Commands can be written whenever the FDC is in 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. The user sends a Sense Interrupt Status
command which returns an invalid command error. Refer
to Table 18 or explanations of the various symbols used.
Table 19 lists the required parameters and the results
associated with each command that the FDC is capable
Table 18 - Description of Command Symbols
Cylinder Address The currently selected address; 0 to 255.
The pattern to be written in each sector data field during formatting.
D0, D1, D2, Drive Select 0-3
Designates which drives are perpendicular drives on the
Perpendicular Mode Command. A "1" indicates a perpendicular
Direction Control 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 Disk Drive Select
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.
When this bit is "1" the "DTL" parameter of the Verify command
becomes SC (number of sectors per track).
This active low bit when a 0, enables the FIFO. A "1" disables the
Enable Implied 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.
End of Track
The final sector number of the current track.
Alters Gap 2 length when using Perpendicular Mode.