PCjs Machines
Home of the original IBM PC emulator for browsers.
IBM PC DOS 1.10 with PCjs Debugger
[PCjs Machine "ibm5150-256k"]
Waiting for machine "ibm5150-256k" to load....
PC DOS 1.10 Boot Sector
The boot sector of the PC DOS 1.10 disk image contains the following bytes:
00000000 eb 27 90 08 00 14 00 00 00 00 00 00 00 00 00 00 |.'..............|
00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000020 00 00 00 00 00 00 00 cd 19 fa 8c c8 8e d8 33 d2 |..............3.|
00000030 8e d2 bc 00 7c fb b8 60 00 8e d8 8e c0 33 d2 8b |....|..`.....3..|
00000040 c2 cd 13 72 69 e8 85 00 72 dd 2e 83 3e 03 7c 08 |...ri...r...>.|.|
00000050 74 06 2e c6 06 64 7d 02 bb 00 00 2e 8b 0e 03 7c |t....d}........||
00000060 51 b0 09 2a c1 b4 00 8b f0 56 33 d2 33 c0 8a c5 |Q..*.....V3.3...|
00000070 2e f6 36 64 7d 8a e8 8a f4 8b c6 b4 02 cd 13 72 |..6d}..........r|
00000080 2d 5e 59 2e 29 36 05 7c 74 1f 8b c6 2e f7 26 65 |-^Y.)6.|t.....&e|
00000090 7d 03 d8 fe c5 b1 01 51 be 08 00 2e 3b 36 05 7c |}......Q....;6.||
000000a0 7c 05 2e 8b 36 05 7c eb c0 ea 00 00 60 00 be 67 ||...6.|.....`..g|
000000b0 7d e8 02 00 eb fe 32 ff 2e ac 24 7f 74 0b 56 b4 |}.....2...$.t.V.|
000000c0 0e bb 07 00 cd 10 5e eb ef c3 e9 33 ff bb 00 00 |......^....3....|
000000d0 b9 04 00 b8 01 02 cd 13 1e 72 33 8c c8 8e d8 bf |.........r3.....|
000000e0 00 00 b9 0b 00 26 80 0d 20 26 80 4d 20 20 47 e2 |.....&.. &.M G.|
000000f0 f4 bf 00 00 be 8b 7d b9 0b 00 fc f3 a6 75 0f bf |......}......u..|
00000100 20 00 be 97 7d b9 0b 00 f3 a6 75 02 1f c3 be 1b | ...}.....u.....|
00000110 7d e8 a2 ff b4 00 cd 16 1f f9 c3 0d 0a 4e 6f 6e |}............Non|
00000120 2d 53 79 73 74 65 6d 20 64 69 73 6b 20 6f 72 20 |-System disk or |
00000130 64 69 73 6b 20 65 72 72 6f 72 0d 0a 52 65 70 6c |disk error..Repl|
00000140 61 63 65 20 61 6e 64 20 73 74 72 69 6b 65 20 61 |ace and strike a|
00000150 6e 79 20 6b 65 79 20 77 68 65 6e 20 72 65 61 64 |ny key when read|
00000160 79 0d 0a 00 01 00 02 0d 0a 44 69 73 6b 20 42 6f |y........Disk Bo|
00000170 6f 74 20 66 61 69 6c 75 72 65 0d 0a 00 4d 69 63 |ot failure...Mic|
00000180 72 6f 73 6f 66 74 2c 49 6e 63 20 69 62 6d 62 69 |rosoft,Inc ibmbi|
00000190 6f 20 20 63 6f 6d 30 69 62 6d 64 6f 73 20 20 63 |o com0ibmdos c|
000001a0 6f 6d 30 05 c6 06 77 2f ff 83 7e fc 00 75 0b 80 |om0...w/..~..u..|
000001b0 7e f7 3b 75 05 c6 06 76 2f ff 89 ec 5d ca 04 00 |~.;u...v/...]...|
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 |................|
This is also the boot sector as it appears on any other single-sided 160Kb diskette formatted by PC DOS 1.10. However, PC DOS 1.10 added supported for a second format: double-sided 320Kb diskettes. And on those larger diskettes, the boot sector is slightly different:
1c1
< 00000000 eb 27 90 08 00 14 00 00 00 00 00 00 00 00 00 00 |.'..............|
---
> 00000000 eb 27 90 03 01 14 00 00 00 00 00 00 00 00 00 00 |.'..............|
The bytes at offset 0x0003 (0x7C03 when the boot sector is loaded into memory) contain the initial sector and head numbers of IBMBIO.COM. On a 160Kb diskette, the first data sector is at sector #8 and head #0, but since a 320Kb diskette is allocated a larger root directory (7 sectors instead of 4), that pushes the first data sector out to sector #3 and head #1. The total number of sectors to read (0x14) remains the same as PC DOS 1.00, but that value is now stored at offset 0x0005 (0x7C05) instead of 0x0002 (0x7C02).
