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MS-DOS-kernel/boot/oemboot.asm at master · Programuserbash/MS-DOS-kernel · GitHub
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;
; File:
; oemboot.asm
; 2004, Kenneth J. Davis
; Copyright (c) 200?, <add name here>
; Description:
; OEM boot sector for FreeDOS compatible with IBM's (R) PC-DOS,
; and Microsoft's (R) MS-DOS. It may work with older OpenDOS/DR-DOS,
; although the standard FreeDOS boot sector is needed with ver 7+
; releases. May work with other versions of DOS that use
; IBMBIO.COM/IBMDOS.COM pair. This boot sector loads only up
; to 58 sectors (29KB) of the kernel (IBMBIO.COM) to 0x70:0 then
; jumps to it. As best I can tell, PC-DOS (and MS-DOS up to version
; 6.xx behaves similar) expects on entry for:
; ch = media id byte in the boot sector
; dl = BIOS drive booted from (0x00=A:, 0x80=C:, ...)
; ax:bx = the starting (LBA) sector of cluster 2 (ie the 1st
; data sector, which is 0x0000:0021 for FAT12)
; ?note? IBMBIO.COM/IO.SYS may use ax:bx and cluster # stored
; elsewhere (perhaps dir entry still at 0x50:0) to determine
; starting sector for full loading of kernel file.
; it also expects the boot sector (in particular the BPB)
; to still be at 0x0:7C00, the directory entry for IBMBIO.COM
; (generally first entry of first sector of the root directory)
; at 0x50:0 (DOS Data Area). The original boot sector may update
; the floppy disk parameter table (int 1Eh), but we don't so
; may fail for any systems where the changes (???) are needed.
; If the above conditions are not met, then IBMBIO.COM will
; print the not a bootable disk error message.
;
; For MS-DOS >= 7 (ie Win9x DOS) the following conditions
; must be met:
; bp = 0x7C00, ie offset boot sector loaded at
; [bp-4] = the starting (LBA) sector of cluster 2 (ie the 1st
; data sector [this is the same as ax:bx for earlier versions
; and dx:ax in Win9x boot sector]
; The starting cluster of the kernel file is stored in
; di for FAT 12/16 (where si is a don't care) and si:di
; for FAT 32.
; The values for ax,bx,cx,dx,ds and the stack do not
; seem to be important (used by IO.SYS) and so may be any value
; (though dx:ax=[data_start], cx=0, bx=0x0f00 on FAT12 or
; 0x0700 on FAT32, ds=0, ss:sp=0:7b??)
; the boot time stack may store the original int1E floppy
; parameter table, otherwise nothing else important seems
; stored there and I am unsure if even this value is used
; beyond boot sector code.
;
; This boot sector only supports FAT12/FAT16 as PC-DOS
; does not support FAT32 and newer FAT32 capable DOSes
; probably have different boot requirements; also do NOT
; use it to boot the FreeDOS kernel as it expects to be
; fully loaded by boot sector (> 29KB & usually to 0x60:0).
;
; WARNING: PC-DOS has additional requirements, in particular,
; it may expect that IBMBIO.COM and IBMDOS.COM be the 1st
; two entries in the root directory (even before the label)
; and that they occupy the 1st consecutive data sectors.
; Newer releases may support other positions, but still
; generally should occupy consecutive sectors. These conditions
; can usually be met by running sys on a freshly formatted
; and un-label'd disk.
;
;
; Derived From:
; boot.asm
; DOS-C boot
;
; Copyright (c) 1997, 2000-2004
; Svante Frey, Jim Hall, Jim Tabor, Bart Oldeman,
; Tom Ehlert, Eric Auer, Luchezar Georgiev, Jon Gentle
; and Michal H. Tyc (DR-DOS adaptation, boot26dr.asm)
; All Rights Reserved
;
; This file is part of FreeDOS.
;
; DOS-C 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; either version
; 2, or (at your option) any later version.
;
; DOS-C is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
; the GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public
; License along with DOS-C; see the file COPYING. If not,
; write to the Free Software Foundation, 675 Mass Ave,
; Cambridge, MA 02139, USA.
