CP/M 4.1 disc formats

CP/M 4.1 (DOS Plus) allows the use of two filesystems - CP/M and DOS. The version I have seen (supplied with the Amstrad PC1512) cannot handle larger floppies than 360k (CP/M) / 1.2Mb (DOS), or larger hard drive partitions than 32Mb.

For implementation details (ie: how CP/M calls behave on a DOS system) there is a separate document on the DOS filesystem .

CP/M filesystem

The Disc Parameter block is

	DEFW	spt	;Number of physical sectors per track. 
			;This differs from previous versions.
	DEFB	bsh	;Block shift. 3 => 1k, 4 => 2k, 5 => 4k....
	DEFB	blm	;Block mask. 7 => 1k, 0Fh => 2k, 1Fh => 4k...
	DEFB	exm	;Extent mask, see later
	DEFW	dsm	;(no. of blocks on the disc)-1
	DEFW	drm	;(no. of directory entries)-1
	DEFB	al0	;Directory allocation bitmap, first byte
	DEFB	al1	;Directory allocation bitmap, second byte
			;These bitmaps are zero if the disc has a DOS format.
	DEFW	cks	;Checksum vector size, 0 or 8000h for a fixed disc.
			;No. directory entries/4, rounded up.
	DEFW	off	;Offset, number of reserved tracks
	DEFB	psh	;Physical sector shift, 0 => 128-byte sectors
			;1 => 256-byte sectors  2 => 512-byte sectors...
	DEFB	phm	;Physical sector mask,  0 => 128-byte sectors
			;1 => 256-byte sectors, 3 => 512-byte sectors...

The directory allocation bitmap is interpreted as:

       al0              al1
b7b6b5b4b3b2b1b0 b7b6b5b4b3b2b1b0
 1 1 1 1 0 0 0 0  0 0 0 0 0 0 0 0

 - ie, in this example, the first 4 blocks of the disc contain the directory.

The DPB is not stored on the disc. The BIOS generates it when a disc is logged in.

CP/M 4.1 directory

The CP/M 4.1 directory has four types of entry:

Files:

0U F1 F2 F3 F4 F5 F6 F7 F8 T1 T2 T3 EX S1 S2 RC   .FILENAMETYP....
AL AL AL AL AL AL AL AL AL AL AL AL AL AL AL AL   ................

The meanings of the bytes are:

0U

User number. 0-15. The user number allows multiple files of the same name to coexist on the disc.
User number = 0E5h => File deleted

Fn

filename

Tn

filetype. The characters used for these are 7-bit ASCII.
The top bit of T1 (often referred to as T1') is set if the file is read-only.
T2' is set if the file is a system file (this corresponds to "hidden" on other systems). System files with user number 0 can be read from any user number.
T3' is set if the file has been backed up.

EX

Extent counter, low byte - takes values from 0-31

S2

Extent counter, high byte.

An extent is the portion of a file controlled by one directory entry. If a file takes up more blocks than can be listed in one directory entry, it is given multiple entries, distinguished by their EX and S2 bytes. The formula is: Entry number = ((32*S2)+EX) / (exm+1) where exm is the extent mask value from the Disc Parameter Block.

S1

Last Record Byte Count. The number of bytes used in the last 128-byte record of the file, with 0 meaning 128.

RC

Number of records (1 record=128 bytes) used in this extent, low byte. The total number of records used in this extent is

(EX & exm) * 128 + RC

If RC is 80h, this extent is full and there may be another one on the disc. File lengths are optionally saved exactly (using the S1 byte); this is mainly apparent in files copied from DOS-formatted media.

AL

Allocation. Each AL is the number of a block on the disc. If an AL number is zero, that section of the file has no storage allocated to it (ie it does not exist). For example, a 3k file might have allocation 5,6,8,0,0.... - the first 1k is in block 5, the second in block 6, the third in block 8.
AL numbers can either be 8-bit (if there are fewer than 256 blocks on the disc) or 16-bit (stored low byte first).

Disc label

20 F1 F2 F3 F4 F5 F6 F7 F8 T1 T2 T3 LB PB RR MT     LABENAMETYP....
P1 P2 P3 P4 P5 P6 P7 P8 D1 D1 D1 D1 D2 D2 D2 D2    ................

20h - Characteristic number of a disc label
F1-F8, T1-T3 - Label name, 7-bit ASCII
LB - Label byte. Bit 0 set => Label exists
                 Bit 4 set => Time stamp on create --+
                 Bit 5 set => Time stamp on update   +--These 2 are mutually
                 Bit 6 set => Time stamp on access --+   exclusive
                 Bit 7 set => Password protection enabled
PB - Used to decode the label password
RR - Reserved, set to zero.
MT - Media type, set to zero for CP/M. If you search for the disc label on 
    a DOS-formatted drive, a fake disc label will be generated with MT set
    to 80h.
P1-P8 - password, rather feebly encrypted.
D1 - Label create datestamp
D2 - Label update datestamp

Date stamps

If date stamps are in use, then every fourth directory entry will be a date stamp entry, containing stamps for the preceding three entries.

21 D1 D1 D1 D1 D2 D2 D2 D2 M1 00 D3 D3 D3 D3 D4    !...............
D4 D4 D4 M2 00 D5 D5 D5 D5 D6 D6 D6 D6 M3 00 00    ................

21h - Characteristic number of a date stamp.
D1  - File 1 create OR access date
D2  - File 1 update date
D3  - File 2 create OR access date
D4  - File 2 update date
D5  - File 3 create OR access date
D6  - File 3 update date
M1  - File 1 password mode
M2  - File 2 password mode
M3  - File 3 password mode
00  - Reserved.

The format of a date stamp is:

	DW	day	;Julian day number, stored low byte first. 
			;Day 1 = 1 Jan 1978.
	DB	hour	;BCD hour, eg 13h => 13:xx 
	DB	min	;BCD minute

Password control

1U F1 F2 F3 F4 F5 F6 F7 F8 T1 T2 T3 PM PB RR RR   .FILENAMETYP....
P1 P2 P3 P4 P5 P6 P7 P8 RR RR RR RR RR RR RR RR   ................

1U = 16+User number (ie 16-31). The user number will be the number of
    the file to which the password belongs.
F1-F8 - Filename of the file to which the password belongs
T1-T3 - Filetype of the file to which the password belongs
PM    - Password mode byte
         Bit 7 set => Password required to read from file
         Bit 6 set => Password required to write to file
         Bit 5 set => Password required to delete file
PB    - Used to decode the password
P1-P8 - The password, rather feebly encrypted.
RR    - Reserved, set to 0.

Password encryption system

This system is extremely simple:

When making the password, add all 8 bytes together (packing with spaces if necessary). This becomes PB (the decode byte). XOR each byte with PB and store them backwards in the directory (ie the last byte becomes P1).
To decode the password, XOR PB with the 8 bytes of the password and read it off backwards.

DOS filesystem

The DOS filesystem can be either FAT12 or FAT16. The format is exactly as in PCDOS 2.11, except:

Byte 0Ch of the directory entry (which is not used by PCDOS) holds the four "user attributes" F1'-F4':
- Bit 7: F1'
- Bit 6: F2'
- Bit 5: F3'
- Bit 4: F4'
DRDOS-style passwords are not supported.

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