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Revision as of 21:02, 22 May 2021 (view source) osdev>Basicfreak No edit summary ← Older edit		Latest revision as of 04:58, 9 June 2024 (view source) Xtex (talk \| contribs) m (Bot: Replace deprecated source tag with syntaxhighlight)
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Line 19: Therefore IP is unaffected by all this, and the code itself is still limited to 64KiB. <~~source~~syntaxhighlight lang="asm"> ; Assembly example Line 25: ; partcopy boot.bin 0 200 -f0 [ORG 0x7c00] ; add to offsets start: ~~start:~~ xor ax, ax ; make it zero mov ds, ax ; DS=0 mov ss, ax ; stack starts at seg 0 Line 41 ⟶ 42: or al,1 ; set pmode bit mov cr0, eax jmp 0x8:pmode pmode: ~~jmp $+2 ; tell 386/486 to not crash~~ mov dsbx, bx 0x10 ; 8hselect =descriptor ~~1000b~~2▼ mov bxds, ~~0x08~~bx ; ~~select~~10h ~~descriptor~~= 110000b ▲ mov ds, bx ; 8h = 1000b and al,0xFE ; back to realmode mov cr0, eax ; by toggling bit again jmp 0x0:unreal unreal: pop ds ; get back old segment sti Line 63 ⟶ 66: dd gdt ;start of table gdt : dd 0,0 ; entry 0 is always unused ~~flatdesc~~ codedesc: db 0xff, 0xff, 0, 0, 0, ~~10010010b~~10011010b, ~~11001111b~~00000000b, 0 flatdesc: db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0 gdt_end: times 510-($-$$) db 0 ; fill sector w/ 0's dw 0xAA55 ; Required by some BIOSes </syntaxhighlight> ~~</source>~~ * At least on the 486SL the jmp $+2 instruction is needed after toggling the PM bit off, not just on. - BASICFreak ===Huge Unreal Mode=== Huge Unreal Mode enables code over 64KiB. However, it is more difficult to implement as real mode interrupts do not automatically save the high 16 bits of EIP. Initialization is simple though, you just load a code segment with a 4GiB limit: <~~source~~syntaxhighlight lang="asm"> ; Assembly example Line 81 ⟶ 84: ; partcopy boot.bin 0 200 -f0 [ ORG 0x7c00] ; add to offsets start: xor ax, ax ; make it zero Line 111 ⟶ 114: times 510-($-$$) db 0 ; fill sector w/ 0's dw 0xAA55 ; Required by some BIOSes </syntaxhighlight> ~~</source>~~ WARNING: this may not work on some emulators or some hardware. Line 127 ⟶ 130: For an example of an Unreal Mode [[bootloader]] implementation with Smaller C, look at [https://github.com/fysnet/FYSOS/tree/master/loader FYSOS]. [[Category:X86 CPU]] [[Category:Operating Modes]] [[Category:FAQ]]