COMMAND SET/DESCRIPTIONS
Commands can be written whenever the FDC is in
The user sends a Sense Interrupt Status
the command phase. Each command has a
unique set of needed parameters and status
results. The FDC checks to see that the first byte
is a valid command and, if valid, proceeds with the
command. If it is invalid, an interrupt is issued.
command which returns an invalid command error.
Refer to TABLE 20 for explanations of the various
symbols used. TABLE 21 lists the required
parameters and the results associated with each
command that the FDC is capable of performing.
TABLE 20 - DESCRIPTION OF COMMAND SYMBOLS
SYMBOL
NAME
DESCRIPTION
C
Cylinder Address
Data Pattern
The currently selected address; 0 to 255.
D
The pattern to be written in each sector data field during formatting.
D0, D1
Drive Select 0-1
Designates which drives are perpendicular drives on the
Perpendicular Mode Command. A "1" indicates a perpendicular
drive.
DIR
Direction Control
Disk Drive Select
If this bit is 0, then the head will step out from the spindle during a
relative seek. If set to a 1, the head will step in toward the spindle.
DS0, DS1
DS1 DS0 Drive Selected
0
0
0
1
Drive 0
Drive 1
DTL
Special Sector
Size
By setting N to zero (00), DTL may be used to control the number of
bytes transferred in disk read/write commands. The sector size (N =
0) is set to 128. If the actual sector (on the diskette) is larger than
DTL, the remainder of the actual sector is read but is not passed to
the host during read commands; during write commands, the
remainder of the actual sector is written with all zero bytes. The CRC
check code is calculated with the actual sector. When N is not zero,
DTL has no meaning and should be set to FF HEX.
EC
Enable Count
Enable FIFO
When this bit is "1" the "DTL" parameter of the Verify command
becomes SC (number of sectors per track).
EFIFO
EIS
This active low bit when a 0, enables the FIFO. A "1" disables the
FIFO (default).
Enable Implied
Seek
When set, a seek operation will be performed before executing any
read or write command that requires the C parameter in the
command phase. A "0" disables the implied seek.
EOT
GAP
GPL
End of Track
The final sector number of the current track.
Alters Gap 2 length when using Perpendicular Mode.
Gap Length
The Gap 3 size. (Gap 3 is the space between sectors excluding the
VCO synchronization field).
H/HDS
Head Address
Selected head: 0 or 1 (disk side 0 or 1) as encoded in the sector ID
field.
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