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403 lines
14 KiB
NASM
403 lines
14 KiB
NASM
; FAT.ASM
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; FAT12/16 Boot Sector
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; Copyright (c) 1998, 2001, 2002 Brian Palmer
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; This is a FAT12/16 file system boot sector
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; that searches the entire root directory
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; for the file freeldr.sys and loads it into
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; memory.
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;
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; The stack is set to 0000:7BF2 so that the first
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; WORD pushed will be placed at 0000:7BF0
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;
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; The DWORD at 0000:7BFC or BP-04h is the logical
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; sector number of the start of the data area.
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;
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; The DWORD at 0000:7BF8 or BP-08h is the total
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; sector count of the boot drive as reported by
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; the computers bios.
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;
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; The WORD at 0000:7BF6 or BP-0ah is the offset
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; of the ReadSectors function in the boot sector.
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;
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; The WORD at 0000:7BF4 or BP-0ch is the offset
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; of the ReadCluster function in the boot sector.
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;
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; The WORD at 0000:7BF2 or BP-0eh is the offset
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; of the PutChars function in the boot sector.
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;
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; When it locates freeldr.sys on the disk it will
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; load the first sector of the file to 0000:8000
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; With the help of this sector we should be able
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; to load the entire file off the disk, no matter
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; how fragmented it is.
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;
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; We load the entire FAT table into memory at
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; 7000:0000. This improves the speed of floppy disk
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; boots dramatically.
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BootSectorStackTop equ 0x7bf2
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DataAreaStartHigh equ 0x2
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DataAreaStartLow equ 0x4
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BiosCHSDriveSizeHigh equ 0x6
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BiosCHSDriveSizeLow equ 0x8
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BiosCHSDriveSize equ 0x8
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ReadSectorsOffset equ 0xa
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ReadClusterOffset equ 0xc
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PutCharsOffset equ 0xe
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org 7c00h
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segment .text
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bits 16
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start:
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jmp short main
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nop
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OEMName db 'FrLdr1.0'
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BytesPerSector dw 512
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SectsPerCluster db 1
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ReservedSectors dw 1
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NumberOfFats db 2
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MaxRootEntries dw 224
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TotalSectors dw 2880
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MediaDescriptor db 0f0h
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SectorsPerFat dw 9
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SectorsPerTrack dw 18
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NumberOfHeads dw 2
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HiddenSectors dd 0
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TotalSectorsBig dd 0
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BootDrive db 0
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Reserved db 0
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ExtendSig db 29h
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SerialNumber dd 00000000h
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VolumeLabel db 'NO NAME '
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FileSystem db 'FAT12 '
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main:
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xor ax,ax
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mov ss,ax
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mov bp,7c00h
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mov sp,BootSectorStackTop ; Setup a stack
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mov ds,ax ; Make DS correct
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mov es,ax ; Make ES correct
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cmp BYTE [BYTE bp+BootDrive],BYTE 0xff ; If they have specified a boot drive then use it
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jne GetDriveParameters
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mov [BYTE bp+BootDrive],dl ; Save the boot drive
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GetDriveParameters:
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mov ah,08h
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mov dl,[BYTE bp+BootDrive] ; Get boot drive in dl
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int 13h ; Request drive parameters from the bios
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jnc CalcDriveSize ; If the call succeeded then calculate the drive size
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; If we get here then the call to the BIOS failed
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; so just set CHS equal to the maximum addressable
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; size
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mov cx,0ffffh
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mov dh,cl
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CalcDriveSize:
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; Now that we have the drive geometry
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; lets calculate the drive size
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mov bl,ch ; Put the low 8-bits of the cylinder count into BL
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mov bh,cl ; Put the high 2-bits in BH
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shr bh,6 ; Shift them into position, now BX contains the cylinder count
