312 lines
6.4 KiB
NASM
312 lines
6.4 KiB
NASM
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;
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;Copyright (c) 1995 Microsoft Corporation
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;
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;Module Name:
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;
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; bootfix.asm
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;
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;Abstract:
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;
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; The code in this "image" is responsible for checking if is appropriate
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; for us to boot from CD. We want to boot from CD whenever we don't have
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; a valid active partition or when the user pressed CTRL key during the
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; boot process.
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;
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;Author:
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;
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; Calin Negreanu (calinn) 25-May-1998
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;
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;Environment:
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;
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; Image has been loaded at 2000:0000 by ETFS boot code.
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; Real mode
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; ISO 9660 El Torito no-emulation CD-ROM Boot support
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; DL = El Torito drive number we booted from
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;
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;Revision History:
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;
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; Calin Negreanu (calinn) 18-Feb-1999
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;
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;--
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page ,132
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title bootfix
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name bootfix
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.8086
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CODE SEGMENT
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ASSUME CS:CODE,DS:CODE,SS:NOTHING,ES:NOTHING
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ORG 0000H
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_BootFix label byte
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MaxCodeSize EQU 1024
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Part_Active EQU 0
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Part_Type EQU 4
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Data_PartType EQU 0 ;address of partition type inside BootData structure
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LoadSeg EQU 3000H ;we load MBR here
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SectSize EQU 512
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EntriesOnMbr EQU 4
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MbrDataOff EQU 01BEH
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VolbootOrg EQU 7c00h
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JMPFAR MACRO DestOfs,DestSeg
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db 0eah
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dw OFFSET DestOfs
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dw DestSeg
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endm
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START:
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;
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; we already have a valid stack set by the original ETFS boot sector
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; we only need to set ds and es
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;
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push ds
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push es
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mov ax,cs
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mov ds,ax
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mov es,ax
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;
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; read partition table from hdd 80h at LoadSeg:0000
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;
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push es
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mov ax,LoadSeg
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mov es,ax
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mov bx,0000h
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mov ax,0201h ;read function, one sector
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mov cx,0001h
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mov dx,0080h
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int 13h
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jnc MbrOk
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;
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; there was an error, boot from CD
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;
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pop es
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jmp CdBoot
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MbrOk:
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pop es
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;
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; now it's the time to loop and find the active partition table
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;
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push es
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mov ax,LoadSeg
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mov es,ax
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mov cx,EntriesOnMbr ;number of partitions in partition table
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mov bp,MbrDataOff ;01beh
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LoopActive:
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;
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; 00 - inactive, 80h active, others-invalid
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;
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cmp BYTE PTR es:[bp].Part_Active,00H
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jl CheckInactive
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jne BadMbr
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add bp,16
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loop LoopActive
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;
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; no active partition found. boot from CD
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;
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pop es
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jmp CdBoot
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CheckInactive:
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push bp
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InactiveLoop:
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add bp,16
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dec cx
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jz ActiveFound
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cmp BYTE PTR es:[bp].Part_Active,00H
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je InactiveLoop
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pop bp
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BadMbr:
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;
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; bad mbr was found. boot from CD
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;
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pop es
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jmp CdBoot
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ActiveFound:
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pop bp
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pop es
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;
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; let's see if we can display UI (that is if MsgPressKey is not empty
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;
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mov si, OFFSET MsgPressKey
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lodsb
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mov UIAllowed, al
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push si
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mov si,OFFSET MsgPressKey
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call PrintMsg
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pop si
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;
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; read all available keys from the queue (prevents us from booting from CD when there
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; is some garbage there
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;
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mov cx, 80h
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FlushQueue:
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mov ah, 01h
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int 16h
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jz QueueEmpty
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mov ah, 00h
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int 16h
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loop FlushQueue
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QueueEmpty:
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;
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; hook int08
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;
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cli
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push es
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xor ax,ax
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mov es,ax
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mov bx,08h * 4
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mov ax,es:[bx]
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mov WORD PTR [OldInt08 ], ax
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mov ax,es:[bx+2]
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mov WORD PTR [OldInt08+2], ax
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mov WORD PTR es:[bx],OFFSET NewInt08
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mov WORD PTR es:[bx+2],cs
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pop es
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sti
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;
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; loop until the delay ticks is 0. Check for a key pressed (if UI), or for CTRL pressed (if no UI)
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;
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CheckKey:
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cmp UIAllowed, 0
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je CheckCTRL
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mov ah, 01h
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int 16h
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jnz KeyPressed
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CheckCTRL:
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mov ah,02h
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int 16h
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and al,00000100b
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jnz KeyPressed
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NotPressed:
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cmp DotTicks, 0
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jg AddDot
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push si
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mov si,OFFSET MsgDot
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call PrintMsg
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pop si
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mov DotTicks, 18
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AddDot:
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cmp DelayTicks, 0
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jne CheckKey
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call UnhookInt08
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jmp BootFromHD
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UnhookInt08:
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cli
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push es
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xor ax,ax
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mov es,ax
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mov bx,08h * 4
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mov ax,WORD PTR [OldInt08]
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mov es:[bx],ax
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mov ax,WORD PTR [OldInt08+2]
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mov es:[bx+2],ax
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pop es
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sti
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ret
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KeyPressed:
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call UnhookInt08
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jmp CdBoot
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BootFromHD:
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;
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; let's move the mbr code to 0000:7c00 and jump there
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;
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mov ax,LoadSeg
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mov ds,ax
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xor ax,ax
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mov es,ax
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xor si,si
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mov di,VolBootOrg
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mov cx,SectSize
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cld
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rep movsb
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mov dl,80h
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JMPFAR VolbootOrg,0000H
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CdBoot:
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;
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; return to caller code
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;
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pop es
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pop ds
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retf
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NewInt08:
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pushf
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cli
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cmp WORD PTR cs:[DelayTicks], 0
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je default08
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dec WORD PTR cs:[DotTicks]
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dec WORD PTR cs:[DelayTicks]
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default08:
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popf
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push WORD PTR cs:[OldInt08+2]
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push WORD PTR cs:[OldInt08]
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retf
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;
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;EXPECTS DS:SI - MESSAGE ADDR
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;
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PrintMsg proc near
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push ax
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push bx
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cmp UIAllowed, 0
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je PrintMsgEnd
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PrintMsgLoop:
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lodsb
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cmp al,0
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je PrintMsgEnd
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mov ah,0eh
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mov bx,0007h
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int 10h
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jmp PrintMsgLoop
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PrintMsgEnd:
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pop bx
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pop ax
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ret
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PrintMsg endp
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include bootfix.inc ; message text
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UIAllowed db 0
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MsgDot db "."
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db 0
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OldInt08 dd ?
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DelayTicks dw 4*18+1 ; 4 seconds
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DotTicks dw 18
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.errnz ($-_BootFix) GT (MaxCodeSize - 2) ;FATAL: bootfix code is too large
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org MaxCodeSize - 2
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db 55h,0aah
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CODE ENDS
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END START
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