More boot sector README updates
This commit is contained in:
Jeff Parsons
Jeff Parsons
parent
6d16f582c7
commit
6d8090649f
3 changed files with 203 additions and 109 deletions
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@ -117,8 +117,8 @@ The boot sector of the COMPAQ MS-DOS 1.11 disk image contains the following byte
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000001e0 00 00 00 00 00 00 00 e7 01 00 00 00 00 00 00 00 |................|
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000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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Unlike most DOS boot sectors, this boot sector doesn't start with an Intel x86 JMP instruction. Let's take a
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look at the code, using the PCjs Debugger to stop at the first instruction:
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Unlike most DOS boot sectors, this boot sector doesn't start with an Intel x86 JMP instruction.
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Let's take a look at the code, using the PCjs Debugger to stop at the first instruction:
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>> bp 0:7c00
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bp &0000:7C00 set
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@ -134,44 +134,38 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
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&0000:7C04 B8C007 MOV AX,07C0
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&0000:7C07 8ED0 MOV SS,AX
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&0000:7C09 FB STI
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;
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; At this point, the stack has been set to an unused region INSIDE the boot sector, because
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; 07C0:01E7 is the same as 0000:7C00+01E7 or 0000:7DE7. This puts a fairly low limit on stack
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; usage: as more stack is used, the COMPAQ copyright string will be trashed first, then boot
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; sector data, then boot sector code.
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;
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; Curiously, the value 01E7 is also stored at 07C0:01E7. Also curious is the choice of an ODD
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; address for the stack (although, for the 8088, this presumably didn't affect performance).
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;
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At this point, the stack has been set to an unused region INSIDE the boot sector, because
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07C0:01E7 is the same as 0000:7C00+01E7 or 0000:7DE7. This puts a fairly low limit on stack
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usage: as more stack is used, the COMPAQ copyright string will be trashed first, then boot
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sector data, then boot sector code.
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Curiously, the value 01E7 is also stored at 07C0:01E7. Also curious is the choice of an ODD
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address for the stack (although, for the 8088, this presumably didn't affect performance).
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After the next two instructions, all of DS:0, ES:0, and SS:0 point to the beginning of the
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boot sector (just like CS:7C00):
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&0000:7C0A 8ED8 MOV DS,AX
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&0000:7C0C 8EC0 MOV ES,AX
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;
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; All of DS:0, ES:0, and SS:0 now point to the beginning of the boot sector, as does CS:7C00.
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;
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&0000:7C0E 33C0 XOR AX,AX ; perform a disk reset
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This resets the drive:
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&0000:7C0E 33C0 XOR AX,AX ; perform a drive reset
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&0000:7C10 CD13 INT 13
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;
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; Read 1 sector (the 4th sector, which is the first sector of the directory) into memory immediately
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; above the boot sector (07C0:0200).
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;
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Next, read 1 sector (the 4th sector, which is the first sector of the directory) into memory
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immediately above the boot sector (07C0:0200):
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&0000:7C12 B80102 MOV AX,0201
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&0000:7C15 BB0002 MOV BX,0200
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&0000:7C18 B90400 MOV CX,0004
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&0000:7C1B 33D2 XOR DX,DX
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&0000:7C1D E88B00 CALL 7CAB ; call DISK_IO
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&0000:7C1D E88B00 CALL 7CAB ; read the disk
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&0000:7C20 EB1C JMP 7C3E
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;
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; ERROR: print error message and start over.
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;
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; Setting DS:BX to 07C0:00DA references the same memory as CS:7CDA:
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;
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; >> db cs:7cda cs:7d22
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; &0000:7CDA 0A 0D 4E 6F 6E 2D 53 79-73 74 65 6D 20 64 69 73 ..Non-System dis
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; &0000:7CEA 6B 20 6F 72 20 64 69 73-6B 20 65 72 72 6F 72 0A k or disk error.
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; &0000:7CFA 0D 52 65 70 6C 61 63 65-20 61 6E 64 20 73 74 72 .Replace and str
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; &0000:7D0A 69 6B 65 20 61 6E 79 20-6B 65 79 20 77 68 65 6E ike any key when
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; &0000:7D1A 20 72 65 61 64 79 0A 0D-24 ready..$
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;
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Next, the error handler and restart sequence:
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&0000:7C22 BBDA00 MOV BX,00DA
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&0000:7C25 8A07 MOV AL,[BX]
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&0000:7C27 3C24 CMP AL,24
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@ -188,51 +182,62 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
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&0000:7C3B EBC3 JMP 7C00 ; start over
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&0000:7C3D C3 RET ; unreachable RET?
