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CRC
Two
eRe
(cyclic redundancy check) bytes are generated
in the
eRe
register during a write operation for an ID
field or a data field. The bit structure of the
eRe
bytes
is determined by an algebraic formula applied to the bit
structure of the field being written. These 2
eRe
bytes
are written on the diskette immediately following the
field. The data written on the diskette during a write
operation is retained in a buffer for use during the verify
read operation. During the verify read operation, the
data is compared bit for bit with the data in the buffer,
as it is read from the diskette. If any bit read from the
diskette does not compare with the corresponding bit in
the buffer, a data check error occurs. Also, as the data
is read from the diskette, 2
eRe
bytres are built in the
eRe
register. When the 2
eRe
bytes (written on the
diskette during me write operation) are read, they are
subtracted from the 2
eRe
bytes that were built in the
eRe
register during the verify read operation. The
eRe
register is then tested for a content of
O.
If the
eRe
register equals
0,
the field was written correctly. If the
eRe
register does not equal
0,
a
eRe
error occurs.
During a read operation, 2
eRe
bytes are again
generated in the
eRe
register for
10
fields and data
fields. The bit structure of the
eRe
bytes is determined
by an algebraic formula applied to the bit structure of
the field being read. After the complete field is read,
the 2
eRe
bytes on the diskette are read and compared
to the 2
eRe
bytes from the
eRe
register. If the
eRe
bytes are exactly the same, the field has been read
correctly. If they are not exacly the same, a
eRe
error
occurs.
GAP 2
This Gap is used to separate the
10
field from the data
field. Gap 2 consists of 11 bytes of hex FF in FM
format and 22 bytes of hex FF in MFM format.
AM 2
AM 2 is the address mark that identifies the following
field as either a data field or a control field. If the
following field is a data field, AM 2 (in FM format)
contains hex FB. In MFM format, it contains hex
A 1 A 1 A 1 FB. If the following field is a control field, AM
2 (in FM format) contains hex Fa. In MFM format, it
contains hex A 1 A 1 A 1 F8.
3-22
Data Field
The data field contains the data record.
Control Field
The control field contains data to control the reading of
that particular record.
GAP 3
Gap
3
separates one sector from another. This gap
contains hex
FF
in
FM
format. In
MFM
format,
it contains hex 4E.
Record
Gap
Record
Format
Size
Length
FM
27
bytes
128 bytes
FM
42 bytes
256 bytes
FM
58 bytes
512 bytes
MFM
54 bytes
256 bytes
MFM
84
bytes
512 bytes
MFM
116 bytes
1024 bytes
GAP 4
Gap 4 occurs after the last record of the last sector of a
track and separates that record from the index pulse.
This gap contains a variable number of bytes of hex FF
in FM format and a variable number of bytes of hex 4E
in MFM format. The actual number of bytes depends on
the speed of the diskette. The length is variable to allow
interchangeability of diskettes between diskette drives.
Index
An index pulse occurs each time the index hole in the
diskette passes the light emitting diode/phototransistor
(LED/PTX) of the diskette drive. The index pulse
indicates to the adapter that sector 1 of that particular
track will be the next sector to reach the read / write
heads.
....
...
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