;
;
; +--------+
; | CLUSTER|
; | LIST |
; |--------| 0000:7F00
; |LBA PKT |
; |--------| 0000:7E00 (0:BP+200)
; |BOOT SEC| contains BPB
; |ORIGIN |
; |--------| 0000:7C00 (0:BP)
; |VARS | only known is 1st data sector (start of cluster 2)
; |--------| 0000:7BFC (DS:[BP-4])
; |STACK | minimal 256 bytes (1/2 sector)
; |- - - - |
; |KERNEL | kernel loaded here (max 58 sectors, 29KB)
; |LOADED | also used as FAT buffer
; |--------| 0070:0000 (0:0700)
; |DOS DA/ | DOS Data Area,
; |ROOT DIR| during boot contains directory entries
; |--------| 0000:0500
; |BDA | BIOS Data Area
; +--------+ 0000:0400
; |IVT | Interrupt Vector Table
; +--------+ 0000:0000
; NOTE: sys must be updated if magic offsets change
%assign
ISFAT1216DUAL
1
%include
"magic.mac"
CPU
8086
; enable assembler warnings to limit instruction set
;%define ISFAT12 1 ; only 1 of these should be set,
;%define ISFAT16 1 ; defines which FAT is supported
%define
TRYLBAREAD
1
; undefine to use only CHS int 13h
%define
SETROOTDIR
1
; if defined dir entry copied to 0:500
%define
LOOPONERR
1
; if defined on error simply loop forever
;%define RETRYALWAYS 1 ; if defined retries read forever
;%define WINBOOT 1 ; use win9x kernel calling conventions (name & jmp addr)
;%define MSCOMPAT 1 ; sets default filename to MSDOS IO.SYS
%ifdef
WINBOOT
; if set also change from PC-DOS to
%ifndef
MSCOMPAT
; kernel name to MS-DOS kernel name
%define
MSCOMPAT
%endif
%endif
segment .text
%define
BASE
0x7c00
; boot sector originally at 0x0:BASE
%define
LOADSEG
0x0070
; segment to load kernel at LOADSEG:0
%define
LOADEND
0x07b0
; limit reads to below this segment
; LOADSEG+29KB, else data overwritten
%define
FATBUF
bp
-
0x7500
; offset of temporary buffer for FAT
; chain 0:FATBUF = 0:0700 = LOADSEG:0
%define
ROOTDIR
0x7C00
-
0x7700
; offset to buffer for root directory
; entry of kernel 0:ROOTDIR
%define
CLUSTLIST
bp
+
0x0300
; zero terminated list of clusters
; that the kernel occupies
; Some extra variables
; using bp-Entry+variable_name generates smaller code than using just
; variable_name, where bp is initialized to Entry, so bp-Entry equals 0
%define
LBA_PACKET
bp
+
0x0200
; immediately after boot sector
%define
LBA_SIZE word
[
LBA_PACKET
]
; size of packet, should be 10h
%define
LBA_SECNUM word
[
LBA_PACKET
+
2
]
; number of sectors to read
%define
LBA_OFF LBA_PACKET
+
4
; buffer to read/write to
%define
LBA_SEG LBA_PACKET
+
6
%define
LBA_SECTOR_0 word
[
LBA_PACKET
+
8
]
; LBA starting sector #
%define
LBA_SECTOR_16 word
[
LBA_PACKET
+
10
]
%define
LBA_SECTOR_32 word
[
LBA_PACKET
+
12
]
%define
LBA_SECTOR_48 word
[
LBA_PACKET
+
14
]
%define
PARAMS LBA_PACKET
+
0x10
;%define RootDirSecs PARAMS+0x0 ; # of sectors root dir uses
%define
fat_start PARAMS
+
0x2
; first FAT sector
;%define root_dir_start PARAMS+0x6 ; first root directory sector
%define
first_cluster PARAMS
+
0x0a
; starting cluster of kernel file
%define
data_start
bp
-
4
; first data sector (win9x expects here)
;-----------------------------------------------------------------------
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
drive
bp
+
0x24
; drive number
%define
extBoot
bp
+
0x26
; extended boot signature
%define
volid
bp
+
0x27
%define
vollabel
bp
+
0x2b
%define
filesys
bp
+
0x36
;-----------------------------------------------------------------------
; times 0x3E-$+$$ db 0
;
; Instead of zero-fill,
; initialize BPB with values suitable for a 1440 K floppy
;
db
'IBM 5.0'
; OEM label
dw
512
; bytes per sector
db
1
; sectors per cluster
dw
1
; reserved sectors
db
2
; number of FATs
dw
224
; root directory entries
dw
80
*
36
; total sectors on disk
db
0xF0
; media descriptor
dw
9
; sectors per 1 FAT copy
dw
18
; sectors per track
dw
2
; number of heads
dd
0
; hidden sectors
dd
0
; big total sectors
db
0
; boot unit
db
0
; reserved
db
0x29
; extended boot record id
dd
0x12345678
; volume serial number
db
'NO NAME '
; volume label
times
36h
-
($
-
$$) db
0
; The filesystem ID is used by lDOS's instsect (by ecm)
; by default to validate that the filesystem matches.