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and cl,3fh ; Mask off cylinder bits from sector count
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; CL now contains sectors per track and DH contains head count
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movzx eax,dh ; Move the heads into EAX
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movzx ebx,bx ; Move the cylinders into EBX
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movzx ecx,cl ; Move the sectors per track into ECX
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inc eax ; Make it one based because the bios returns it zero based
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inc ebx ; Make the cylinder count one based also
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mul ecx ; Multiply heads with the sectors per track, result in edx:eax
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mul ebx ; Multiply the cylinders with (heads * sectors) [stored in edx:eax already]
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; We now have the total number of sectors as reported
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; by the bios in eax, so store it in our variable
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mov [BYTE bp-BiosCHSDriveSize],eax
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; Now we must find our way to the first sector of the root directory
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xor ax,ax
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xor cx,cx
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mov al,[BYTE bp+NumberOfFats] ; Number of fats
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mul WORD [BYTE bp+SectorsPerFat] ; Times sectors per fat
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add ax,WORD [BYTE bp+HiddenSectors]
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adc dx,WORD [BYTE bp+HiddenSectors+2] ; Add the number of hidden sectors
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add ax,WORD [BYTE bp+ReservedSectors] ; Add the number of reserved sectors
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adc dx,cx ; Add carry bit
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mov WORD [BYTE bp-DataAreaStartLow],ax ; Save the starting sector of the root directory
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mov WORD [BYTE bp-DataAreaStartHigh],dx ; Save it in the first 4 bytes before the boot sector
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mov si,WORD [BYTE bp+MaxRootEntries] ; Get number of root dir entries in SI
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pusha ; Save 32-bit logical start sector of root dir
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; DX:AX now has the number of the starting sector of the root directory
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; Now calculate the size of the root directory
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xor dx,dx
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mov ax,0020h ; Size of dir entry
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mul si ; Times the number of entries
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mov bx,[BYTE bp+BytesPerSector]
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add ax,bx
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dec ax
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div bx ; Divided by the size of a sector
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; AX now has the number of root directory sectors
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add [BYTE bp-DataAreaStartLow],ax ; Add the number of sectors of the root directory to our other value
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adc [BYTE bp-DataAreaStartHigh],cx ; Now the first 4 bytes before the boot sector contain the starting sector of the data area
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popa ; Restore root dir logical sector start to DX:AX
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LoadRootDirSector:
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mov bx,7e0h ; We will load the root directory sector
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mov es,bx ; Right after the boot sector in memory
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xor bx,bx ; We will load it to [0000:7e00h]
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xor cx,cx ; Zero out CX
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inc cx ; Now increment it to 1, we are reading one sector
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xor di,di ; Zero out di
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push es ; Save ES because it will get incremented by 20h
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call ReadSectors ; Read the first sector of the root directory
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pop es ; Restore ES (ES:DI = 07E0:0000)
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SearchRootDirSector:
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cmp [es:di],ch ; If the first byte of the directory entry is zero then we have
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jz ErrBoot ; reached the end of the directory and FREELDR.SYS is not here so reboot
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pusha ; Save all registers
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mov cl,0xb ; Put 11 in cl (length of filename in directory entry)
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mov si,filename ; Put offset of filename string in DS:SI
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repe cmpsb ; Compare this directory entry against 'FREELDR SYS'
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popa ; Restore all the registers
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jz FoundFreeLoader ; If we found it then jump
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dec si ; SI holds MaxRootEntries, subtract one
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jz ErrBoot ; If we are out of root dir entries then reboot
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add di,BYTE +0x20 ; Increment DI by the size of a directory entry
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cmp di,0200h ; Compare DI to 512 (DI has offset to next dir entry, make sure we haven't gone over one sector)
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jc SearchRootDirSector ; If DI is less than 512 loop again
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jmp short LoadRootDirSector ; Didn't find FREELDR.SYS in this directory sector, try again
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FoundFreeLoader:
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; We found freeldr.sys on the disk
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; so we need to load the first 512
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; bytes of it to 0000:8000
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; ES:DI has dir entry (ES:DI == 07E0:XXXX)
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mov ax,WORD [es:di+1ah] ; Get start cluster
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push ax ; Save start cluster
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push WORD 800h ; Put 800h on the stack and load it
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pop es ; Into ES so that we load the cluster at 0000:8000
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call ReadCluster ; Read the cluster
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pop ax ; Restore start cluster of FreeLoader
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; Save the addresses of needed functions so
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; the helper code will know where to call them.
