PCjs Machines
Home of the original IBM PC emulator for browsers.
IBM PC DOS 1.00 with PCjs Debugger
IBM PC, 64Kb RAM, Dual Floppy Drives, MDA, Debugger
Monochrome Display
Caps
Num
Scroll
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10
Esc
1 !
2 @
3 #
4 $
5 %
6 ^
7 &
8 *
9 (
0 )
- _
= +
Back
-
Num
Scroll
Tab
Q
W
E
R
T
Y
U
I
O
P
[ {
] }
↵
7
↑ 8
9
Ctrl
A
S
D
F
G
H
J
K
L
; :
' "
` ~
← 4
5
→ 6
+
Shift
\ |
Z
X
C
V
B
N
M
, <
. >
/ ?
Shift
PrtSc
1
↓ 2
3
Alt
Space
Caps
Ins 0
Del .
Esc
Tab
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10
Num
Scroll
1 !
2 @
3 #
4 $
5 %
6 ^
7 &
8 *
9 (
0 )
- _
= +
Back
-
+
Q
W
E
R
T
Y
U
I
O
P
[ {
] }
↵
7
↑ 8
9
A
S
D
F
G
H
J
K
L
; :
' "
` ~
← 4
5
→ 6
\ |
Z
X
C
V
B
N
M
, <
. >
/ ?
Shift
PrtSc
1
↓ 2
3
Ctrl
Shift
Alt
Space
Caps
Ins 0
Del .
PCjs Debugger
Speed:
Stopped
AX
0000
BX
0000
CX
0000
DX
0000
SP
0000
BP
0000
SI
0000
DI
0000
DS
0000
ES
0000
SS
0000
CS
0000
IP
0000
V
0
D
0
I
0
T
0
S
0
Z
0
A
0
P
0
C
0
SW1
1
2
3
4
5
6
7
8
SW2
1
2
3
4
5
6
7
8
64K, No FPU, Color Monitor, 2 Floppy Drives
PCx86 v2.23 © 2012-2024 Jeff Parsons
PC DOS 1.00 Boot Sector
The boot sector of the PC DOS 1.00 disk image contains the following bytes:
00000000 eb 2f 14 00 00 00 60 00 20 37 2d 4d 61 79 2d 38 |./....`. 7-May-8|
00000010 31 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |1...............|
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000030 00 fa 8c c8 8e d8 ba 00 00 8e d2 bc 00 7c fb a1 |.............|..|
00000040 06 7c 8e d8 8e c0 ba 00 00 8b c2 cd 13 72 41 e8 |.|...........rA.|
00000050 58 00 72 fb 2e 8b 0e 02 7c 51 bb 00 00 33 d2 b9 |X.r.....|Q...3..|
00000060 08 00 be 01 00 56 b0 01 b4 02 cd 13 72 22 5e 58 |.....V......r"^X|
00000070 e8 e7 00 2b c6 74 14 fe c5 b1 01 be 08 00 3b c6 |...+.t........;.|
00000080 73 04 8b f0 eb 01 96 56 50 eb dd 2e ff 2e 04 7c |s......VP......||
00000090 be 44 7d b8 42 7d 50 32 ff ac 24 7f 74 0b 56 b4 |.D}.B}P2..$.t.V.|
000000a0 0e bb 07 00 cd 10 5e eb f0 c3 bb 00 00 b9 04 00 |......^.........|
000000b0 b8 01 02 cd 13 1e 72 34 8c c8 8e d8 bf 00 00 b9 |......r4........|
000000c0 0b 00 26 80 0d 20 26 80 8d 20 00 20 47 e2 f3 bf |..&.. &.. . G...|
000000d0 00 00 be 76 7d b9 0b 00 fc f3 a6 75 0f bf 20 00 |...v}......u.. .|
000000e0 be 82 7d b9 0b 00 f3 a6 75 02 1f c3 be f9 7c e8 |..}.....u.....|.|
000000f0 a5 ff b4 00 cd 16 1f f9 c3 0d 0a 4e 6f 6e 2d 53 |...........Non-S|
00000100 79 73 74 65 6d 20 64 69 73 6b 20 6f 72 20 64 69 |ystem disk or di|
00000110 73 6b 20 65 72 72 6f f2 0d 0a 52 65 70 6c 61 63 |sk erro...Replac|
00000120 65 20 61 6e 64 20 73 74 72 69 6b 65 20 61 6e 79 |e and strike any|
00000130 20 6b 65 79 20 77 68 65 6e 20 72 65 61 64 f9 0d | key when read..|
00000140 0a 00 cd 18 0d 0a 44 69 73 6b 20 42 6f 6f 74 20 |......Disk Boot |
00000150 66 61 69 6c 75 72 e5 0d 0a 00 50 52 8b c6 bf 00 |failur....PR....|
00000160 02 f7 e7 03 d8 5a 58 c3 52 6f 62 65 72 74 20 4f |.....ZX.Robert O|
