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Assembly Source File | 1995-05-19 | 8.8 KB | 253 lines

; Simple boot strap loader for floppies (only) ; Assemble: tasm boot ; Link: tlink /t boot, boot.bin ;....................................................................... NUL equ 00h ; NUL terminator LF equ 0ah ; line feed CR equ 0dh ; carriage return BOOT equ 7c00h ; offset address of boot sector load point cseg segment assume cs:cseg org 0 db BOOT dup (?) ; skip to load point start: jmp boot_code ; skip over data ;....................................................................... ; required data header for boot sector oem_name db "FLOPPY " ; varies bytes_per_sector dw ? ; 512 sectors_per_cluster db ? ; 1 or 2 reserved_sectors dw ? ; 1 FATs db ? ; 2 root_directories dw ? ; usually 112 or 224 total_sectors dw ? ; varies media_descriptor db ? ; varies sectors_per_FAT dw ? ; varies sectors_per_track dw ? ; varies heads dw ? ; usually 2 hidden_sectors dd 0 large_vol_tsectors dd 0 physical_drive db 0 reserved db 0 extended_boot_sig db 29h serial_number dd ? volume_label db "NO NAME " fat_type db "FAT12 " ;....................................................................... ; boot sector program code boot_code: cli ; disable interrupts xor ax, ax ; set DS, ES and SS to segment 0 mov ds, ax mov es, ax mov ss, ax mov sp, BOOT ; put stack just below boot code sti ; enable interrupts assume ds:cseg, es:cseg, ss:cseg int 13h ; reset the disk controller jc reset_error ; jump if error mov si, offset boot_msg ; display boot message call puts ; compute starting sector for directory entries xor ah, ah mov al, byte ptr FATs mul word ptr sectors_per_FAT add ax, word ptr reserved_sectors mov word ptr directory_sector, ax ; save result ; read the directory sector mov bx, 0500h ; directory data will go here call read_sector ; read the first directory sector jc read_error ; jump if error ; check first directory entry for IO.SYS (MS-DOS systems) mov di, bx ; BX still points to buffer start mov si, offset iosys ; file name: IO.SYS mov cx, 11 ; file names are 11 characters long rep cmpsb ; compare jnz system_error ; jump if not first entry ; check second directory entry for MSDOS.SYS (MS-DOS systems) lea di, [bx+32] ; get address of next directory entry mov si, offset msdos ; file name: MSDOS.SYS mov cx, 11 ; file names are 11 characters long rep cmpsb ; compare jnz system_error ; jump if not second entry ; compute starting sector for data (comes after the root directory entries) mov ax, 32 ; 32 bytes per directory entry mul word ptr root_directories div word ptr bytes_per_sector add ax, word ptr directory_sector mov word ptr data_sector, ax ; save it mov cx, 3 ; first 3 sectors of io.sys mov bx, 0700h ; will go here ; read the 3 required sectors read_loop: push ax ; save logical sector number push bx ; save buffer address push cx ; save count call read_sector ; read 1 sector pop cx ; restore registers pop bx pop ax jc read_error ; jump if error inc ax ; bump sector# to next sector add bx, word ptr bytes_per_sector ; advance pointer loop read_loop ; continue ; set up the parameters for the BIOS loader mov ch, byte ptr media_descriptor mov dl, byte ptr physical_drive xor ax, ax mov bx, word ptr data_sector jmp far ptr bios_load ; do it ;....................................................................... reset_error: mov si, offset reset_msg jmp short no_good system_error: mov si, offset system_msg jmp short no_good read_error: mov si, offset read_msg jmp short no_good no_good: call puts ; print error message mov si, offset retry_msg call puts ; print retry message xor ah, ah ; wait for key press int 16h int 19h ; try booting again mov si, offset reboot_msg ; just in case it comes back jmp no_good ; (it shouldn't) ;....................................................................... ; write a message to the screen ; address of message in register SI puts proc near lodsb ; get one character or al, al ; check for terminating NUL jnz disp ; continue ret ; else done disp: mov ah, 0eh ; write character in teletype mode mov bx, 0007h ; video page 0, normal white int 10h ; BIOS video function jmp puts puts endp ; read a sector from the disk ; buffer address in register BX ; logical sector number in register AX read_sector proc near ; translate logical sector number to sector, track, and head xor dx, dx div word ptr sectors_per_track inc dl ; sector numbering starts at 1 mov byte ptr sector, dl ; save remainder: sector xor dx, dx ; quotient still in AX div word ptr heads mov byte ptr head, dl ; save remainder: head mov word ptr track, ax ; save quotient: track ; set up for disk read mov dx, word ptr track mov cl, 06 ; bits 8 & 9 of the track shl dh, cl ; into upper 2 bits of sector or dh, byte ptr sector ; stuff in the sector mov cx, dx ; move result into CX xchg ch, cl ; CH<-track, CL<-sector mov dl, byte ptr physical_drive ; DL<-drive mov dh, byte ptr head ; DH<-head mov ax, 0201h ; read one sector int 13h ret read_sector endp ;....................................................................... ; data used by program iosys db "IO SYS" msdos db "MSDOS SYS" boot_msg db "Now booting from floppy.", CR, LF, NUL reset_msg db "Disk controller error.", CR, LF, NUL read_msg db "Disk read error.", CR, LF, NUL system_msg db "No system on disk.", CR, LF, NUL reboot_msg db "Reboot failed.", CR, LF, NUL retry_msg db "Press any key to reboot.", CR, LF, NUL track dw 0 head db 0 sector db 0 directory_sector dw 0 data_sector dw 0 ; fill-out program size to 510 bytes (remaining 2 are for boot signature) fill db (512-(offset fill-offset start)-2) dup (0) signature dw 0AA55h cseg ends ;....................................................................... bios segment at 0070h assume cs:bios org 0000h bios_load label far bios ends end start