85 lines
2 KiB
NASM
85 lines
2 KiB
NASM
;
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; This file is designed to run as a ROM replacement, but it has a .COM extension because it's
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; also designed to run as a COM file under DOS (hence the "org 0x100").
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;
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cpu 386
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org 0x100
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section .text
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bits 16
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DPL0 equ 0
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DPL1 equ 1
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DPL2 equ 2
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DPL3 equ 3
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ACC_TYPE_SEG equ 0x1000
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ACC_TYPE_CODE equ 0x0800
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ACC_TYPE_READABLE equ 0x0200
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ACC_TYPE_WRITABLE equ 0x0200
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ACC_TYPE_CODE_READABLE equ 0x1a00
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ACC_TYPE_DATA_WRITABLE equ 0x1200
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EXT_BIG equ 0x0040
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CSEG_REAL equ 0xf000
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CSEG_PROT equ 0x0008
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DSEG_PROT equ 0x0010
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CR0_MSW_PE equ 0x0001
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;
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; descDT defines a descriptor, given a base (%1), limit (%2), type (%3), dpl (%4), and ext (%5)
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;
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%macro defDesc 1-5 0,0,0,0
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dw (%2 & 0x0000ffff)
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dw (%1 & 0x0000ffff)
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dw ((%1 & 0x00ff0000) >> 16) | %3 | (%4 << 13)
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dw ((%2 & 0x000f0000) >> 16) | %5 | ((%1 & 0xff000000) >> 16)
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%endmacro
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start: mov eax,0x44332211
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mov ecx,0x88776655
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mul ecx
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div ecx
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jnz near goProt ; apparently we have to tell NASM "near" because this is a forward reference
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times 32768 nop ; lots of NOPs to force a 16-bit conditional jump
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romGDT: defDesc 0 ; the first descriptor in any descriptor table is always a dud (it corresponds to the null descriptor)
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defDesc 0x000f0000,0x0000ffff,ACC_TYPE_CODE_READABLE
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defDesc 0x00000000,0x000fffff,ACC_TYPE_DATA_WRITABLE
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goProt: lgdt [cs:romGDT]
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mov eax,cr0
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or eax,CR0_MSW_PE
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mov cr0,eax
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jmp dword CSEG_PROT:inProt
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inProt: mov ax,DSEG_PROT
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mov ds,ax
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;
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; Do some protected-mode tests...
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;
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goReal: mov eax,cr0
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and eax,~CR0_MSW_PE
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mov cr0,eax
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jmp dword CSEG_REAL:inReal
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inReal: or eax,1
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jnz start ; apparently we do NOT have to say "near" here since this is a backward reference
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;
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; Fill the remaining space with NOPs until we get to target offset 0xFFF0.
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; Note that we subtract 0x100 from the target offset because we're ORG'ed at 0x100.
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;
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times 0xfff0-0x100-($-$$) nop
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bits 16
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jmp CSEG_REAL:start
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db 0x20
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db '04/04/15'
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db 0xFC ; 0000FFFE FC (Model ID byte)
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db 0x00 ; 0000FFFF 00 (location of checksum byte)
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