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Here's the error message (setting DS:BX to 07C0:00DA references the same memory as CS:7CDA):
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>> db cs:7cda cs:7d22
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&0000:7CDA 0A 0D 4E 6F 6E 2D 53 79-73 74 65 6D 20 64 69 73 ..Non-System dis
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&0000:7CEA 6B 20 6F 72 20 64 69 73-6B 20 65 72 72 6F 72 0A k or disk error.
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&0000:7CFA 0D 52 65 70 6C 61 63 65-20 61 6E 64 20 73 74 72 .Replace and str
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&0000:7D0A 69 6B 65 20 61 6E 79 20-6B 65 79 20 77 68 65 6E ike any key when
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&0000:7D1A 20 72 65 61 64 79 0A 0D-24 ready..$
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Next, we have the code to validate the directory contents:
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&0000:7C3E FC CLD
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&0000:7C3F 8BF3 MOV SI,BX
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&0000:7C41 BFC400 MOV DI,00C4 ; ES:DI -> 7C0:C4 (aka 0:7CC4)
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&0000:7C44 B90B00 MOV CX,000B
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&0000:7C47 F3 REPZ
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&0000:7C48 A6 CMPSB
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&0000:7C49 75D7 JNZ 7C22 ; if first DIRENTRY was not "IOSYS.COM", jump to ERROR
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&0000:7C49 75D7 JNZ 7C22 ; if first DIRENTRY was not "IOSYS.COM", jump to error handler
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&0000:7C4B 83C615 ADD SI,0015
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&0000:7C4E B90B00 MOV CX,000B
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&0000:7C51 F3 REPZ
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&0000:7C52 A6 CMPSB
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&0000:7C53 75CD JNZ 7C22 ; if second DIRENTRY was not "IOSYS.COM", jump to ERROR
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&0000:7C53 75CD JNZ 7C22 ; if second DIRENTRY was not "IOSYS.COM", jump to error handler
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&0000:7C55 B80102 MOV AX,0201
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&0000:7C58 BB0002 MOV BX,0200
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&0000:7C5B B90200 MOV CX,0002
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&0000:7C5E 33D2 XOR DX,DX
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&0000:7C60 E84800 CALL 7CAB ; call DISK_IO to read 1st FAT sector
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;
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; At this point, the directory sector is gone, and all this code did with it was verify the names of
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; the first two entries. It did not examine or record their starting cluster numbers. And in the FAT
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; sector just read, only one byte (the first byte) will be examined next: the media ID byte.
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;
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; Further reading will rely on on two hard-coded tables inside the boot sector: one table at DS:126 (CS:7D26)
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; if the FAT media ID byte is 0xFE (160Kb diskette), or another table at DS:13C (CS:7D3C) if the FAT media ID
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; byte is 0xFF (320Kb diskette). If the FAT media ID byte is neither, off to ERROR we go.
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;
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; Here's the table for the 160Kb diskette:
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;
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; >> dw cs:7d26 l11.
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; &0000:7D26 0201 0008 0000 0208 0200 0101 0000 0208 ................
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; &0000:7D36 1000 0201 0000 ......
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;
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; and here's the table for the 320Kb diskette:
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;
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; >> dw cs:7d3c l11.
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; &0000:7D3C 0206 0003 0100 0208 0C00 0101 0000 0203 ................
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; &0000:7D4C 1000 0101 0100 ......
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;
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; These tables are then followed by a DWORD jump address:
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;
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; >> dw cs:7d52 l2
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; &0000:7D52 0000 0060 ..`.
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;
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; which means that the "JMP FAR [0152]" instruction will jump to 60:0 after the reads are complete.
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;
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&0000:7C60 E84800 CALL 7CAB ; read 1st FAT sector
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At this point, the directory sector is gone, and all this code did with it was verify the names of
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the first two entries. It did not examine or record their starting cluster numbers. And in the FAT
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sector just read, only one byte (the first byte) will be examined next: the media ID byte.