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mov WORD [BYTE bp-ReadSectorsOffset],ReadSectors ; Save the address of ReadSectors
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mov WORD [BYTE bp-ReadClusterOffset],ReadCluster ; Save the address of ReadCluster
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mov WORD [BYTE bp-PutCharsOffset],PutChars ; Save the address of PutChars
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; Now AX has start cluster of FreeLoader and we
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; have loaded the helper code in the first 512 bytes
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; of FreeLoader to 0000:8000. Now transfer control
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; to the helper code. Skip the first three bytes
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; because they contain a jump instruction to skip
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; over the helper code in the FreeLoader image.
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;jmp 0000:8003h
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jmp 8003h
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; Displays an error message
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; And reboots
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ErrBoot:
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mov si,msgFreeLdr ; FreeLdr not found message
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call PutChars ; Display it
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Reboot:
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mov si,msgAnyKey ; Press any key message
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call PutChars ; Display it
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xor ax,ax
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int 16h ; Wait for a keypress
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int 19h ; Reboot
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PutChars:
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lodsb
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or al,al
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jz short Done
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mov ah,0eh
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mov bx,07h
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int 10h
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jmp short PutChars
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Done:
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retn
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; Displays a bad boot message
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; And reboots
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BadBoot:
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mov si,msgDiskError ; Bad boot disk message
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call PutChars ; Display it
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jmp short Reboot
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; Reads cluster number in AX into [ES:0000]
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ReadCluster:
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; StartSector = ((Cluster - 2) * SectorsPerCluster) + ReservedSectors + HiddenSectors;
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dec ax ; Adjust start cluster by 2
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dec ax ; Because the data area starts on cluster 2
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xor ch,ch
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mov cl,BYTE [BYTE bp+SectsPerCluster]
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mul cx ; Times sectors per cluster
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add ax,[BYTE bp-DataAreaStartLow] ; Add start of data area
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adc dx,[BYTE bp-DataAreaStartHigh] ; Now we have DX:AX with the logical start sector of OSLOADER.SYS
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xor bx,bx ; We will load it to [ES:0000], ES loaded before function call
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;mov cl,BYTE [BYTE bp+SectsPerCluster]; Sectors per cluster still in CX
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;call ReadSectors
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;ret
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; Reads logical sectors into [ES:BX]
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; DX:AX has logical sector number to read
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; CX has number of sectors to read
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ReadSectors:
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; We can't just check if the start sector is
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; in the BIOS CHS range. We have to check if
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; the start sector + length is in that range.
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pusha
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dec cx
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add ax,cx
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adc dx,byte 0
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cmp dx,WORD [BYTE bp-BiosCHSDriveSizeHigh] ; Check if they are reading a sector within CHS range
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ja ReadSectorsLBA ; No - go to the LBA routine
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jb ReadSectorsCHS ; Yes - go to the old CHS routine
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cmp ax,WORD [BYTE bp-BiosCHSDriveSizeLow] ; Check if they are reading a sector within CHS range
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jbe ReadSectorsCHS ; Yes - go to the old CHS routine
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ReadSectorsLBA:
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popa
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ReadSectorsLBALoop:
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pusha ; Save logical sector number & sector count
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o32 push byte 0
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push dx ; Put 64-bit logical
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push ax ; block address on stack
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push es ; Put transfer segment on stack
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push bx ; Put transfer offset on stack
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push byte 1 ; Set transfer count to 1 sector
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push byte 0x10 ; Set size of packet to 10h
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mov si,sp ; Setup disk address packet on stack
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; We are so totally out of space here that I am forced to
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; comment out this very beautifully written piece of code
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; It would have been nice to have had this check...
