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CP/M on the MZ-80K - 
Disk Parameter Block & Bootstrap Code 
written by Maurice Hawes / SUC/UK
Source: SUC-magazine July 2003, Volume 23 Number 2, p. 10  
Sharp Users Club - MZ-80K Section - CP/M via DPB.COM


Most of this issue is taken up with a general look at the CP/M and PCP/M Systems on Sharp MZ-computers. Here, I concentrate on the Disk Parameter Block used by Crystal Research in their 5.25" 35-track CP/M 2.2 system for the MZ-80K, as displayed by DPB.COM:-

Sectors per Track  
16
Block Shift ( BSH )  
3
Block Mask ( BLM )  
7
Extent Mask ( EXM )  
1
Total Blocks ( DSM )  
129
Directory Entries ( DRM )  
63
Allocation 0 (AL0)  
C0
Allocation 1 (AL1)  
0
Cylinder Offset (OFS)  
5

( Crystal CP/M, like most CP/M systems, uses 128-byte logical sectors )

16 Sectors per Track gives a track capacity of 128 * 16 = 2K - exactly the same as in MZ-80K Disk Basic. The SUC program DISKEDIT.K7 may therefore be used to examine a Crystal CP/M disk, and edit it if necessary.

In Crystal MZ-80K CP/M 2.2, each side of a 5.25" disk holds 35 'Logical Tracks'. A double-sided disk is therefore seen as 70 'Logical Tracks' and the overall capacity of such a disk is
128 * 16 * 70 = 143,360 bytes ( 140K ).

BSH = 3 and BLM = 7 both give a block size of 1K ( 1024 bytes )

EXM = 0 implies that, in this case, a 'Physical Extent' is the same as a 'Logical Extent' i.e. they are both 16K.

DSM = 129 implies that a disk contains 130 CP/M blocks ( i.e. 130K ):

DRM = 63 implies that the Directory can hold 64 entries.

AL0 - AL1 = 1100 0000 implies that the Directory is allocated the first 2 CP/M blocks ( i.e. Blocks 00 and 01, total 2K ).

OFS = 5 implies that the first CP/M block ( Block 00 ) is at the start of Track 5, and that Tracks 0 - 4 are reserved for bootstrap & system files.

Further Information


The 'User Area' is 130K, but 2K of this is allocated to the Directory, so the actual space available for user files is 128K per disk.

FILES.COM in XTAL MZ-80K CP/M confirms this figure.

DISKEDIT reveals the following sections of code on Tracks 0-4:-

Bootstrap Loader   Tr.0 Sec.1
CCP   Tr.0 Sec.9 to Tr.1 Sec.8
BDOS   Tr.1 Sec.9 to Tr.3 Sec.4
BIOS   Tr.3 Sec.5 to Tr.4 Sec.9


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last updated August 18, 2003
SUC / UK: Maurice Hawes

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