source: bootcd/isolinux/syslinux-6.03/dosutil/copybs.asm

; -*- fundamental -*- (asm-mode sucks)
; -----------------------------------------------------------------------
;
;   Copyright 1998-2008 H. Peter Anvin - All Rights Reserved
;
;   This program is free software; you can redistribute it and/or modify
;   it under the terms of the GNU General Public License as published by
;   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
;   Boston MA 02111-1307, USA; either version 2 of the License, or
;   (at your option) any later version; incorporated herein by reference.
;
; -----------------------------------------------------------------------

;
; copybs.asm
;
; Small DOS program to copy the boot sector from a drive
; to a file
;
; Usage: copybs <drive>: <file>
;

                absolute 0
pspInt20:               resw 1
pspNextParagraph:       resw 1
                        resb 1          ; reserved
pspDispatcher:          resb 5
pspTerminateVector:     resd 1
pspControlCVector:      resd 1
pspCritErrorVector:     resd 1
                        resw 11         ; reserved
pspEnvironment:         resw 1
                        resw 23         ; reserved
pspFCB_1:               resb 16
pspFCB_2:               resb 16
                        resd 1          ; reserved
pspCommandLen:          resb 1
pspCommandArg:          resb 127

                section .text
                org 100h                        ; .COM format
_start:
                mov ax,3000h                    ; Get DOS version
                int 21h
                xchg al,ah
                mov [DOSVersion],ax
                cmp ax,0200h                    ; DOS 2.00 minimum
                jae dosver_ok
                mov dx,msg_ancient_err
                jmp die

                section .bss
                alignb 2
DOSVersion:     resw 1

                section .text
;
; Scan command line for a drive letter followed by a colon
;
dosver_ok:
                xor cx,cx
                mov si,pspCommandArg
                mov cl,[pspCommandLen]

cmdscan1:       jcxz bad_usage                  ; End of command line?
                lodsb                           ; Load character
                dec cx
                cmp al,' '                      ; White space
                jbe cmdscan1
                or al,020h                      ; -> lower case
                cmp al,'a'                      ; Check for letter
                jb bad_usage
                cmp al,'z'
                ja bad_usage
                sub al,'a'                      ; Convert to zero-based index
                mov [DriveNo],al                ; Save away drive index

                section .bss
DriveNo:        resb 1

                section .text
;
; Got the leading letter, now the next character must be a colon
;
got_letter:     jcxz bad_usage
                lodsb
                dec cx
                cmp al,':'
                jne bad_usage
;
; Got the colon; now we should have at least one whitespace
; followed by a filename
;
got_colon:      jcxz bad_usage
                lodsb
                dec cx
                cmp al,' '
                ja bad_usage

skipspace:      jcxz bad_usage
                lodsb
                dec cx
                cmp al,' '
                jbe skipspace

                mov di,FileName
copyfile:       stosb
                jcxz got_cmdline
                lodsb
                dec cx
                cmp al,' '
                ja copyfile
                jmp short got_cmdline

;
; We end up here if the command line doesn't parse
;
bad_usage:      mov dx,msg_unfair
                jmp die

                section .data
msg_unfair:     db 'Usage: copybs <drive>: <filename>', 0Dh, 0Ah, '$'

                section .bss
                alignb 4
FileName        resb 256

;
; Parsed the command line OK.  Get device parameter block to get the
; sector size.
;
                struc DPB
dpbDrive:       resb 1
dpbUnit:        resb 1
dpbSectorSize:  resw 1
dpbClusterMask: resb 1
dpbClusterShift: resb 1
dpbFirstFAT:    resw 1
dpbFATCount:    resb 1
dpbRootEntries: resw 1
dpbFirstSector: resw 1
dpbMaxCluster:  resw 1
dpbFATSize:     resw 1
dpbDirSector:   resw 1
dpbDriverAddr:  resd 1
dpbMedia:       resb 1
dpbFirstAccess: resb 1
dpbNextDPB:     resd 1
dpbNextFree:    resw 1
dpbFreeCnt:     resw 1
                endstruc

                section .bss
                alignb 2
SectorSize      resw 1

                section .text
got_cmdline:
                xor al,al                       ; Zero-terminate filename
                stosb

                mov dl,[DriveNo]
                inc dl                          ; 1-based
                mov ah,32h
                int 21h                         ; Get Drive Parameter Block

                and al,al
                jnz filesystem_error

                mov dx,[bx+dpbSectorSize]       ; Save sector size
;
; Read the boot sector.
;
                section .data
                align 4, db 0
DISKIO          equ $
diStartSector:  dd 0                            ; Absolute sector 0
diSectors:      dw 1                            ; One sector
diBuffer:       dw SectorBuffer                 ; Buffer offset
                dw 0                            ; Buffer segment

                section .text
read_bootsect:
                mov ax,cs                       ; Set DS <- CS
                mov ds,ax

                mov [SectorSize],dx             ; Saved sector size from above

                cmp word [DOSVersion],0400h     ; DOS 4.00 has a new interface
                jae .new
.old:
                mov bx,SectorBuffer
                mov cx,1                        ; One sector
                jmp short .common
.new:
                mov [diBuffer+2],ax             ; == DS
                mov bx,DISKIO
                mov cx,-1
.common:
                xor dx,dx                       ; Absolute sector 0
                mov al,[DriveNo]
                int 25h                         ; DOS absolute disk read
                pop ax                          ; Remove flags from stack
                jc disk_read_error

;
; Open the file and write the boot sector to the file.
;
                mov dx,FileName
                mov cx,0020h                    ; Attribute = ARCHIVE
                mov ah,3Ch                      ; Create file
                int 21h
                jc file_write_error

                mov bx,ax
                push ax                         ; Handle

                mov cx,[SectorSize]
                mov dx,SectorBuffer
                mov ah,40h                      ; Write file
                int 21h
                jc file_write_error
                cmp ax,[SectorSize]
                jne file_write_error

                pop bx                          ; Handle
                mov ah,3Eh                      ; Close file
                int 21h
                jc file_write_error
;
; We're done!
;
                mov ax,4C00h                    ; exit(0)
                int 21h

;
; Error routine jump
;
filesystem_error:
                mov dx,msg_filesystem_err
                jmp short die
disk_read_error:
                mov dx,msg_read_err
                jmp short die
file_write_error:
                mov dx,msg_write_err
die:
                push cs
                pop ds
                push dx
                mov dx,msg_error
                mov ah,09h
                int 21h
                pop dx

                mov ah,09h                      ; Write string
                int 21h

                mov ax,4C01h                    ; Exit error status
                int 21h

                section .data
msg_error:              db 'ERROR: $'
msg_ancient_err:        db 'DOS version 2.00 or later required', 0Dh, 0Ah, '$'
msg_filesystem_err:     db 'Filesystem not found on disk', 0Dh, 0Ah, '$'
msg_read_err:           db 'Boot sector read failed', 0Dh, 0Ah, '$'
msg_write_err:          db 'File write failed', 0Dh, 0Ah, '$'

                section .bss
                alignb 4
SectorBuffer:   resb 4096