00000170 27 52 65 61 72 20 69 62 6d 62 69 6f 20 20 63 6f |'Rear ibmbio co|
00000180 6d b0 69 62 6d 64 6f 73 20 20 63 6f 6d b0 c9 00 |m.ibmdos com...|
00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
A number of people have already delved into the guts of the PC DOS 1.00 boot sector, including:
- Reverse-Engineering DOS 1.0 – Part 1: The Boot Sector (May 7, 2009 by Michael Steil)
- The Boot Sector of IBM® Personal Computer™ DOS Version 1.00 (August 2, 2005 by Daniel B. Sedory)
Let’s start with Michael Steil’s well-written reverse-engineered source code (designed for NASM, not ASM or MASM), because it includes lots of commentary, allowing the boot sector to more or less speak for itself:
;-----------------------------------------------------------------------------
; DOS 1.0 Boot Sector (disk image MD5 73c919cecadf002a7124b7e8bfe3b5ba)
; http://www.pagetable.com/
;-----------------------------------------------------------------------------
org 0x7C00
jmp short start
;-----------------------------------------------------------------------------
os_numsectors dw 20 ; how many sectors to read
os_offset dw 0 ; segment to load code into
os_segment dw 0x60 ; offset to load code into
db " 7-May-81",0 ; timestamp
times 31 db 0 ; padding
;-----------------------------------------------------------------------------
start cli
mov ax, cs
mov ds, ax ; DS := CS
mov dx, 0
mov ss, dx ; SS := 0000
mov sp, 0x7C00 ; stack below code
sti
mov ax, [os_segment]
mov ds, ax
mov es, ax ; ES := DS := where to load DOS
mov dx, 0
mov ax, dx
int 0x13 ; reset drive 0
jc disk_error
again call check_sys_files ; check for presence of IBMDOS/IBMBIO
jc again ; not found, try another disk
mov cx, [cs:os_numsectors]
push cx ; remaining sectors
mov bx, 0
xor dx, dx ; drive 0, head 0
mov cx, 8 ; track 0, sector 8
mov si, 1 ; read 1 sector in first found
push si
mov al, 1 ; 1 sector
read_loop mov ah, 2
int 0x13 ; read sector(s)
jc disk_error
pop si ; sectors read
pop ax ; remaining sectors
call add_si_sectors ; bx += si*512
sub ax, si ; remaining -= read
jz done ; none left
inc ch ; next track
mov cl, 1 ; start at sector 1
mov si, 8 ; read up to 8 sectors
cmp ax, si ; how many are left to read?
jae at_least_8_left ; at least 8
mov si, ax ; only read remaining amount
jmp short skip
at_least_8_left xchg ax, si ; read 8 sectors this time
skip push si ; number of remaining sectors
push ax ; number of sectors to read this time
jmp read_loop ; next read
done jmp far [cs:os_offset]; jump to IBMBIO.COM
disk_error mov si, FAILURE ; string to print
mov ax, rom_basic ; put return address of "int 18" code
push ax ; onto stack
;-----------------------------------------------------------------------------
; print zero-terminated string pointed to by DS:SI
;-----------------------------------------------------------------------------
print xor bh, bh ; XXX unnecessary
print_loop lodsb
and al, 0x7F ; clear bit 7 XXX why is it set?
