this, so the code itself is still limited to 64k.
; Assembly example
; nasmw boot.asm -o boot.bin
; partcopy boot.bin 0 200 -f0
[ORG 0x7c00] ; add to offsets
start: xor ax, ax ; make it zero
mov ds, ax ; DS=0
mov ss, ax ; stack starts at 0
mov sp, 0x9c00 ; 200h past code start
cli ; no interrupt
push ds ; save real mode
lgdt [gdtinfo] ; load gdt register
mov eax, cr0 ; switch to pmode by
or al,1 ; set pmode bit
mov cr0, eax
mov bx, 0x08 ; select descriptor 1
mov ds, bx ; 8h = 1000b
and al,0xFE ; back to realmode
mov cr0, eax ; by toggling bit again
pop ds ; get back old segment
sti
mov bx, 0x0f01 ; attrib/char of smiley
mov eax, 0x0b8000 ; note 32 bit offset
mov word [ds:eax], bx
jmp $ ; loop forever
gdtinfo:
dw gdt_end - gdt - 1 ;last byte in table
dd gdt ;start of table
gdt dd 0,0 ; entry 0 is always unused
flatdesc db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0
gdt_end:
times 510-($-$$) db 0 ; fill sector w/ 0's
db 0x55 ; req'd by some BIOSes
db 0xAA