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Further reading will rely on on two hard-coded tables inside the boot sector: one table at DS:126 (CS:7D26)
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if the FAT media ID byte is 0xFE (160Kb diskette), or another table at DS:13C (CS:7D3C) if the FAT media ID
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byte is 0xFF (320Kb diskette). If the FAT media ID byte is neither, off to the error handler we go.
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Here's the table for the 160Kb diskette:
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>> dw cs:7d26 l11.
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&0000:7D26 0201 0008 0000 0208 0200 0101 0000 0208 ................
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&0000:7D36 1000 0201 0000 ......
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and here's the table for the 320Kb diskette:
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>> dw cs:7d3c l11.
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&0000:7D3C 0206 0003 0100 0208 0C00 0101 0000 0203 ................
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&0000:7D4C 1000 0101 0100 ......
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These tables are then followed by a DWORD jump address:
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>> dw cs:7d52 l2
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&0000:7D52 0000 0060 ..`.
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which means that the "JMP FAR [0152]" instruction will jump to 60:0 after the reads are complete.
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&0000:7C63 8A27 MOV AH,[BX]
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&0000:7C65 80FCFE CMP AH,FE
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&0000:7C68 BE2601 MOV SI,0126
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@ -260,9 +265,10 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
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&0000:7CA1 8B5414 MOV DX,[SI+14]
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&0000:7CA4 E80400 CALL 7CAB
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&0000:7CA7 FF2E5201 JMP FAR [0152]
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;
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; DISK_IO: read AL sector(s) identified by the CHS values in CX:DX into ES:BX, using DI as a retry counter.
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;
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The final chunk of code is the disk read interface, which reads AL sector(s) identified by the CHS values in CX:DX
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into ES:BX, using DI as a retry counter:
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&0000:7CAB BF0500 MOV DI,0005
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&0000:7CAE 57 PUSH DI
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&0000:7CAF 50 PUSH AX
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@ -279,7 +285,9 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
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&0000:7CBE 75EE JNZ 7CAE
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&0000:7CC0 58 POP AX ; retry count exhausted
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&0000:7CC1 E95EFF JMP 7C22 ; jump to PRINT function
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And then we have the assorted strings and tables:
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>> db 7cc4 l13c
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&7CC4 49 4F 53 59 53 20 20 20-43 4F 4D 4D 53 44 4F 53 IOSYS COMMSDOS
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&7CD4 20 20 20 43 4F 4D 0A 0D-4E 6F 6E 2D 53 79 73 74 COM..Non-Syst
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@ -310,7 +318,11 @@ relieving the code from dealing with head and track size calculations.
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It's arguably an improvement over the [PC-DOS 1.10 Boot Sector](/disks/pcx86/dos/ibm/1.10/#pc-dos-110-boot-sector),
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because it didn't need to be altered to support both 160Kb and 320Kb diskettes. On the other hand, that flexibility
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also makes it slightly slower to boot, because it must read the first FAT sector to determine the diskette format.
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also makes it slightly slower to boot, because it must read the first FAT sector to determine the diskette format.
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Finally, notice the letters `GAS` in the boot sector at offset 0x7D23. Presumably, they're the initials of
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author, and somewhat less conspicuous and more modest than the `Robert O'Rear` signature in the
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[PC-DOS 1.00 Boot Sector](/disks/pcx86/dos/ibm/1.00/#pc-dos-100-boot-sector).