%ifdef
ISFAT12
%define
FATFS
"FAT12"
%ifdef
ISFAT16
%error
Must select one
FS
%endif
%elifdef
ISFAT16
%define
FATFS
"FAT16"
%else
%define
FATFS
"unknown"
%error
Must select one
FS
%endif
db
FATFS
; filesystem id
times
3Eh
-
($
-
$$) db
32
;-----------------------------------------------------------------------
; ENTRY
;-----------------------------------------------------------------------
real_start:
cli
; disable interrupts until stack ready
cld
; all string operations increment
xor
ax
,
ax
; ensure our segment registers ready
mov
ds
,
ax
; cs=ds=es=ss=0x0000
mov
es
,
ax
mov
ss
,
ax
mov
bp
,
BASE
lea
sp
,
[
bp
-
4
]
; for DOS <7 this may be [bp]
; For compatibility, diskette parameter vector updated.
; lea di [bp+0x3E] ; use 7c3e([bp+3e]) for PC-DOS,
; ;lea di [bp] ; but 7c00([bp]) for DR-DOS 7 bug
; mov bx, 4 * 1eh ; stored at int 1E's vector
; lds si, [bx] ; fetch current int 1eh pointer
; push ds ; store original 1eh pointer at stack top
; push si ; so can restore later if needed
;
; Copy table to new location
; mov cl, 11 ; the parameter table is 11 bytes
; rep movsb ; and copy the parameter block
; mov ds, ax ; restore DS
;
; Note: make desired changes to table here
;
; Update int1E to new location
; mov [bx+2], 0 ; set to 0:bp or 0:bp+3e as appropriate
; mov word [bx], 0x7c3e ; (use 0x7c00 for DR-DOS)
sti
; enable interrupts
; If updated floppy parameter table then must notify BIOS
; Otherwise a reset should not be needed here.
; int 0x13 ; reset drive (AX=0)
;
; Note: some BIOS implementations may not correctly pass drive number
; in DL, however we work around this in SYS.COM by NOP'ing out the use of DL
; (formerly we checked for [drive]==0xff; update sys.c if code moves)
;
magicoffset
"set unit"
,
4Fh
,
4Fh
mov
[
drive
],
dl
; rely on BIOS drive number in DL
; GETDRIVEPARMS: Calculate start of some disk areas.
;
mov
si
,
word
[
nHidden
]
mov
di
,
word
[
nHidden
+
2
]
add
si
,
word
[
bsResSectors
]
adc
di
,
byte
0
; DI:SI = first FAT sector
mov
word
[
fat_start
],
si
mov
word
[
fat_start
+
2
],
di
mov
al
,
[
bsFATs
]
cbw
mul
word
[
sectPerF
at
]
; DX:AX = total number of FAT sectors
add
si
,
ax
adc
di
,
dx
; DI:SI = first root directory sector
push
di
; mov word [root_dir_start+2], di
push
si
; mov word [root_dir_start], si
; Calculate how many sectors the root directory occupies.
mov
bx
,
[
bsBytesPerSec
]
mov
cl
,
5
; divide BX by 32
shr
bx
,
cl
; BX = directory entries per sector
mov
ax
,
[
bsRootDirEnts
]
xor
dx
,
dx
div
bx
; set AX = sectors per root directory
push
ax
; mov word [RootDirSecs], ax
add
si
,
ax
adc
di
,
byte
0
; DI:SI = first data sector
mov
[
data_start
],
si
mov
[
data_start
+
2
],
di
; FINDFILE: Searches for the file in the root directory.
;
; Returns:
; AX = first cluster of file
; First, read the root directory into buffer.
; into the temporary buffer. (max 29KB or overruns stuff)
pop
di
; mov di, word [RootDirSecs]
pop
ax
; mov ax, word [root_dir_start]
pop
dx
; mov dx, word [root_dir_start+2]
mov
bx
,
ROOTDIR
; es:bx = 0:0500
push
es
; save pointer to ROOTDIR
call
readDisk
pop
es
; restore pointer to ROOTDIR
mov
si
,
ROOTDIR
; ds:si = 0:0500
; Search for kernel file name, and find start cluster.
next_entry:
mov
cx
,
11
mov
di
,
filename
push
si
repe
cmpsb
pop
si
mov
ax
,
[
si
+
0x1A
]
; get cluster number from directory entry
je
ffDone
add
si
,
byte
0x20
; go to next directory entry
jc
boot_error
; fail if not found and si wraps
cmp
byte
[
si
],
0
; if the first byte of the name is 0,
jnz
next_entry
; there are no more files in the directory
jmp
boot_error
ffDone:
mov
[
first_cluster
],
ax
; store first cluster number
%ifdef
SETROOTDIR
; copy over this portion of root dir to 0x0:500 for PC-DOS
; (this may allow IBMBIO.COM to start in any directory entry)
mov
di
,
ROOTDIR
; es:di = 0:0500
mov
cx
,
32
; limit to this 1 entry (rest don't matter)
rep
movsw
%endif
; GETFATCHAIN:
;
; Reads the FAT chain and stores it in a temporary buffer in the first
; 64 kb. The FAT chain is stored an array of 16-bit cluster numbers,
; ending with 0.