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.
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 EB27 JMP 7C29
>> db 7c00 l200
&0000:7C00 EB 27 90 08 00 14 00 00-00 00 00 00 02 01 01 00 .'..............
&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 CD-19 FA 8C C8 8E D8 33 D2 ..............3.
&0000:7C30 8E D2 BC 00 7C FB B8 60-00 8E D8 8E C0 33 D2 8B ....|..`.....3..
&0000:7C40 C2 CD 13 72 69 E8 85 00-72 DD 2E 83 3E 03 7C 08 ...ri...r...>.|.
&0000:7C50 74 06 2E C6 06 64 7D 02-BB 00 00 2E 8B 0E 03 7C t....d}........|
&0000:7C60 51 B0 09 2A C1 B4 00 8B-F0 56 33 D2 33 C0 8A C5 Q..*.....V3.3...
&0000:7C70 2E F6 36 64 7D 8A E8 8A-F4 8B C6 B4 02 CD 13 72 ..6d}..........r
&0000:7C80 2D 5E 59 2E 29 36 05 7C-74 1F 8B C6 2E F7 26 65 -^Y.)6.|t.....&e
&0000:7C90 7D 03 D8 FE C5 B1 01 51-BE 08 00 2E 3B 36 05 7C }......Q....;6.|
&0000:7CA0 7C 05 2E 8B 36 05 7C EB-C0 EA 00 00 60 00 BE 67 |...6.|.....`..g
&0000:7CB0 7D E8 02 00 EB FE 32 FF-2E AC 24 7F 74 0B 56 B4 }.....2...$t.V.
&0000:7CC0 0E BB 07 00 CD 10 5E EB-EF C3 E9 33 FF BB 00 00 ......^....3....
&0000:7CD0 B9 04 00 B8 01 02 CD 13-1E 72 33 8C C8 8E D8 BF .........r3.....
&0000:7CE0 00 00 B9 0B 00 26 80 0D-20 26 80 4D 20 20 47 E2 .....&.. &.M G.
&0000:7CF0 F4 BF 00 00 BE 8B 7D B9-0B 00 FC F3 A6 75 0F BF ......}......u..
&0000:7D00 20 00 BE 97 7D B9 0B 00-F3 A6 75 02 1F C3 BE 1B ...}.....u.....
&0000:7D10 7D E8 A2 FF B4 00 CD 16-1F F9 C3 0D 0A 4E 6F 6E }............Non
&0000:7D20 2D 53 79 73 74 65 6D 20-64 69 73 6B 20 6F 72 20 -System disk or
&0000:7D30 64 69 73 6B 20 65 72 72-6F 72 0D 0A 52 65 70 6C disk error..Repl
&0000:7D40 61 63 65 20 61 6E 64 20-73 74 72 69 6B 65 20 61 ace and strike a
&0000:7D50 6E 79 20 6B 65 79 20 77-68 65 6E 20 72 65 61 64 ny key when read
&0000:7D60 79 0D 0A 00 01 00 02 0D-0A 44 69 73 6B 20 42 6F y........Disk Bo
&0000:7D70 6F 74 20 66 61 69 6C 75-72 65 0D 0A 00 4D 69 63 ot failure...Mic
&0000:7D80 72 6F 73 6F 66 74 2C 49-6E 63 20 69 62 6D 62 69 rosoft,Inc ibmbi
&0000:7D90 6F 20 20 63 6F 6D 30 69-62 6D 64 6F 73 20 20 63 o com0ibmdos c
&0000:7DA0 6F 6D 30 05 C6 06 77 2F-FF 83 7E FC 00 75 0B 80 om0...w/..~..u..
&0000:7DB0 7E F7 3B 75 05 C6 06 76-2F FF 89 EC 5D CA 04 00 ~.;u...v/...]...
&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.
