Crc; Gap 2; Am2; Data Field - IBM 5280 Maintenance Information Manual

CRC
Two
eRe
(cyclic redundancy check) bytes are generated
in the
eRe
~egister
during a write operation for an ID field
or a data field. The bit structure of the
eRe
byte is deter-
mined by an algebraic formula applied to the bit structure
of the field being written. These two
eRe
bytes are written
on the diskette immediately following the field. During
a write operation, the data written on the diskette is
retained in main storage for use during the write verify
operation.
During the write verify operation, the data is compared
bit for bit with the data in the main storage as it is read
from the diskette. If any bit read from the. diskette does
not compare with the corresponding bit in the main
storage, a write verify error occurs. Also, as the data is
read from the diskette, two
eRe
bytes are built in the
eRe
register. When the two
eRe
bytes (written on the
diskette during the write operation) are read, they are
subtracted from the two
eRe
bytes that were built in the
eRe
register during the write verify 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, two
eRe
bytes are generated in
the
eRe
register for the ID 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. As the field is being read, the two
eRe
bytes
on the diskette are read and compared to the two bytes
from the
eRe
register. If the
eRe
bytes are exactly the
same, the field has been read correctly. If they are not
exactly the same, a
eRe
error occurs.
Gap 2
This gap separates the ID field from the data field. In FM
format, gap 2 consists of 11 bytes of hex 4E. In MFM
format, gap 2 consists of 22 bytes of hex F F.
AM2
AM 2 is the address mark that identifies the following
field as either a data field or a control field. If the field
following the address mark is a data field, AM 2 will
contain the following:
FM format
MFM format
Hex FB
Hex A1A1A1FB
If the field following the address mark is a control field,
AM 2 will contain the following:
FM format
MFM format
Data Field
Hex F8
Hex A1A1A1F8
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. The following
chart shows what is contained in gap 3:
Sector Size
128 bytes
256 bytes
512 bytes
1024 bytes
Gap 4
Field Contents
in FM
27 bytes of hex F F
42 bytes of hex F F
58 bytes of hex FF
Field Contents
inMFM
54 bytes of hex
84 bytes of hex
116 bytes of hex
This gap occurs after the last record of the last sector of
a track and separates that record from the index pulse. In
FM format, this gap contains a variable number of bytes of
hex F F. In MFM format, this gap contains a variable
number of bytes of hex 4E. The actual number of bytes
depends on the speed of the diskette. The length of this
field can vary to let the diskette interchange 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 diskette control circuitry that sector 1 of
that particular track will be the next sector to reach the
read/write heads.
Theory 75