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or by receipt of F5 in MFM. An F7 pattern will generate two
CRC characters in FM or MFM. As a consequence, the
patterns F5 thru FE must not appear in the gaps, data
fields, or ID fields. Also, CRC's must be generated by an F7
pattern.
Disks may be formatted in IBM 3740 or System 34 formats
with sector lengths of 128, 256, 512, or 1024 bytes.
TYPE IV COMMANDS
The Forced Interrupt command is generally used to ter·
minate a multiple sector read or write command or to in·
sure Type I status in the status register. This command can
be loaded into the command register at any time. If there is
a current command under execution (busy status bit set)
the command will be terminated and the busy status bit
reset.
The lower four bits of the command determine the condi-
tional interrupt as follows:
10
=
Not-Ready to Ready Transition
11
=
Ready to Not-Ready Transition
12
=
Every Index Pulse
13
=
Immediate Interrupt
The conditional interrupt is enabled when the correspond-
ing bit positions of the command (13 . 10) are set to a 1.
Then, when the condition for interrupt is met, the INTRQ
line will go high signifying that the condition specified has
occurred. If 13 - 10 are all set to zero (HEX DO), no interrupt
will occur but any command presently under execution will
be immediately terminated. When using the immediate
READ
NO
mACK
NO
ADDRESS
YES
MAAK DETECTEC
NO
TYPE III COMMAND
Read Track/Address
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