Unreal Mode: Difference between revisions

'''Unreal mode''' consistconsists of breaking the '64Kb'64KiB limit of real mode segments, but(while still keepingretaining 16-bit bits instructioninstructions and the <tt>segment * 16 + offset</tt> addressaddressing formationmode) by tweaking the [[Descriptor Cache|descriptor caches]].

Unreal mode is usually recommended in the two following cases :

* youYou're trying to extend a legacy 16-bitsbit DOS program so that it can deal with larger data and neither vm86[[Virtual 8086 Mode]], nor xms isare suitable for your needs.

* youYou're trying to load something that will run in 32 bits-bit mode and which is larger than 640Kb640K (sotherefore you cannot load it in conventional memory) and you don't want to bother withwriting a protected mode disk driver called from pmode yet, andbut you doalso notwant wishto toavoid switchswitching between real and protected mode forto copyingcopy chunks from the conventional memory buffer tointo the highextended 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 MiB blocks will be unavailable.

In protected mode, the bits 3-15 in the segment registerregisters arerepresent an index into [[GDT|the global descriptor table]]. That's why in thisthe following code 0x08 = 1000b gets you theentry #1 (entry #0 is ALWAYS a null descriptor).

When this(in protected mode) a segment register givenis loaded with a "selector", a "segment descriptor cache register" is filled with the descriptor's 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 So the 64k64KiB limit is no longer valid and 32-bit offsets can be used within theReal real-modeMode addressingto rulesactually (i.e.access shiftareas above 64KiB (<tt>segment 4* bits,16 then+ add32-bit offset</tt>).

TheseThis willwon't not have affectedtouch CS. <br />

Therefore IP is unaffected by all this, soand the code itself is still limited to 64k64KiB.

<sourcesyntaxhighlight lang="asm">

; nasmwnasm boot.asm -o boot.bin

[ORG 0x7c00] ; add to offsets

start: xor ax, ax ; make it zero

mov bxds, 0x08bx ; select10h descriptor= 110000b

gdt : dd 0,0 ; entry 0 is always unused

flatdesc codedesc: db 0xff, 0xff, 0, 0, 0, 10010010b10011010b, 11001111b00000000b, 0

db 0x55 dw 0xAA55 ; req'dRequired by some BIOSes

Huge Unreal Mode enables code over 64K64KiB. 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 with a 4GiB limit:

<sourcesyntaxhighlight lang="asm">

; nasmwnasm boot.asm -o boot.bin

[ ORG 0x7c00] ; add to offsets

db 0x55 dw 0xAA55 ; req'dRequired by some BIOSes

WARNING: this may not work on some emulators or some hardware. This is because of direct 32bit PMODE -> (Un)real mode.