Operating System Development-baby tutorial 7

Unreal Mode

A. K. A big real or voodoo Mode

While this code is largely just a party trick, understanding it gives a gentle intro to protected mode concepts and possibly avoids some headaches later on 'cause you skipped over this kind of stuff.

The single descriptor in the Global Descriptor Table at the bottom is laid out to match babystep6. the 'SIZE' given is 1 MB, the base address is
0x0, and the bit fields you can work out yourself.

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 so the 64 K limit is no longer valid and 32-bit offsets can be used with the real-mode addressing rules (I. e. shift segment 4 bits, then add offset ).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 64 K.

AsmExample: ;==========================================; 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 unusedflatdesc    db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0gdt_end:    times 510-($-$$) db 0  ; fill sector w/ 0's   db 0x55          ; req'd by some BIOSes   db 0xAA;==========================================