More boot sector README updates
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Jeff Parsons
Jeff Parsons
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3 changed files with 203 additions and 109 deletions
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@ -74,7 +74,7 @@ size of IBMDOS.COM is 6656, so the total number of bytes consumed by those two f
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### PC-DOS 1.10 Boot Sector
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The boot sector of the PC-DOS 1.11 disk image contains the following bytes:
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The boot sector of the PC-DOS 1.10 disk image contains the following bytes:
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00000000 eb 27 90 08 00 14 00 00 00 00 00 00 00 00 00 00 |.'..............|
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00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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@ -124,6 +124,11 @@ a 320Kb diskette is allocated a larger root directory (7 sectors instead of 4),
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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
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value is now stored at offset 0x0005 (0x7C05) instead of 0x0002 (0x7C02).
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Interestingly, the [COMPAQ MS-DOS 1.11 Boot Sector](/disks/pcx86/dos/compaq/1.11/#compaq-ms-dos-111-boot-sector)
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is completely different from all PC-DOS 1.x boot sectors. The COMPAQ boot sector supports both 160Kb and 320Kb
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formats with less code and without requiring "patches", although that flexibility did require reading the first
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FAT sector, which PC-DOS 1.x boot sectors did not do.
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Using the PCjs Debugger, we can examine the boot sector in its native environment:
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bp &0000:7C00 hit
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@ -131,6 +136,46 @@ Using the PCjs Debugger, we can examine the boot sector in its native environmen
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AX=0000 BX=7C00 CX=0004 DX=0000 SP=0100 BP=E4B7 SI=0000 DI=0044
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SS=0030 DS=0040 ES=0000 PS=F296 V0 D0 I1 T0 S1 Z0 A1 P1 C0
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&0000:7C00 EB27 JMP 7C29
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>> db 7c00 l200
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&0000:7C00 EB 27 90 08 00 14 00 00-00 00 00 00 02 01 01 00 .'..............
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&0000:7C10 02 40 00 40 01 FE 01 00-08 00 01 00 00 00 00 00 .@.@............
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&0000:7C20 00 00 00 00 00 00 00 CD-19 FA 8C C8 8E D8 33 D2 ..............3.
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&0000:7C30 8E D2 BC 00 7C FB B8 60-00 8E D8 8E C0 33 D2 8B ....|..`.....3..
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&0000:7C40 C2 CD 13 72 69 E8 85 00-72 DD 2E 83 3E 03 7C 08 ...ri...r...>.|.
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&0000:7C50 74 06 2E C6 06 64 7D 02-BB 00 00 2E 8B 0E 03 7C t....d}........|
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&0000:7C60 51 B0 09 2A C1 B4 00 8B-F0 56 33 D2 33 C0 8A C5 Q..*.....V3.3...
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&0000:7C70 2E F6 36 64 7D 8A E8 8A-F4 8B C6 B4 02 CD 13 72 ..6d}..........r
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&0000:7C80 2D 5E 59 2E 29 36 05 7C-74 1F 8B C6 2E F7 26 65 -^Y.)6.|t.....&e
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&0000:7C90 7D 03 D8 FE C5 B1 01 51-BE 08 00 2E 3B 36 05 7C }......Q....;6.|
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&0000:7CA0 7C 05 2E 8B 36 05 7C EB-C0 EA 00 00 60 00 BE 67 |...6.|.....`..g
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&0000:7CB0 7D E8 02 00 EB FE 32 FF-2E AC 24 7F 74 0B 56 B4 }.....2...$t.V.
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&0000:7CC0 0E BB 07 00 CD 10 5E EB-EF C3 E9 33 FF BB 00 00 ......^....3....
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&0000:7CD0 B9 04 00 B8 01 02 CD 13-1E 72 33 8C C8 8E D8 BF .........r3.....
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&0000:7CE0 00 00 B9 0B 00 26 80 0D-20 26 80 4D 20 20 47 E2 .....&.. &.M G.
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&0000:7CF0 F4 BF 00 00 BE 8B 7D B9-0B 00 FC F3 A6 75 0F BF ......}......u..
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&0000:7D00 20 00 BE 97 7D B9 0B 00-F3 A6 75 02 1F C3 BE 1B ...}.....u.....
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&0000:7D10 7D E8 A2 FF B4 00 CD 16-1F F9 C3 0D 0A 4E 6F 6E }............Non
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&0000:7D20 2D 53 79 73 74 65 6D 20-64 69 73 6B 20 6F 72 20 -System disk or
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&0000:7D30 64 69 73 6B 20 65 72 72-6F 72 0D 0A 52 65 70 6C disk error..Repl
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&0000:7D40 61 63 65 20 61 6E 64 20-73 74 72 69 6B 65 20 61 ace and strike a
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&0000:7D50 6E 79 20 6B 65 79 20 77-68 65 6E 20 72 65 61 64 ny key when read
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&0000:7D60 79 0D 0A 00 01 00 02 0D-0A 44 69 73 6B 20 42 6F y........Disk Bo
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&0000:7D70 6F 74 20 66 61 69 6C 75-72 65 0D 0A 00 4D 69 63 ot failure...Mic
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&0000:7D80 72 6F 73 6F 66 74 2C 49-6E 63 20 69 62 6D 62 69 rosoft,Inc ibmbi
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&0000:7D90 6F 20 20 63 6F 6D 30 69-62 6D 64 6F 73 20 20 63 o com0ibmdos c
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&0000:7DA0 6F 6D 30 05 C6 06 77 2F-FF 83 7E FC 00 75 0B 80 om0...w/..~..u..
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&0000:7DB0 7E F7 3B 75 05 C6 06 76-2F FF 89 EC 5D CA 04 00 ~.;u...v/...]...
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&0000:7DC0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&0000:7DD0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&0000:7DE0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&0000:7DF0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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NOTE: PCjs adds a BPB to the boot sector, which is why the first 32 bytes of the boot sector
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appear slightly different from the original [dump](#pc-dos-110-boot-sector) above. The addition of
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a BPB makes the disk image mountable by modern operating systems and does not otherwise affect the
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operation of the boot sector or any of the code below.
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>> tr
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AX=0000 BX=7C00 CX=0004 DX=0000 SP=0100 BP=E4B7 SI=0000 DI=0044
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SS=0030 DS=0040 ES=0000 PS=F296 V0 D0 I1 T0 S1 Z0 A1 P1 C0
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