HP 9800 SERIES - Software Compatibility

2-12 Theory of Operation
Track Numbering. Each track has a physical address as previously described. There is also
a logical track address associated with each good track. The logical track address
is written in
the
ID field of each sector on the track. If a disc has no bad tracks, the logical address of a
track is the same as the physical address.
A
4isc with N bad tracks can be made to look like a 77 - N track disc with no bad tracks. To
do this, the logical track address stored in the ID field of each sector of every bad track is set
to !FF. Tracks of this type are known as invisible tracks. All visible tracks are then sequen-
tially assigned logical track numbers. Logical Track 0 is the outermost good track, not
necessarily physical Track O.
Format Differences
Table 2-3 summarizes the principal differences between the IBM Format and the HP For-
mat. Details of these differences are provided in the following paragraphs.
Coding. The
IBM Format uses a single-density encoding scheme known as frequency
modulation
(FM). The rules for FM coding are as follows:
• A 0 bit cell has no data transition.
• A 1 bit cell has a data transition.
• Every bit cell has a clock transition.
The minimum distance between transitions
is one-half the bit cell, that is, the distance from
a clock transition to a
data transition.
Table 2-3.
Format Differences
Feature
HP Format IBM Format
Usable Physical Tracks 0-76
0-76
Sectors Per Track 30 26
Sector Numbering 0-29
1 - 26
Bytes Per Sector
256 128
Data Order
LS Byte First
MS Byte First
LS Bit
Frrst MS Bit First
Coding
MMFM
FM
Precompensation Required
Yes
No
Sync-Up Bytes
Four Bytes of
roo Six Bytes of ! 00
And
Four Bytes of !FF
Address Marks Extra Clock Missing Clock
Transitions
Transitions
CRC Includes Address Mark No
Yes