Unreal mode consists of breaking the 64KiB limit of real mode segments (while retaining 16-bit instructions and the segment * 16 + offset addressing mode) by tweaking the descriptor caches.
Unreal mode is usually recommended in the two following cases:
- You're trying to extend a legacy 16-bit DOS program so that it can deal with larger data and neither Virtual 8086 Mode, nor xms are suitable for your needs.
- You're trying to load something that will run in 32-bit mode which is larger than 640K (therefore you cannot load it in conventional memory) and you don't want to bother writing a protected mode disk driver yet, but you also want to avoid switching between real and protected mode to copy chunks from the conventional memory buffer into extended memory.
You still will not have full access to all physical RAM if you do not have the A20 Line enabled; all the "odd" 1 MiB blocks will be unavailable.
To do this, you need to set the descriptor cache's limits for your segment register(s) to any value higher than 64KiB (usually a full 4GiB (0xffffffff)).
In protected mode, bits 3-15 in the segment registers represent an index into the global descriptor table. That's why in the following code 0x08 = 1000b gets you entry #1 (entry #0 is ALWAYS a null descriptor).
When (in protected mode) a segment register is 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 So the 64KiB limit is no longer valid and 32-bit offsets can be used in Real Mode to actually access areas above 64KiB (segment * 16 + 32-bit offset).
Big Unreal Mode
This won't touch CS.
Therefore IP is unaffected by all this, and the code itself is still limited to 64KiB.
; Assembly example ; nasm 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 0x8:pmode pmode: mov bx, 0x10 ; select descriptor 2 mov ds, bx ; 10h = 10000b and al,0xFE ; back to realmode mov cr0, eax ; by toggling bit again jmp 0x0:unreal unreal: 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 codedesc: db 0xff, 0xff, 0, 0, 0, 10011010b, 00000000b, 0 flatdesc: db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0 gdt_end: times 510-($-$$) db 0 ; fill sector w/ 0's dw 0xAA55 ; Required by some BIOSes
Huge Unreal Mode
Huge Unreal Mode enables code over 64KiB. However, it is more difficult to implement as real mode interrupts do not automatically save the high 16 bits of EIP. Initialization is simple though, you just load a code segment with a 4GiB limit:
; Assembly example ; nasm boot.asm -o boot.bin ; partcopy boot.bin 0 200 -f0 ORG 0x7c00 ; add to offsets start: xor ax, ax ; make it zero ... ; As before mov cr0, eax jmp 0x8:pmode pmode: mov bx, 0x10 ; select descriptor 2, instead of 1 mov ds, bx ; 10h = 10000b and al,0xFE ; back to realmode mov cr0, eax ; by toggling bit again jmp 0x0:huge_unreal huge_unreal: ... ;As before gdtinfo: dw gdt_end - gdt - 1 ;last byte in table dd gdt ;start of table gdt dd 0,0 ; entry 0 is always unused flatcode db 0xff, 0xff, 0, 0, 0, 10011010b, 10001111b, 0 flatdata db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0 gdt_end: times 510-($-$$) db 0 ; fill sector w/ 0's dw 0xAA55 ; Required by some BIOSes
WARNING: this may not work on some emulators or some hardware.
The Smaller C compiler supports unreal mode. It produces MZ executables for unreal mode (can be loaded with BootProg).
The code and the stack are to be located below the 1MB mark and the stack size is limited by 64KB (IOW, there's nothing unusual about CS:(E)IP, SS:(E)SP, it's a natural setup for MZ executables in DOS). The DS and ES segment registers are set to 0, so C pointers can work as flat 32-bit physical addresses and address data or memory-mapped devices anywhere in the first 4GB of memory.
The startup code of these executables performs the necessary relocation (there are only custom relocations and no standard MZ relocations, which may simplify loading of the executables) and sets up unreal mode before passing control to the equivalent of main(). See srclib/c0du.asm and other C/assembly code under srclib in the compiler source tree for how to write bits of assembly code for unreal mode (look for asm("inline asm code") under #ifdef __UNREAL__).
You can try out unreal mode in DOS (e.g. in DOSBox, VirtualBox + FreeDOS) as the compiler fully supports the DOS + unreal mode combo in its C library. tests/vesalfb.c is a simple example of setting up a VESA graphics mode with the linear frame buffer enabled and drawing something on the screen in unreal mode.
For an example of an Unreal Mode bootloader implementation with Smaller C, look at FYSOS.