jz ret0 ; zero-termination
push si
mov ah, 0x0E
mov bx, 7 ; light grey, text page 0
int 0x10 ; write character
pop si
jmp print_loop
ret0 retn
;-----------------------------------------------------------------------------
; test for IBMBIO.COM and IBMDOS.COM in the first two directory entries
;-----------------------------------------------------------------------------
check_sys_files mov bx, 0 ; read to address 0 in the DOS segment
mov cx, 4 ; track 0, sector 4
mov ax, 0x0201
int 0x13 ; read 1 sector
push ds
jc non_system_disk ; error case
mov ax, cs
mov ds, ax ; DS := CS
mov di, 0
mov cx, 11 ; convert 11 bytes of first two
to_lower or byte [es:di], 0x20; directory entries to lowercase
or byte [es:di+0x20], 0x20
nop ; XXX original assembler wasted a byte
inc di
loop to_lower
mov di, 0 ; first entry
mov si, IBMBIO_COM
mov cx, 11
cld
rep cmpsb ; compare first entry with IBMBIO.COM
jnz non_system_disk
mov di, 0x20 ; second entry
mov si, IBMDOS_COM
mov cx, 11
rep cmpsb ; compare second entry with IBMDOS.COM
jnz non_system_disk
pop ds
retn ; return with carry clear
non_system_disk mov si, NON_SYSTEM_DISK
call print
mov ah, 0
int 0x16 ; wait for key
pop ds
stc
retn ; return with carry set
;-----------------------------------------------------------------------------
NON_SYSTEM_DISK db 13,10
db "Non-System disk or disk erro",'r'+0x80
db 13,10
db "Replace and strike any key when read",'y'+0x80
db 13,10,0
;-----------------------------------------------------------------------------
rom_basic int 0x18 ; ROM BASIC
;-----------------------------------------------------------------------------
FAILURE db 13,10
db "Disk Boot failur",'e'+0x80
db 13,10,0
;-----------------------------------------------------------------------------
add_si_sectors push ax ; bx += si*512
push dx
mov ax, si
mov di, 512
mul di
add bx, ax
pop dx
pop ax
retn
;-----------------------------------------------------------------------------
db "Robert O'Rear "
IBMBIO_COM db "ibmbio com"
db 0xB0 ; XXX unused
IBMDOS_COM db "ibmdos com"
db 0xB0, 0xC9 ; XXX unused
;-----------------------------------------------------------------------------
times 512-($-$$) db 0
;-----------------------------------------------------------------------------
Using the PCjs Debugger, we can examine the boot sector in its native environment:
bp &0000:7C00 hit
stopped
AX=0000 BX=7C00 CX=0004 DX=0000 SP=0100 BP=E4B7 SI=0000 DI=0044
SS=0030 DS=0040 ES=0000 PS=F296 V0 D0 I1 T0 S1 Z0 A1 P1 C0
&0000:7C00 EB2F JMP 7C31
>> db 7c00 l200
&0000:7C00 EB 2F 14 00 00 00 60 00-20 37 2D 00 02 01 01 00 ./....`. 7-.....
&0000:7C10 02 40 00 40 01 FE 01 00-08 00 01 00 00 00 00 00 .@.@............
&0000:7C20 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7C30 00 FA 8C C8 8E D8 BA 00-00 8E D2 BC 00 7C FB A1 .............|..
&0000:7C40 06 7C 8E D8 8E C0 BA 00-00 8B C2 CD 13 72 41 E8 .|...........rA.
&0000:7C50 58 00 72 FB 2E 8B 0E 02-7C 51 BB 00 00 33 D2 B9 X.r.....|Q...3..
&0000:7C60 08 00 BE 01 00 56 B0 01-B4 02 CD 13 72 22 5E 58 .....V......r"^X
&0000:7C70 E8 E7 00 2B C6 74 14 FE-C5 B1 01 BE 08 00 3B C6 ...+.t........;.
&0000:7C80 73 04 8B F0 EB 01 96 56-50 EB DD 2E FF 2E 04 7C s......VP......|
&0000:7C90 BE 44 7D B8 42 7D 50 32-FF AC 24 7F 74 0B 56 B4 .D}.B}P2..$t.V.
&0000:7CA0 0E BB 07 00 CD 10 5E EB-F0 C3 BB 00 00 B9 04 00 ......^.........
&0000:7CB0 B8 01 02 CD 13 1E 72 34-8C C8 8E D8 BF 00 00 B9 ......r4........
&0000:7CC0 0B 00 26 80 0D 20 26 80-8D 20 00 20 47 E2 F3 BF ..&.. &.. . G...
&0000:7CD0 00 00 BE 76 7D B9 0B 00-FC F3 A6 75 0F BF 20 00 ...v}......u.. .
