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; boot12.asm FAT12 bootstrap for real mode image or loader
; Version 1.0, Jul 5, 1999
; Sample code
; by John S. Fine [email protected]
; I do not place any restrictions on your use of this source code
; I do not provide any warranty of the correctness of this source code
;_____________________________________________________________________________
;
; Documentation:
;
; I) BASIC features
; II) Compiling and installing
; III) Detailed features and limits
; IV) Customization
;_____________________________________________________________________________
;
; I) BASIC features
;
; This boot sector will load and start a real mode image from a file in the
; root directory of a FAT12 formatted floppy or partition.
;
; Inputs:
; DL = drive number
;
; Outputs:
; The boot record is left in memory at 7C00 and the drive number is patched
; into the boot record at 7C24.
; SS = DS = 0
; BP = 7C00
;_____________________________________________________________________________
;
; II) Compiling and installing
;
; To compile, use NASM
;
; nasm boot12.asm -o boot12.bin
;
; Then you must copy the first three bytes of BOOT12.BIN to the first three
; bytes of the volume and copy bytes 0x3E through 0x1FF of BOOT12.BIN to
; bytes 0x3E through 0x1FF of the volume. Bytes 0x3 through 0x3D of the
; volume should be set by a FAT12 format program and should not be modified
; when copying boot12.bin to the volume.
;
; If you use my PARTCOPY program to install BOOT12.BIN on A:, the
; commands are:
;
; partcopy boot12.bin 0 3 -f0
; partcopy boot12.bin 3e 1c2 -f0 3e
;
; PARTCOPY can also install to a partition on a hard drive. Please read
; partcopy documentation and use it carefully. Careless use could overwrite
; important parts of your hard drive.
;
; You can find PARTCOPY and links to NASM on my web page at
; http://www.erols.com/johnfine/
;_____________________________________________________________________________
;
; III) Detailed features and limits
;
; Most of the limits are stable characteristics of the volume. If you are
; using boot12 in a personal project, you should check the limits before
; installing boot12. If you are using boot12 in a project for general
; distribution, you should include an installation program which checks the
; limits automatically.
;
; CPU: Supports any 8088+ CPU.
;
; Volume format: Supports only FAT12.
;
; Sector size: Supports only 512 bytes per sector.
;
; Drive/Partition: Supports whole drive or any partition of any drive number
; supported by INT 13h.
;
; Diskette parameter table: This code does not patch the diskette parameter
; table. If you boot this code from a diskette that has more sectors per
; track than the default initialized by the BIOS then the failure to patch
; that table may be a problem. Because this code splits at track boundaries
; a diskette with fewer sectors per track should not be a problem.
;
; File position: The file name may be anywhere in the root directory and the
; file may be any collection of clusters on the volume. There are no
; contiguity requirements. (But see track limit).
;
; Track boundaries: Transfers are split on track boundaries. Many BIOS's
; require that the caller split floppy transfers on track boundaries.
;
; 64Kb boundaries: Transfers are split on 64Kb boundaries. Many BIOS's
; require that the caller split floppy transfers on track boundaries.
;
; Cluster boundaries: Transfers are merged across cluster boundaries whenever
; possible. On some systems, this significantly reduces load time.
;
; Cluster 2 limit: Cluster 2 must start before sector 65536 of the volume.
; This is very likely because only the reserved sectors (usually 1) and
; the FAT's (two of up to 12 sectors each) and the root directory (usually
; either 15 or 32 sectors) precede cluster 2.
;
; Track limit: The entire image file must reside before track 32768 of the
; entire volume. This is true on most media up to 1GB in size. If it is a
; problem it is easy to fix (see boot16.asm). I didn't expect many people
; to put FAT12 partitions beyond the first GB of a large hard drive.
;
; Memory boundaries: The FAT, Root directory, and Image must all be loaded
; starting at addresses that are multiples of 512 bytes (32 paragraphs).
;
; Memory use: The FAT and Root directory must each fit entirely in the
; first 64Kb of RAM. They may overlap.
;
; Root directory size: As released, it supports up to 928 entries in the
; root directory. If ROOT_SEG were changed to 0x7E0 it would support up
; to 1040. Most FAT12 volumes have either 240 or 512 root directory
; entries.
;_____________________________________________________________________________
;
; IV) Customization
;
; The memory usage can be customized by changing the _SEG variables (see
; directly below).
;
; The file name to be loaded and the message displayed in case of error
; may be customized (see end of this file).
;
; The ouput values may be customized. For example, many loaders expect the
; bootsector to leave the drive number in DL. You could add "mov dl,[drive]"
; at the label "eof:".
;
; Some limits (like maximum track) may be removed. See boot16.asm for
; comparison.
;
; Change whatever else you like. The above are just likely possibilities.
;_____________________________________________________________________________
; Change the _SEG values to customize memory use during the boot.
