; +--------+ ; | | ; | | ; |--------| 4000:0000 ; | | ; | FAT | ; | | ; |--------| 2000:0000 ; |BOOT SEC| ; |RELOCATE| ; |--------| 1FE0:0000 ; | | ; | | ; | | ; | | ; |--------| ; |BOOT SEC| ; |ORIGIN | 07C0:0000 ; |--------| ; | | ; | | ; | | ; |--------| ; |KERNEL | ; |LOADED | ; |--------| 0060:0000 ; | | ; +--------+ ;%define MULTI_SEC_READ 1 segment .text %define BASE 0x7c00 org BASE Entry: jmp short real_start nop ; bp is initialized to 7c00h %define bsOemName bp+0x03 ; OEM label %define bsBytesPerSec bp+0x0b ; bytes/sector %define bsSecPerClust bp+0x0d ; sectors/allocation unit %define bsResSectors bp+0x0e ; # reserved sectors %define bsFATs bp+0x10 ; # of fats %define bsRootDirEnts bp+0x11 ; # of root dir entries %define bsSectors bp+0x13 ; # sectors total in image %define bsMedia bp+0x15 ; media descrip: fd=2side9sec, etc... %define sectPerFat bp+0x16 ; # sectors in a fat %define sectPerTrack bp+0x18 ; # sectors/track %define nHeads bp+0x1a ; # heads %define nHidden bp+0x1c ; # hidden sectors %define nSectorHuge bp+0x20 ; # sectors if > 65536 %define xsectPerFat bp+0x24 ; Sectors/Fat %define xrootClst bp+0x2c ; Starting cluster of root directory %define drive bp+0x40 ; Drive number times 0x5a-$+$$ db 0 %define LOADSEG 0x0060 %define FATSEG 0x2000 %define fat_sector bp+0x48 ; last accessed sector of the FAT %define loadsegoff_60 bp+0x5a ; FAR pointer = 60:0 dw 0 %define loadseg_60 bp+0x5c dw LOADSEG %define fat_start bp+0x5e ; first FAT sector %define data_start bp+0x62 ; first data sector %define fat_secmask bp+0x66 ; number of clusters in a FAT sector - 1 %define fat_secshift bp+0x68 ; fat_secmask+1 = 2^fat_secshift ;----------------------------------------------------------------------- ; ENTRY ;----------------------------------------------------------------------- real_start: cld cli sub ax, ax mov ds, ax mov bp, 0x7c00 mov ax, 0x1FE0 mov es, ax mov si, bp mov di, bp mov cx, 0x0100 rep movsw ; move boot code to the 0x1FE0:0x0000 jmp word 0x1FE0:cont cont: mov ds, ax mov ss, ax lea sp, [bp-0x20] sti mov [drive], dl ; BIOS passes drive number in DL ; call print ; db "Loading ",0 ; Calc Params ; Fat_Start mov si, word [nHidden] mov di, word [nHidden+2] add si, word [bsResSectors] adc di, byte 0 mov word [fat_start], si mov word [fat_start+2], di ; Data_Start mov al, [bsFATs] cbw push ax mul word [xsectPerFat+2] add di, ax pop ax mul word [xsectPerFat] add ax, si adc dx, di mov word[data_start], ax mov word[data_start+2], dx ; fat_secmask mov ax, word[bsBytesPerSec] shr ax, 1 shr ax, 1 dec ax mov word [fat_secmask], ax ; fat_secshift ; cx = temp ; ax = fat_secshift xchg ax, cx ; cx = 0 after movsw inc cx secshift: inc ax shr cx, 1 cmp cx, 1 jne secshift mov byte [fat_secshift], al dec cx ; FINDFILE: Searches for the file in the root directory. ; ; Returns: ; DX:AX = first cluster of file mov word [fat_sector], cx ; CX is 0 after "dec" mov word [fat_sector + 2], cx mov ax, word [xrootClst] mov dx, word [xrootClst + 2] ff_next_cluster: push dx ; save cluster push ax call convert_cluster jc boot_error ; EOC encountered ff_next_sector: push bx ; save sector count les bx, [loadsegoff_60] call readDisk push dx ; save sector push ax mov ax, [bsBytesPerSec] ; Search for KERNEL.SYS file name, and find start cluster. ff_next_entry: mov cx, 11 mov si, filename mov di, ax sub di, 0x20 repe cmpsb jz ff_done sub ax, 0x20 jnz ff_next_entry pop ax ; restore sector pop dx pop bx ; restore sector count dec bx jnz ff_next_sector ff_find_next_cluster: pop ax ; restore current cluster pop dx call next_cluster jmp short ff_next_cluster ff_done: mov ax, [es:di+0x1A-11] ; get