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;CheckInt13hExtensions: ; Now make sure this computer supports extended reads
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; mov ah,0x41 ; AH = 41h
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; mov bx,0x55aa ; BX = 55AAh
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; mov dl,[BYTE bp+BootDrive] ; DL = drive (80h-FFh)
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; int 13h ; IBM/MS INT 13 Extensions - INSTALLATION CHECK
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; jc PrintDiskError ; CF set on error (extensions not supported)
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; cmp bx,0xaa55 ; BX = AA55h if installed
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; jne PrintDiskError
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; test cl,1 ; CX = API subset support bitmap
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; jz PrintDiskError ; Bit 0, extended disk access functions (AH=42h-44h,47h,48h) supported
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; Good, we're here so the computer supports LBA disk access
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; So finish the extended read
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mov dl,[BYTE bp+BootDrive] ; Drive number
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mov ah,42h ; Int 13h, AH = 42h - Extended Read
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int 13h ; Call BIOS
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jc BadBoot ; If the read failed then abort
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add sp,byte 0x10 ; Remove disk address packet from stack
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popa ; Restore sector count & logical sector number
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inc ax ; Increment Sector to Read
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adc dx,byte 0
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push bx
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mov bx,es
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add bx,byte 20h ; Increment read buffer for next sector
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mov es,bx
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pop bx
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loop ReadSectorsLBALoop ; Read next sector
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ret
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; Reads logical sectors into [ES:BX]
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; DX:AX has logical sector number to read
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; CX has number of sectors to read
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; CarryFlag set on error
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ReadSectorsCHS:
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popa
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ReadSectorsCHSLoop:
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pusha
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xchg ax,cx
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xchg ax,dx
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xor dx,dx
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div WORD [BYTE bp+SectorsPerTrack]
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xchg ax,cx
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div WORD [BYTE bp+SectorsPerTrack] ; Divide logical by SectorsPerTrack
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inc dx ; Sectors numbering starts at 1 not 0
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xchg cx,dx
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div WORD [BYTE bp+NumberOfHeads] ; Number of heads
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mov dh,dl ; Head to DH, drive to DL
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mov dl,[BYTE bp+BootDrive] ; Drive number
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mov ch,al ; Cylinder in CX
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ror ah,2 ; Low 8 bits of cylinder in CH, high 2 bits
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; in CL shifted to bits 6 & 7
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or cl,ah ; Or with sector number
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mov ax,0201h
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int 13h ; DISK - READ SECTORS INTO MEMORY
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; AL = number of sectors to read, CH = track, CL = sector
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; DH = head, DL = drive, ES:BX -> buffer to fill
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; Return: CF set on error, AH = status (see AH=01h), AL = number of sectors read
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jc BadBoot
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popa
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inc ax ;Increment Sector to Read
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jnz NoCarryCHS
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inc dx
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NoCarryCHS:
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push bx
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mov bx,es
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add bx,byte 20h
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mov es,bx
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pop bx
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; Increment read buffer for next sector
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loop ReadSectorsCHSLoop ; Read next sector
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ret
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msgDiskError db 'Disk error',0dh,0ah,0
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msgFreeLdr db 'freeldr.sys not found',0dh,0ah,0
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; Sorry, need the space...
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;msgAnyKey db 'Press any key to restart',0dh,0ah,0
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msgAnyKey db 'Press any key',0dh,0ah,0
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filename db 'FREELDR SYS'
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times 509-($-$$) db 0 ; Pad to 509 bytes
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BootPartition:
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db 0
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BootSignature:
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dw 0aa55h ; BootSector signature
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