gm82c765b ETC-unknow, gm82c765b Datasheet - Page 33

• Manufacturer Part Number: gm82c765b
• Description: Floppy Disk Subsystem Controller
• Manufacturer: ETC-unknow

The Scan commands allow data which is being
read from the diskette to be compared against
data which is being supplied from the main
system. The FDC compares the data on a
byte-by-byte basis and looks for a sector of data
which meets the conditions of D FDD=D
Processor, D FDD<D Processor, or D FDD>D
Processor. The hexidecimal byte of FF either
from memory or from FDD can be used as a
mask byte because it always meets the condition
of the comparison. One’s complement arithmetic
is used for comparison (FF-largest number,
00=smallest number). After a whole sector of
data is compared, if the conditions art not met,
the sector number is incremented (R+STP R),
and the scan operation is continued. The scan
operation continues until one of the following
conditions occur: the conditions for scan are met
(equal, low, or high), the last sector on the track
is reached (EOT), on the terminal count signal is
received.
If the conditions for scan are met, then the FDC
sets the SH (Scan Hit) flag of Status Register 2 to
a 1 (high) and terminates the Scan command. If
the conditions for scan are not met between the
starting sector (as specified by R) and the last
sector on the cylinder (EOT), then the FDC sets
the SN (Scan Not Satisfied) flag of Status
Register 2 to a 1 (high) and terminates the Scan
command. The receipt of a Terminal Count
signal from the processor or DMA controller
during the scan operation will cause the FDC to
complete the comparison of the particular byte
which is in process and then to terminate the
command. Table 30 shows the status of bits SH
and SN under various conditions of Scan.
If the FDC encounters a Deleted Data Address
mark on one of the sectors (and K=0), then it
regards the sector as the last sector on the
cylinder, sets the CM (Control mark) flag of
Status Register 2 to a 1 (high) and terminates the
command. If SK=1, the FDC skips the sector
with the Deleted Address mark and reads the
next sector. In the second case (SK=1), the FDC
sets the CM (Control mark) flag of Status
Register 2 to a 1 (high) in order to show that a
Scan Commands
33
delected sector had been encountered.
TABLE 30. STATUS of BITS SH AND SN
When either the STP (contiguous sectors=01,or
alternate sectors=02) sectors are read or the MT
(Multitrack) is programmed, it is necessary to
remember that the last sector on the track must be
read. For example, if STP=02, MT=0, the sectors
are numbered sequentially 1 through 26 and the
Scan command is started at sector 21, the
following will happen: sectors 21,23 and 25will
be read, then the next sector (26) will be skipped
and the index hole will be encountered before the
EOT value of 26 can be read. This will result in
an abnormal termination of the command. If the
EOT had been set at 25 or the scanning started at
sector 20, then the Scan command would be
completed in a normal manner.
During the Scan command, data is supplied by
either the processor or DMA controller for
comparison against the data read from the
diskette. In order to avoid having the OR
(Overrun) flag set in Status Register 1, it is
necessary to have the data available in less then
27 us (FM mode) or 13 us (MFM mode). If an
Overrun occurs, the FDC ends the command with
bits 7 and 6 of Status Register 0 set to 0 and 1,
respectively.
SEEK
Scan Equal
Scan low
Or Equal
Scan High or
Equal
Command
Status Register
2-SN
Bit
0
1
0
0
1
0
0
1
2
3-SH
Bit
1
0
0
0
0
1
0
0
D
D
D
D
D
D
D
D
GM82C765B
FDD
FDD
FDD
FDD
FDD
FDD
FDD
FDD
Comments
>D
>D
<D
D
D
D
D
D
Pr
Pr
Pr
Pr
Pr
Pr
Pr
Pr
ocessor
ocessor
ocessor
ocessor
ocessor
ocessor
ocessor
ocessor