cluster number mov dx, [es:di+0x14-11] c4: sub bx, bx ; ES points to LOADSEG c5: push dx push ax push bx call convert_cluster jc boot_success mov di, bx pop bx c6: call readDisk dec di jnz c6 pop ax pop dx call next_cluster jmp short c5 boot_error: xor ah,ah int 0x16 ; wait for a key int 0x19 ; reboot the machine ; input: ; DX:AX - cluster ; output: ; DX:AX - next cluster ; CX = 0 ; modify: ; DI next_cluster: push es mov di, ax and di, [fat_secmask] mov cx, [fat_secshift] cn_loop: shr dx,1 rcr ax,1 dec cx jnz cn_loop ; DX:AX fat sector where our ; cluster resides ; DI - cluster index in this ; sector shl di,1 ; DI - offset in the sector shl di,1 add ax, [fat_start] adc dx, [fat_start+2] ; DX:AX absolute fat sector push bx mov bx, FATSEG mov es, bx sub bx, bx cmp ax, [fat_sector] jne cn1 ; if the last fat sector we ; read was this, than skip cmp dx,[fat_sector+2] je cn_exit cn1: mov [fat_sector],ax ; save the fat sector number, mov [fat_sector+2],dx ; we are going to read call readDisk cn_exit: pop bx mov ax, [es:di] ; DX:AX - next cluster mov dx, [es:di + 2] ; pop es ret boot_success: mov bl, [drive] jmp far [loadsegoff_60] ; Convert cluster to the absolute sector ;input: ; DX:AX - target cluster ;output: ; DX:AX - absoulute sector ; BX - [bsSectPerClust] ;modify: ; CX convert_cluster: cmp dx,0x0fff jne c3 cmp ax,0xfff8 jb c3 ; if cluster is EOC (carry is set), do ret stc ret c3: mov cx, dx ; sector = (cluster - 2)*clussize + ; + data_start sub ax, 2 sbb cx, byte 0 ; CX:AX == cluster - 2 mov bl, [bsSecPerClust] sub bh, bh xchg cx, ax ; AX:CX == cluster - 2 mul bx ; first handle high word ; DX must be 0 here xchg ax, cx ; then low word mul bx add dx, cx ; DX:AX target sector add ax, [data_start] adc dx, [data_start + 2] ret ; prints text after call to this function. print_1char: xor bx, bx ; video page 0 mov ah, 0x0E ; else print it int 0x10 ; via TTY mode print: pop si ; this is the first character print1: lodsb ; get token push si ; stack up potential return address cmp al, 0 ; end of string? jne print_1char ; until done ret ; and jump to it ;input: ; DX:AX - 32-bit DOS sector number ; ES:BX - destination buffer ;output: ; ES:BX points one byte after the last byte read. ; DX:AX - next sector ;modify: ; ES if DI * bsBytesPerSec >= 65536, CX readDisk: read_next: push dx push ax ; ; translate sector number to BIOS parameters ; ; ; abs = sector offset in track ; + head * sectPerTrack offset in cylinder ; + track * sectPerTrack * nHeads offset in platter ; xchg ax, cx mov al, [sectPerTrack] mul byte [nHeads] xchg ax, cx ; cx = nHeads * sectPerTrack <= 255*63 ; dx:ax = abs div cx ; ax = track, dx = sector + head * sectPertrack xchg ax, dx ; dx = track, ax = sector + head * sectPertrack div byte [sectPerTrack] ; dx = track, al = head, ah = sector mov cx, dx ; cx = track, al = head, ah = sector ; the following manipulations are necessary in order to ; properly place parameters into registers. ; ch = cylinder number low 8 bits ; cl = 7-6: cylinder high two bits ; 5-0: sector mov dh, al ; save head into dh for bios xchg ch, cl ; set cyl no low 8 bits ror cl, 1 ; move track high bits into ror cl, 1 ; bits 7-6 (assumes top = 0) inc ah ; sector offset from 1 or cl, ah ; merge sector into cylinder mov ax, 0x0201 mov dl, [drive] int 0x13 pop ax pop dx jnc read_ok ; jump if no error xor ah, ah ; else, reset floppy int 0x13 jmp short read_next read_ok: add bx, word [bsBytesPerSec] jnc no_incr_es ; if overflow... mov cx, es add ch, 0x10 ; ...add 1000h to ES mov es, cx no_incr_es: add ax,byte 1 adc dx,byte 0 ret times 0x01f1-$+$$ db 0 filename db "KERNEL SYS",0,0 sign dw 0xAA55