>> tr
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
&0000:7C29 FA CLI ;cycles=15
>> u 7c2a 7d1a
&0000:7C2A 8CC8 MOV AX,CS
&0000:7C2C 8ED8 MOV DS,AX
&0000:7C2E 33D2 XOR DX,DX
&0000:7C30 8ED2 MOV SS,DX
&0000:7C32 BC007C MOV SP,7C00
&0000:7C35 FB STI
&0000:7C36 B86000 MOV AX,0060
&0000:7C39 8ED8 MOV DS,AX
&0000:7C3B 8EC0 MOV ES,AX
&0000:7C3D 33D2 XOR DX,DX
&0000:7C3F 8BC2 MOV AX,DX
&0000:7C41 CD13 INT 13
&0000:7C43 7269 JC 7CAE
&0000:7C45 E88500 CALL 7CCD
&0000:7C48 72DD JC 7C27
&0000:7C4A 2E CS:
&0000:7C4B 833E037C08 CMP [7C03],0008
&0000:7C50 7406 JZ 7C58
&0000:7C52 2E CS:
&0000:7C53 C606647D02 MOV [7D64],02
&0000:7C58 BB0000 MOV BX,0000
&0000:7C5B 2E CS:
&0000:7C5C 8B0E037C MOV CX,[7C03]
&0000:7C60 51 PUSH CX
&0000:7C61 B009 MOV AL,09
&0000:7C63 2AC1 SUB AL,CL
&0000:7C65 B400 MOV AH,00
&0000:7C67 8BF0 MOV SI,AX
&0000:7C69 56 PUSH SI
&0000:7C6A 33D2 XOR DX,DX
&0000:7C6C 33C0 XOR AX,AX
&0000:7C6E 8AC5 MOV AL,CH
&0000:7C70 2E CS:
&0000:7C71 F636647D DIV BYTE [7D64]
&0000:7C75 8AE8 MOV CH,AL
&0000:7C77 8AF4 MOV DH,AH
&0000:7C79 8BC6 MOV AX,SI
&0000:7C7B B402 MOV AH,02
&0000:7C7D CD13 INT 13
&0000:7C7F 722D JC 7CAE
&0000:7C81 5E POP SI
&0000:7C82 59 POP CX
&0000:7C83 2E CS:
&0000:7C84 2936057C SUB [7C05],SI
&0000:7C88 741F JZ 7CA9
&0000:7C8A 8BC6 MOV AX,SI
&0000:7C8C 2E CS:
&0000:7C8D F726657D MUL WORD [7D65]
&0000:7C91 03D8 ADD BX,AX
&0000:7C93 FEC5 INC CH
&0000:7C95 B101 MOV CL,01
&0000:7C97 51 PUSH CX
&0000:7C98 BE0800 MOV SI,0008
&0000:7C9B 2E CS:
&0000:7C9C 3B36057C CMP SI,[7C05]
&0000:7CA0 7C05 JL 7CA7
&0000:7CA2 2E CS:
&0000:7CA3 8B36057C MOV SI,[7C05]
&0000:7CA7 EBC0 JMP 7C69
&0000:7CA9 EA00006000 JMP &0060:0000
&0000:7CAE BE677D MOV SI,7D67
&0000:7CB1 E80200 CALL 7CB6
&0000:7CB4 EBFE JMP 7CB4
The next chunk of code is similar to disk_error in the PC DOS 1.00 Boot Sector:
&0000:7CB6 32FF XOR BH,BH
&0000:7CB8 2E CS:
&0000:7CB9 AC LODSB
&0000:7CBA 247F AND AL,7F
&0000:7CBC 740B JZ 7CC9
&0000:7CBE 56 PUSH SI
&0000:7CBF B40E MOV AH,0E
&0000:7CC1 BB0700 MOV BX,0007
&0000:7CC4 CD10 INT 10
&0000:7CC6 5E POP SI
&0000:7CC7 EBEF JMP 7CB8
&0000:7CC9 C3 RET
&0000:7CCA E933FF JMP 7C00
The next chunk of code is identical to check_sys_files in the PC DOS 1.00 Boot Sector:
&0000:7CCD BB0000 MOV BX,0000
&0000:7CD0 B90400 MOV CX,0004
&0000:7CD3 B80102 MOV AX,0201
&0000:7CD6 CD13 INT 13
&0000:7CD8 1E PUSH DS
&0000:7CD9 7233 JC 7D0E
&0000:7CDB 8CC8 MOV AX,CS
&0000:7CDD 8ED8 MOV DS,AX
&0000:7CDF BF0000 MOV DI,0000
&0000:7CE2 B90B00 MOV CX,000B
&0000:7CE5 26 ES:
&0000:7CE6 800D20 OR [DI],20
&0000:7CE9 26 ES:
&0000:7CEA 804D2020 OR [DI+20],20
&0000:7CEE 47 INC DI
&0000:7CEF E2F4 LOOP 7CE5
&0000:7CF1 BF0000 MOV DI,0000
&0000:7CF4 BE8B7D MOV SI,7D8B
&0000:7CF7 B90B00 MOV CX,000B
&0000:7CFA FC CLD
&0000:7CFB F3 REPZ
&0000:7CFC A6 CMPSB
&0000:7CFD 750F JNZ 7D0E
&0000:7CFF BF2000 MOV DI,0020
&0000:7D02 BE977D MOV SI,7D97
&0000:7D05 B90B00 MOV CX,000B
&0000:7D08 F3 REPZ
&0000:7D09 A6 CMPSB
&0000:7D0A 7502 JNZ 7D0E
&0000:7D0C 1F POP DS
&0000:7D0D C3 RET
&0000:7D0E BE1B7D MOV SI,7D1B
&0000:7D11 E8A2FF CALL 7CB6
&0000:7D14 B400 MOV AH,00
&0000:7D16 CD16 INT 16
&0000:7D18 1F POP DS
&0000:7D19 F9 STC
&0000:7D1A C3 RET