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### Mounting the COMPAQ MS-DOS 1.11 Diskette
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@ -78,6 +78,41 @@ CHKDSK reports:
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### PC-DOS 1.00 Boot Sector
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The boot sector of the PC-DOS 1.00 disk image contains the following bytes:
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00000000 eb 2f 14 00 00 00 60 00 20 37 2d 4d 61 79 2d 38 |./....`. 7-May-8|
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00000010 31 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |1...............|
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00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000030 00 fa 8c c8 8e d8 ba 00 00 8e d2 bc 00 7c fb a1 |.............|..|
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00000040 06 7c 8e d8 8e c0 ba 00 00 8b c2 cd 13 72 41 e8 |.|...........rA.|
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00000050 58 00 72 fb 2e 8b 0e 02 7c 51 bb 00 00 33 d2 b9 |X.r.....|Q...3..|
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00000060 08 00 be 01 00 56 b0 01 b4 02 cd 13 72 22 5e 58 |.....V......r"^X|
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00000070 e8 e7 00 2b c6 74 14 fe c5 b1 01 be 08 00 3b c6 |...+.t........;.|
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00000080 73 04 8b f0 eb 01 96 56 50 eb dd 2e ff 2e 04 7c |s......VP......||
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00000090 be 44 7d b8 42 7d 50 32 ff ac 24 7f 74 0b 56 b4 |.D}.B}P2..$.t.V.|
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000000a0 0e bb 07 00 cd 10 5e eb f0 c3 bb 00 00 b9 04 00 |......^.........|
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000000b0 b8 01 02 cd 13 1e 72 34 8c c8 8e d8 bf 00 00 b9 |......r4........|
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000000c0 0b 00 26 80 0d 20 26 80 8d 20 00 20 47 e2 f3 bf |..&.. &.. . G...|
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000000d0 00 00 be 76 7d b9 0b 00 fc f3 a6 75 0f bf 20 00 |...v}......u.. .|
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000000e0 be 82 7d b9 0b 00 f3 a6 75 02 1f c3 be f9 7c e8 |..}.....u.....|.|
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000000f0 a5 ff b4 00 cd 16 1f f9 c3 0d 0a 4e 6f 6e 2d 53 |...........Non-S|
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00000100 79 73 74 65 6d 20 64 69 73 6b 20 6f 72 20 64 69 |ystem disk or di|
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00000110 73 6b 20 65 72 72 6f f2 0d 0a 52 65 70 6c 61 63 |sk erro...Replac|
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00000120 65 20 61 6e 64 20 73 74 72 69 6b 65 20 61 6e 79 |e and strike any|
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00000130 20 6b 65 79 20 77 68 65 6e 20 72 65 61 64 f9 0d | key when read..|
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00000140 0a 00 cd 18 0d 0a 44 69 73 6b 20 42 6f 6f 74 20 |......Disk Boot |
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00000150 66 61 69 6c 75 72 e5 0d 0a 00 50 52 8b c6 bf 00 |failur....PR....|
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00000160 02 f7 e7 03 d8 5a 58 c3 52 6f 62 65 72 74 20 4f |.....ZX.Robert O|
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00000170 27 52 65 61 72 20 69 62 6d 62 69 6f 20 20 63 6f |'Rear ibmbio co|
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00000180 6d b0 69 62 6d 64 6f 73 20 20 63 6f 6d b0 c9 00 |m.ibmdos com...|
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00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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A number of people have already delved into the guts of the PC-DOS 1.00 boot sector, including:
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* [Reverse-Engineering DOS 1.0 – Part 1: The Boot Sector (May 7, 2009 by Michael Steil)](http://www.pagetable.com/?p=165)
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@ -265,46 +300,43 @@ Using the PCjs Debugger, we can examine the boot sector in its native environmen
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&0000:7C00 EB2F JMP 7C31
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>> db 7c00 l200
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&7C00 EB 2F 14 00 00 00 60 00-20 37 2D 4D 61 79 2D 38 ./....`. 7-May-8
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&7C10 31 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 1...............
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&7C20 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7C30 00 FA 8C C8 8E D8 BA 00-00 8E D2 BC 00 7C FB A1 .............|..
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&7C40 06 7C 8E D8 8E C0 BA 00-00 8B C2 CD 13 72 41 E8 .|...........rA.
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&7C50 58 00 72 FB 2E 8B 0E 02-7C 51 BB 00 00 33 D2 B9 X.r.....|Q...3..
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&7C60 08 00 BE 01 00 56 B0 01-B4 02 CD 13 72 22 5E 58 .....V......r"^X
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&7C70 E8 E7 00 2B C6 74 14 FE-C5 B1 01 BE 08 00 3B C6 ...+.t........;.
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&7C80 73 04 8B F0 EB 01 96 56-50 EB DD 2E FF 2E 04 7C s......VP......|
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&7C90 BE 44 7D B8 42 7D 50 32-FF AC 24 7F 74 0B 56 B4 .D}.B}P2..$t.V.
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&7CA0 0E BB 07 00 CD 10 5E EB-F0 C3 BB 00 00 B9 04 00 ......^.........
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&7CB0 B8 01 02 CD 13 1E 72 34-8C C8 8E D8 BF 00 00 B9 ......r4........
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&7CC0 0B 00 26 80 0D 20 26 80-8D 20 00 20 47 E2 F3 BF ..&.. &.. . G...