; When planning memory use, remember:
;
; *) Each of ROOT_SEG, FAT_SEG, and IMAGE_SEG must be divisible by 0x20
;
; *) None of ROOT, FAT or IMAGE should overlap the boot code itself, or
; its stack. That means: avoid paragraphs 0x7B0 to 0x7DF.
;
; *) The FAT area must not overlap the IMAGE area. Either may overlap
; the ROOT area; But, if they do then the root will not remain in
; memory for possible reuse by the next stage.
;
; *) The FAT area and the root area must each fit within the first 64Kb
; excluding BIOS area (paragraphs 0x60 to 0xFFF).
;
; *) A FAT12 FAT can be up to 6Kb (0x180 paragraphs).
;
; *) A FAT12 root directory is typically either 0x1E0 or 0x400 paragraphs
; long, but larger sizes are possible.
;
; *) The code will be two bytes shorter when FAT_SEG is 0x800 than when it
; is another value. (If you reach the point of caring about two bytes).
;
%define ROOT_SEG 0x60
%define FAT_SEG 0x800
%define IMAGE_SEG 0x1000
%if ROOT_SEG & 31
%error "ROOT_SEG must be divisible by 0x20"
%endif
%if ROOT_SEG > 0xC00
%error "Root directory must fit within first 64Kb"
%endif
%if FAT_SEG & 31
%error "FAT_SEG must be divisible by 0x20"
%endif
%if FAT_SEG > 0xE80
%error "FAT must fit within first 64Kb"
%endif
%if IMAGE_SEG & 31
%error "IMAGE_SEG must be divisible by 0x20"
%endif
; The following %define directives declare the parts of the FAT12 "DOS BOOT
; RECORD" that are used by this code, based on BP being set to 7C00.
;
%define sc_p_clu bp+0Dh ;byte Sectors per cluster
%define sc_b4_fat bp+0Eh ;word Sectors (in partition) before FAT
%define fats bp+10h ;byte Number of FATs
%define dir_ent bp+11h ;word Number of root directory entries
%define sc_p_fat bp+16h ;word Sectors per FAT
%define sc_p_trk bp+18h ;word Sectors per track
%define heads bp+1Ah ;word Number of heads
%define sc_b4_prt bp+1Ch ;dword Sectors before partition
%define drive bp+24h ;byte Drive number
org 0x7C00
entry:
jmp short begin
; --------------------------------------------------
; data portion of the "DOS BOOT RECORD"
; ----------------------------------------------------------------------
brINT13Flag DB 90H ; 0002h - 0EH for INT13 AH=42 READ
brOEM DB 'MSDOS5.0' ; 0003h - OEM ID - Windows 95B
brBPS DW 512 ; 000Bh - Bytes per sector
brSPC DB 1 ; 000Dh - Sector per cluster
brSc_b4_fat DW 1 ; 000Eh - Reserved sectors
brFATs DB 2 ; 0010h - FAT copies
brRootEntries DW 0E0H ; 0011h - Root directory entries
brSectorCount DW 2880 ; 0013h - Sectors in volume, < 32MB
brMedia DB 240 ; 0015h - Media descriptor
brSPF DW 9 ; 0016h - Sectors per FAT
brSc_p_trk DW 18 ; 0018h - Sectors per head/track
brHPC DW 2 ; 001Ah - Heads per cylinder
brSc_b4_prt DD 0 ; 001Ch - Hidden sectors
brSectors DD 0 ; 0020h - Total number of sectors
brDrive DB 0 ; 0024h - Physical drive no.
DB 0 ; 0025h - Reserved (FAT32)
DB 29H ; 0026h - Extended boot record sig (FAT32)
brSerialNum DD 404418EAH ; 0027h - Volume serial number
brLabel DB 'Joels disk ' ; 002Bh - Volume label
brFSID DB 'FAT12 ' ; 0036h - File System ID
;------------------------------------------------------------------------
begin:
xor ax, ax
mov ds, ax
mov ss, ax
mov sp, 0x7C00
mov bp, sp
mov [drive], dl ;Drive number
mov al, [fats] ;Number of FATs
mul word [sc_p_fat] ; * Sectors per FAT
add ax, [sc_b4_fat] ; + Sectors before FAT
;AX = Sector of Root directory
mov si, [dir_ent] ;Max root directory entries
mov cl, 4
dec si
shr si, cl
inc si ;SI = Length of root in sectors
mov di, ROOT_SEG/32 ;Buffer (paragraph / 32)
call read_16 ;Read root directory
push ax ;Sector of cluster two
%define sc_clu2 bp-2 ;Later access to the word just pushed is via bp
mov dx, [dir_ent] ;Number of directory entries
push ds
pop es
mov di, ROOT_SEG*16
search:
dec dx ;Any more directory entries?
