Unreal Mode: Difference between revisions
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fix bug: far JMP is required to enter/exit protected mode |
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; partcopy boot.bin 0 200 -f0 |
; partcopy boot.bin 0 200 -f0 |
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ORG 0x7c00 ; add to offsets |
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start: |
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xor ax, ax ; make it zero |
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mov ds, ax ; DS=0 |
mov ds, ax ; DS=0 |
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mov ss, ax ; stack starts at seg 0 |
mov ss, ax ; stack starts at seg 0 |
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or al,1 ; set pmode bit |
or al,1 ; set pmode bit |
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mov cr0, eax |
mov cr0, eax |
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jmp 0x8:pmode |
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pmode: |
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jmp $+2 ; tell 386/486 to not crash |
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| ⚫ | |||
mov |
mov ds, bx ; 10h = 10000b |
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| ⚫ | |||
and al,0xFE ; back to realmode |
and al,0xFE ; back to realmode |
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mov cr0, eax ; by toggling bit again |
mov cr0, eax ; by toggling bit again |
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jmp 0x0:unreal |
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unreal: |
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pop ds ; get back old segment |
pop ds ; get back old segment |
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sti |
sti |
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dd gdt ;start of table |
dd gdt ;start of table |
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gdt |
gdt: dd 0,0 ; entry 0 is always unused |
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codedesc: db 0xff, 0xff, 0, 0, 0, 10011010b, 00000000b, 0 |
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flatdesc: db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0 |
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gdt_end: |
gdt_end: |
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dw 0xAA55 ; Required by some BIOSes |
dw 0xAA55 ; Required by some BIOSes |
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</source> |
</source> |
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* At least on the 486SL the jmp $+2 instruction is needed after toggling the PM bit off, not just on. - BASICFreak |
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===Huge Unreal Mode=== |
===Huge Unreal Mode=== |
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; partcopy boot.bin 0 200 -f0 |
; partcopy boot.bin 0 200 -f0 |
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ORG 0x7c00 ; add to offsets |
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start: xor ax, ax ; make it zero |
start: xor ax, ax ; make it zero |
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