;
; The file must fit in conventional memory, so it can't be larger than
; 640 kb. The sector size must be at least 512 bytes, so the FAT chain
; can't be larger than 2.5 KB (655360 / 512 * 2 = 2560).
;
; Call with: AX = first cluster in chain
; Load the complete FAT into memory. The FAT can't be larger
; than 128 kb
lea
bx
,
[
FATBUF
]
; es:bx = 0:0700
mov
di
,
[
sectPerF
at
]
mov
ax
,
word
[
fat_start
]
mov
dx
,
word
[
fat_start
+
2
]
call
readDisk
; Set ES:DI to the temporary storage for the FAT chain.
push
ds
pop
es
lea
di
,
[
CLUSTLIST
]
; Set DS:0 to FAT data we loaded
mov
ax
,
LOADSEG
mov
ds
,
ax
; ds:0 = 0x70:0 = 0:FATBUF
mov
ax
,
[
first_cluster
]
; restore first cluster number
push
ds
; store LOADSEG
next_clust:
stosw
; store cluster number
mov
si
,
ax
; SI = cluster number
%ifdef
ISFAT12
; This is a FAT-12 disk.
fat_12:
add
si
,
si
; multiply cluster number by 3...
add
si
,
ax
shr
si
,
1
; ...and divide by 2
lodsw
; If the cluster number was even, the cluster value is now in
; bits 0-11 of AX. If the cluster number was odd, the cluster
; value is in bits 4-15, and must be shifted right 4 bits. If
; the number was odd, CF was set in the last shift instruction.
mov
cl
,
4
; always initialise shift counter
jc
fat_odd
; is odd, only shift down -->
shl
ax
,
cl
; shift up (effectively masks off
; the highest 4 bits)
fat_odd:
shr
ax
,
cl
cmp
ax
,
0x0ff8
; check for EOF
jb
next_clust
; continue if not EOF
%endif
%ifdef
ISFAT16
; This is a FAT-16 disk. The maximal size of a 16-bit FAT
; is 128 kb, so it may not fit within a single 64 kb segment.
fat_16:
mov
dx
,
LOADSEG
add
si
,
si
; multiply cluster number by two
jnc
first_half
; if overflow...
add
dh
,
0x10
; ...add 64 kb to segment value
first_half:
mov
ds
,
dx
; DS:SI = pointer to next cluster
lodsw
; AX = next cluster
cmp
ax
,
0xfff8
; >= FFF8 = 16-bit EOF
jb
next_clust
; continue if not EOF
%endif
finished:
; Mark end of FAT chain with 0, so we have a single
; EOF marker for both FAT-12 and FAT-16 systems.
xor
ax
,
ax
stosw
push
cs
pop
ds
; loadFile: Loads the file into memory, one cluster at a time.
pop
es
; set ES:BX to load address 70:0
xor
bx
,
bx
lea
si
,
[
CLUSTLIST
]
; set DS:SI to the FAT chain
cluster_next:
lodsw
; AX = next cluster to read
or
ax
,
ax
; EOF?
jne
load_next
; no, continue
; dl set to drive by readDisk
mov
ch
,
[
bsMedia
]
; ch set to media id
mov
ax
,
[
data_start
+
2
]
; ax:bx set to 1st data sector
mov
bx
,
[
data_start
]
;
mov
di
,
[
first_cluster
]
; set di (si:di on FAT32) to starting cluster #
%ifdef
WINBOOT
jmp
LOADSEG:
0x0200
; yes, pass control to kernel
%else
jmp
LOADSEG:
0000
; yes, pass control to kernel
%endif
magicoffset
"load jump ofs"
,
11Ah
,
118h
,
-
4
; failed to boot
boot_error:
call
show
; db "Error! Hit a key to reboot.",0
db
"):"
,
0
%ifdef
LOOPONERR
jmp
$
%else
; Note: should restore floppy paramater table address at int 0x1E
xor
ah
,
ah
int
0x13
; reset floppy
int
0x16
; wait for a key
int
0x19
; reboot the machine
%endif
load_next:
dec
ax
; cluster numbers start with 2
dec
ax
mov
di
,
word
[
bsSecPerClust
]
dec
di
; minus one if 256 spc
and
di
,
0xff
; DI = sectors per cluster - 1
inc
di
; = spc
mul
di
add
ax
,
[
data_start
]
adc
dx
,
[
data_start
+
2
]
; DX:AX = first sector to read
call
readDisk
jmp
short cluster_next
; shows text after the call to this function.