js error ;No
mov si, filename ;Name we are searching for
mov cx, 11 ;11 characters long
lea ax, [di+0x20] ;Precompute next entry address
push ax
repe cmpsb ;Compare
pop di
jnz search ;Repeat until match
push word [di-6] ;Starting cluster number
mov ax, [sc_b4_fat] ;Sector number of FAT
mov si, [sc_p_fat] ;Length of FAT
mov di, FAT_SEG/32 ;Buffer (paragraph / 32)
call read_16 ;Read FAT
next:
pop bx ;Cluster number
mov si, bx ;First cluster in this sequence
mov ax, bx ;Last cluster in this sequence
.0:
cmp bx, 0xFF8 ;End of file?
jae .2 ; Yes
inc ax ;Last cluster plus one in sequence
;Look in FAT for next cluster
mov di, bx ;Cluster number
rcr bx, 1 ;1.5 byte entry per cluster
;bx = 0x8000 + cluster/2
;c-bit set for odd clusters
mov bx, [bx+di+FAT_SEG*16-0x8000]
jnc .1
shr bx, 1
shr bx, 1
shr bx, 1
shr bx, 1
.1: and bh, 0xF
cmp ax, bx ;Is the next one contiguous?
je .0 ;Yes: look further ahead
.2: sub ax, si ;How many contiguous in this sequence?
jz eof ;None, must be done.
push bx ;Save next (eof or discontiguous) cluster
mov bl, [sc_p_clu] ;Sectors per cluster
mov bh, 0 ; as a word
mul bx ;Length of sequence in sectors
.3: mov di, IMAGE_SEG/32 ;Destination (paragraph / 32)
add [.3+1], ax ;Precompute next destination
xchg ax, si ;AX = starting cluster ;SI = length in sectors
dec ax
dec ax ;Starting cluster minus two
mul bx ; * sectors per cluster
add ax, [sc_clu2] ; + sector number of cluster two
adc dl, dh ;Allow 24-bit result
call read_32 ;Read it
jmp short next ;Look for more
eof:
jmp IMAGE_SEG:0
error: mov si, errmsg ;Same message for all detected errors
mov ax, 0xE0D ;Start message with CR
mov bx, 7
.1: int 10h
lodsb
test al, al
jnz .1
xor ah, ah
int 16h ;Wait for a key
int 19h ;Try to reboot
read_16:
xor dx, dx
read_32:
;
; Input:
; dx:ax = sector within partition
; si = sector count
; di = destination segment / 32
;
; The sector number is converted from a partition-relative to a whole-disk
; (LBN) value, and then converted to CHS form, and then the sectors are read
; into (di*32):0.
;
; Output:
; dx:ax updated (sector count added)
; di updated (sector count added)
; si = 0
; bp, ds preserved
; bx, cx, es modified
.1: push dx ;(high) relative sector
push ax ;(low) relative sector
add ax, [sc_b4_prt] ;Convert to LBN
adc dx, [sc_b4_prt+2]
mov bx, [sc_p_trk] ;Sectors per track
div bx ;AX = track ;DX = sector-1
sub bx, dx ;Sectors remaining, this track
cmp bx, si ;More than we want?
jbe .2 ;No
mov bx, si ;Yes: Transfer just what we want
.2: inc dx ;Sector number
mov cx, dx ;CL = sector ;CH = 0
cwd ;(This supports up to 32767 tracks
div word [heads] ;Track number / Number of heads
mov dh, dl ;DH = head
xchg ch, al ;CH = (low) cylinder ;AL=0
ror ah, 1 ;rotate (high) cylinder
ror ah, 1
add cl, ah ;CL = combine: sector, (high) cylinder
sub ax, di
and ax, byte 0x7F ;AX = sectors to next 64Kb boundary
jz .3 ;On a 64Kb boundary already
cmp ax, bx ;More than we want?
jbe .4 ;No
.3: xchg ax, bx ;Yes: Transfer just what we want
.4: push ax ;Save length
mov bx, di ;Compute destination seg
push cx
mov cl, 5
shl bx, cl
pop cx
mov es, bx
xor bx, bx ;ES:BX = address
mov dl, [drive] ;DL = Drive number
mov ah, 2 ;AH = Read command
int 13h ;Do it
jc error
pop bx ;Length
pop ax ;(low) relative sector
pop dx ;(high) relative sector
add ax, bx ;Update relative sector
adc dl, dh
add di, bx ;Update destination
sub si, bx ;Update count
jnz .1 ;Read some more
ret
errmsg db 10,"Error Executing FAT12 bootsector",13
db 10,"Press any key to reboot",13,10,0
size equ $ - entry
%if size+11+2 > 512
%error "code is too large for boot sector"
%endif
times (512 - size - 11 - 2) db 0
filename db "LOADER BIN" ;11 byte name
db 0x55, 0xAA ;2 byte boot signature