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.
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 data 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 conventional 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 ...
You still will not have full access to all physical RAM if you do not have have the A20 Line enabled; all the "odd" 1-megabyte blocks will be unavailable.
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 So the 64k limit is no longer valid and 32-bit offsets can be used with the real-mode addressing rules (i.e. shift segment 4 bits, then add offset).
Big Unreal Mode
These will not have affected CS.
Therefore 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 seg 0 mov sp, 0x9c00 ; 2000h past code start, ; making the stack 7.5k in size cli ; no interrupts 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 jmp $+2 ; tell 386/486 to not crash 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-($-$$) db 0 ; fill sector w/ 0's db 0x55 ; req'd by some BIOSes db 0xAA
Huge Unreal Mode
Huge Unreal Mode enables code over 64K. However, it is more difficult to implement as real mode interrupts do not automatically save the high 16 bits of EIP. Initialization is simple though, you just load a 32 bit code segment:
; Assembly example ; nasmw boot.asm -o boot.bin ; partcopy boot.bin 0 200 -f0 [ORG 0x7c00] ; add to offsets start: xor ax, ax ; make it zero ... ; As before mov cr0, eax jmp 0x8:pmode pmode: mov bx, 0x10 ; select descriptor 2, instead of 1 mov ds, bx ; 10h = 10000b and al,0xFE ; back to realmode mov cr0, eax ; by toggling bit again jmp 0x0:huge_unreal huge_unreal: ... ;As before gdtinfo: dw gdt_end - gdt - 1 ;last byte in table dd gdt ;start of table gdt dd 0,0 ; entry 0 is always unused gdt_end: flatcode db 0xff, 0xff, 0, 0, 0, 10011010b, 10001111b, 0 flatdata db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0 times 510-($-$$) db 0 ; fill sector w/ 0's db 0x55 ; req'd by some BIOSes db 0xAA
WARNING: this may not work on some emulators or some hardware. This is because of direct 32bit PMODE -> (Un)real mode.