GM82C803CN Datasheet, PDF (52/84 Pages) List of Unclassifed Manufacturers – 2.88 MB FDC/ Dual UARTs with FIFO/PIO(EPP/ ECP)/ IDE Interface/ S-IR/ PnP

GM82C803CN
Scan commands
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 compared the data on a byte by byte basis and looks for a sector of data which meets the
conditions of DFDD = DHOST , DFDD DHOST , DFDD DHOST . The hexadecimal byte of FF either from memory of 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 conditi-
ons for scan are met [equal , low or high ], the last sector on the track is reached [EOT], or the terminal count signal is rece-
ived. If the conditions for scan are met , then the FDC sets the SH(Scan Hit) flag of Status Register 2 to a 1 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 Register2 to a 1 and terminate the Scan
command . The receipt of a TC signal from the Host or DMA controller during the scan operation will cause the FDC to co-
mparison of the particular byte which in process and then to terminate the command . Table 4-26 shows the status of bits
SH and SN under various conditions of Scan.
Table 4-26. Status of bits SH and SN
Command
Status Register 2
Bit 2 = SN
Bit 3 = SH
0
Scan Equal
1
1
0
Scan Low or
0
0
Equal
0
0
1
0
Scan High or
0
1
Equal
0
0
1
0
Comments
Disk Data = Host Data
Disk Data Host Data
Disk Data = Host Data
Disk Data < Host Data
Disk Data > Host Data
Disk Data = Host Data
Disk Data > Host Data
Disk Data < Host Data
If the FDC encounters a Delete Data Address Mark on one of the sector (and SK=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 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 in order to show that a deleted sector had been encountered.
When either the STP ( contiguous sectors = 01 , or alternate sector = 02 ) sectors are read or the MT( Multi Track) is prog-
rammed , it is necessary to remember that the last sector on the track must be read . For example , if STP = 02 , MT = 0 ,
the sector are numbered sequentially 1 through 26 and the Scan command is started at sector 21 , the following will hap-
pen : sector 21, 23 , aand 25 will be read , then the next sector(26) will be skipped and the index hole will 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 st 25 or the scanning started at sector 20, then the Scan command would be completed in a normal manner.