&0000:7CE0 BE 82 7D B9 0B 00 F3 A6-75 02 1F C3 BE F9 7C E8 ..}.....u.....|.
&0000:7CF0 A5 FF B4 00 CD 16 1F F9-C3 0D 0A 4E 6F 6E 2D 53 ...........Non-S
&0000:7D00 79 73 74 65 6D 20 64 69-73 6B 20 6F 72 20 64 69 ystem disk or di
&0000:7D10 73 6B 20 65 72 72 6F F2-0D 0A 52 65 70 6C 61 63 sk erro...Replac
&0000:7D20 65 20 61 6E 64 20 73 74-72 69 6B 65 20 61 6E 79 e and strike any
&0000:7D30 20 6B 65 79 20 77 68 65-6E 20 72 65 61 64 F9 0D key when read..
&0000:7D40 0A 00 CD 18 0D 0A 44 69-73 6B 20 42 6F 6F 74 20 ......Disk Boot
&0000:7D50 66 61 69 6C 75 72 E5 0D-0A 00 50 52 8B C6 BF 00 failur....PR....
&0000:7D60 02 F7 E7 03 D8 5A 58 C3-52 6F 62 65 72 74 20 4F .....ZX.Robert O
&0000:7D70 27 52 65 61 72 20 69 62-6D 62 69 6F 20 20 63 6F 'Rear ibmbio co
&0000:7D80 6D B0 69 62 6D 64 6F 73-20 20 63 6F 6D B0 C9 00 m.ibmdos com...
&0000:7D90 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7DA0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7DB0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7DC0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7DD0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7DE0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&0000:7DF0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
NOTE: PCjs adds a BPB to the boot sector, which is why the first 32 bytes of the boot sector appear slightly different from the original dump above. The addition of a BPB makes the disk image mountable by modern operating systems and does not otherwise affect the operation of the boot sector or any of the code below.
First, let’s disassemble the code from start (0x7C31):
>> u 7c31
&0000:7C31 FA CLI
&0000:7C32 8CC8 MOV AX,CS
&0000:7C34 8ED8 MOV DS,AX
&0000:7C36 BA0000 MOV DX,0000
&0000:7C39 8ED2 MOV SS,DX
&0000:7C3B BC007C MOV SP,7C00
&0000:7C3E FB STI
&0000:7C3F A1067C MOV AX,[7C06]
&0000:7C42 8ED8 MOV DS,AX
&0000:7C44 8EC0 MOV ES,AX
&0000:7C46 BA0000 MOV DX,0000
&0000:7C49 8BC2 MOV AX,DX
&0000:7C4B CD13 INT 13
&0000:7C4D 7241 JC 7C90
&0000:7C4F E85800 CALL 7CAA
&0000:7C52 72FB JC 7C4F
&0000:7C54 2E CS:
&0000:7C55 8B0E027C MOV CX,[7C02]
&0000:7C59 51 PUSH CX
&0000:7C5A BB0000 MOV BX,0000
&0000:7C5D 33D2 XOR DX,DX
&0000:7C5F B90800 MOV CX,0008
&0000:7C62 BE0100 MOV SI,0001
&0000:7C65 56 PUSH SI
&0000:7C66 B001 MOV AL,01
&0000:7C68 B402 MOV AH,02
&0000:7C6A CD13 INT 13
&0000:7C6C 7222 JC 7C90
&0000:7C6E 5E POP SI
&0000:7C6F 58 POP AX
&0000:7C70 E8E700 CALL 7D5A
&0000:7C73 2BC6 SUB AX,SI
&0000:7C75 7414 JZ 7C8B
&0000:7C77 FEC5 INC CH
&0000:7C79 B101 MOV CL,01
&0000:7C7B BE0800 MOV SI,0008
&0000:7C7E 3BC6 CMP AX,SI
&0000:7C80 7304 JNC 7C86
&0000:7C82 8BF0 MOV SI,AX
&0000:7C84 EB01 JMP 7C87