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&7CD0 00 00 BE 76 7D B9 0B 00-FC F3 A6 75 0F BF 20 00 ...v}......u.. .
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&7CE0 BE 82 7D B9 0B 00 F3 A6-75 02 1F C3 BE F9 7C E8 ..}.....u.....|.
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&7CF0 A5 FF B4 00 CD 16 1F F9-C3 0D 0A 4E 6F 6E 2D 53 ...........Non-S
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&7D00 79 73 74 65 6D 20 64 69-73 6B 20 6F 72 20 64 69 ystem disk or di
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&7D10 73 6B 20 65 72 72 6F F2-0D 0A 52 65 70 6C 61 63 sk erro...Replac
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&7D20 65 20 61 6E 64 20 73 74-72 69 6B 65 20 61 6E 79 e and strike any
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&7D30 20 6B 65 79 20 77 68 65-6E 20 72 65 61 64 F9 0D key when read..
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&7D40 0A 00 CD 18 0D 0A 44 69-73 6B 20 42 6F 6F 74 20 ......Disk Boot
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&7D50 66 61 69 6C 75 72 E5 0D-0A 00 50 52 8B C6 BF 00 failur....PR....
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&7D60 02 F7 E7 03 D8 5A 58 C3-52 6F 62 65 72 74 20 4F .....ZX.Robert O
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&7D70 27 52 65 61 72 20 69 62-6D 62 69 6F 20 20 63 6F 'Rear ibmbio co
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&7D80 6D B0 69 62 6D 64 6F 73-20 20 63 6F 6D B0 C9 00 m.ibmdos com...
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&7D90 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7DA0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7DB0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7DC0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7DD0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7DE0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&7DF0 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
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&0000:7C00 EB 2F 14 00 00 00 60 00-20 37 2D 00 02 01 01 00 ./....`. 7-.....
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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 00-00 00 00 00 00 00 00 00 ................
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&0000:7C30 00 FA 8C C8 8E D8 BA 00-00 8E D2 BC 00 7C FB A1 .............|..
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&0000:7C40 06 7C 8E D8 8E C0 BA 00-00 8B C2 CD 13 72 41 E8 .|...........rA.
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&0000:7C50 58 00 72 FB 2E 8B 0E 02-7C 51 BB 00 00 33 D2 B9 X.r.....|Q...3..
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&0000:7C60 08 00 BE 01 00 56 B0 01-B4 02 CD 13 72 22 5E 58 .....V......r"^X
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&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: When PCjs adds a BPB to the boot sector, the first 32 bytes will actually look like:
|
||||
|
||||
&7C00 EB 2F 14 00 00 00 60 00-20 37 2D 00 02 01 01 00 ./....`. 7-.....
|
||||
&7C10 02 40 00 40 01 FE 01 00-08 00 01 00 00 00 00 00 .@.@............
|
||||
|
||||
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.
|
||||
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](#pc-dos-100-boot-sector) 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):
|
||||
|
||||
|
|
@ -439,6 +471,11 @@ And finally, `IBMBIO_COM` (0x7D76) and `IBMDOS_COM` (0x7D82):
|
|||
&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 1.11 Boot Sector](/disks/pcx86/dos/compaq/1.11/#compaq-ms-dos-111-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.
|
||||
|
||||
### Additional Information From [PC DOS Retro](https://sites.google.com/site/pcdosretro/doshist)
|
||||
|
||||
- distributed on 1 160KB floppy disk
|
||||
|
|
|
|||
|
|
@ -74,7 +74,7 @@ size of IBMDOS.COM is 6656, so the total number of bytes consumed by those two f
|
|||
|
||||
### PC-DOS 1.10 Boot Sector
|
||||
|
||||
The boot sector of the PC-DOS 1.11 disk image contains the following bytes:
|
||||
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 |................|
|
||||
|
|
@ -124,6 +124,11 @@ a 320Kb diskette is allocated a larger root directory (7 sectors instead of 4),
|
|||
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 1.11 Boot Sector](/disks/pcx86/dos/compaq/1.11/#compaq-ms-dos-111-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
|
||||
|
|
@ -131,6 +136,46 @@ Using the PCjs Debugger, we can examine the boot sector in its native environmen
|
|||
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](#pc-dos-110-boot-sector) 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
|
||||
|
|
|
|||
Loading…
Reference in a new issue