show.do_show:
mov
ah
,
0Eh
; show character
int
10h
; via "TTY" mode
show:
pop
si
lodsb
; get character
push
si
; stack up potential return address
cmp
al
,
0
; end of string?
jne
.do_show
; until done
ret
; readDisk: Reads a number of sectors into memory.
;
; Call with: DX:AX = 32-bit DOS sector number
; DI = number of sectors to read
; ES:BX = destination buffer
;
; Returns: CF set on error
; ES:BX points one byte after the last byte read.
; Exits early if LBA_SEG == LOADEND.
readDisk:
push
si
; preserve cluster #
mov
LBA_SECTOR_0
,
ax
mov
LBA_SECTOR_16
,
dx
mov
word
[
LBA_SEG
],
es
mov
word
[
LBA_OFF
],
bx
call
show
db
"."
,
0
read_next:
; initialize constants
mov
LBA_SIZE
,
10h
; LBA packet is 16 bytes
mov
LBA_SECNUM
,
1
; reset LBA count if error
; limit kernel loading to 29KB, preventing stack & boot sector being overwritten
cmp
word
[
LBA_SEG
],
LOADEND
; skip reading if past the end
je
read_skip
; of kernel file buffer
;******************** LBA_READ *******************************
; check for LBA support
%ifdef
TRYLBAREAD
mov
ah
,
041h
;
mov
bx
,
055aah
;
mov
dl
,
[
drive
]
; BIOS drive, 0=A:, 80=C:
test
dl
,
dl
; don't use LBA addressing on A:
jz
read_normal_BIOS
; might be a (buggy)
; CDROM-BOOT floppy emulation
int
0x13
jc
read_normal_BIOS
shr
cx
,
1
; CX must have 1 bit set
sbb
bx
,
0aa55h
-
1
; tests for carry (from shr) too!
jne
read_normal_BIOS
; OK, drive seems to support LBA addressing
lea
si
,[
LBA_PACKET
]
; setup LBA disk block
mov
LBA_SECTOR_32
,
bx
; bx is 0 if extended 13h mode supported
mov
LBA_SECTOR_48
,
bx
mov
ah
,
042h
jmp
short do_int13_read
%endif
read_normal_BIOS:
;******************** END OF LBA_READ ************************
mov
cx
,
LBA_SECTOR_0
mov
dx
,
LBA_SECTOR_16
;
; translate sector number to BIOS parameters
;
;
; abs = sector offset in track
; + head * sectPerTrack offset in cylinder
; + track * sectPerTrack * nHeads offset in platter
;
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)
or
cl
,
ah
; merge sector into cylinder
inc
cx
; make sector 1-based (1-63)
les
bx
,[
LBA_OFF
]
mov
ax
,
0x0201
do_int13_read:
mov
dl
,
[
drive
]
int
0x13
read_finished:
%ifdef
RETRYALWAYS
jnc
read_ok
; jump if no error
xor
ah
,
ah
; else, reset floppy
int
0x13
read_next_chained:
jmp
short read_next
; read the same sector again
%else
jc
boot_error
; exit on error
%endif
read_ok:
mov
ax
,
word
[
bsBytesPerSec
]
mov
cl
,
4
; adjust segment pointer by increasing
shr
ax
,
cl
add
word
[
LBA_SEG
],
ax
; by paragraphs read in (per sector)
add
LBA_SECTOR_0
,
byte
1
adc
LBA_SECTOR_16
,
byte
0
; DX:AX = next sector to read
dec
di
; if there is anything left to read,
%ifdef
RETRYALWAYS
jnz
read_next_chained
; continue
%else
jnz
read_next
; continue
%endif
read_skip:
mov
es
,
word
[
LBA_SEG
]
; load adjusted segment value
; clear carry: unnecessary since adc clears it
pop
si
ret
times
0x01f1
-
$
+
$$ db
0
magicoffset
"kernel name"
,
1F1h
,
1F1h
%ifdef
MSCOMPAT
filename db
"IO SYS"
%else
filename db
"IBMBIO COM"
%endif
db
0
,
0
sign dw
0xAA55
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