&0000:7C86 96 XCHG AX,SI
&0000:7C87 56 PUSH SI
&0000:7C88 50 PUSH AX
&0000:7C89 EBDD JMP 7C68
&0000:7C8B 2E CS:
&0000:7C8C FF2E047C JMP FAR [7C04]
Next, the code for disk_error (0x7C90):
>> u 7c90
&0000:7C90 BE447D MOV SI,7D44
&0000:7C93 B8427D MOV AX,7D42
&0000:7C96 50 PUSH AX
&0000:7C97 32FF XOR BH,BH
&0000:7C99 AC LODSB
&0000:7C9A 247F AND AL,7F
&0000:7C9C 740B JZ 7CA9
&0000:7C9E 56 PUSH SI
&0000:7C9F B40E MOV AH,0E
&0000:7CA1 BB0700 MOV BX,0007
&0000:7CA4 CD10 INT 10
&0000:7CA6 5E POP SI
&0000:7CA7 EBF0 JMP 7C99
&0000:7CA9 C3 RET
Next, the code for check_sys_files (0x7CAA):
>> u 7caa
&0000:7CAA BB0000 MOV BX,0000
&0000:7CAD B90400 MOV CX,0004
&0000:7CB0 B80102 MOV AX,0201
&0000:7CB3 CD13 INT 13
&0000:7CB5 1E PUSH DS
&0000:7CB6 7234 JC 7CEC
&0000:7CB8 8CC8 MOV AX,CS
&0000:7CBA 8ED8 MOV DS,AX
&0000:7CBC BF0000 MOV DI,0000
&0000:7CBF B90B00 MOV CX,000B
&0000:7CC2 26 ES:
&0000:7CC3 800D20 OR [DI],20
&0000:7CC6 26 ES:
&0000:7CC7 808D200020 OR [DI+0020],20
&0000:7CCC 47 INC DI
&0000:7CCD E2F3 LOOP 7CC2
&0000:7CCF BF0000 MOV DI,0000
&0000:7CD2 BE767D MOV SI,7D76
&0000:7CD5 B90B00 MOV CX,000B
&0000:7CD8 FC CLD
&0000:7CD9 F3 REPZ
&0000:7CDA A6 CMPSB
&0000:7CDB 750F JNZ 7CEC
&0000:7CDD BF2000 MOV DI,0020
&0000:7CE0 BE827D MOV SI,7D82
&0000:7CE3 B90B00 MOV CX,000B
&0000:7CE6 F3 REPZ
&0000:7CE7 A6 CMPSB
&0000:7CE8 7502 JNZ 7CEC
&0000:7CEA 1F POP DS
&0000:7CEB C3 RET
&0000:7CEC BEF97C MOV SI,7CF9
&0000:7CEF E8A5FF CALL 7C97
&0000:7CF2 B400 MOV AH,00
&0000:7CF4 CD16 INT 16
&0000:7CF6 1F POP DS
&0000:7CF7 F9 STC
&0000:7CF8 C3 RET
Then NON_SYSTEM_DISK (0x7CF9):
>> db 7cf9 l49
&7CF9 0D 0A 4E 6F 6E 2D 53 79-73 74 65 6D 20 64 69 73 ..Non-System dis
&7D09 6B 20 6F 72 20 64 69 73-6B 20 65 72 72 6F F2 0D k or disk erro..
&7D19 0A 52 65 70 6C 61 63 65-20 61 6E 64 20 73 74 72 .Replace and str
&7D29 69 6B 65 20 61 6E 79 20-6B 65 79 20 77 68 65 6E ike any key when
&7D39 20 72 65 61 64 F9 0D 0A-00
And FAILURE (0x7D44):
>> db 7d44 l16
&7D44 0D 0A 44 69 73 6B 20 42-6F 6F 74 20 66 61 69 6C ..Disk Boot fail
&7D54 75 72 E5 0D 0A 00
And finally, IBMBIO_COM (0x7D76) and IBMDOS_COM (0x7D82):
>> db 7d68
&7D68 52 6F 62 65 72 74 20 4F-27 52 65 61 72 20 69 62 Robert O'Rear ib
&7D78 6D 62 69 6F 20 20 63 6F-6D B0 69 62 6D 64 6F 73 mbio com.ibmdos
&7D88 20 20 63 6F 6D B0 C9 00-00 00 00 00 00 00 00 00 com...........
Interestingly, the COMPAQ MS-DOS 1.10 Boot Sector is completely different from all PC DOS 1.x boot sectors. The COMPAQ boot sector supports both 160Kb and 320Kb formats with less code and without requiring “patches”, although that flexibility did require reading the first FAT sector, which PC DOS 1.x boot sectors did not do.