FDC37B78X 查看數據表（PDF） - SMSC -> Microchip

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FDC37B78X

Enhanced Super I/O Controller with ACPI Support, Real Time Clock and Consumer IR

SMSC -> Microchip 

Version

The Version command checks to see if the

controller is an enhanced type or the older type

(765A). A value of 90 H is returned as the result

byte.

Relative Seek

The command is coded the same as for Seek,

except for the MSB of the first byte and the DIR

bit.

DIR

ACTION

0 Step Head Out

1 Step Head In

DIR Head Step Direction Control

RCN

Relative Cylinder Number that

determines how many tracks to step the

head in or out from the current track

number.

The Relative Seek command differs from the

Seek command in that it steps the head the

absolute number of tracks specified in the

command instead of making a comparison

against an internal register. The Seek

command is good for drives that support a

maximum of 256 tracks. Relative Seeks cannot

be overlapped with other Relative Seeks. Only

one Relative Seek can be active at a time.

Relative Seeks may be overlapped with Seeks

and Recalibrates. Bit 4 of Status Register 0

(EC) will be set if Relative Seek attempts to step

outward beyond Track 0.

As an example, assume that a floppy drive has

300 useable tracks. The host needs to read

track 300 and the head is on any track (0-255).

If a Seek command is issued, the head will stop

at track 255. If a Relative Seek command is

issued, the FDC will move the head the

specified number of tracks, regardless of the

internal cylinder position register (but will

increment the register). If the head was on track

40 (d), the maximum track that the FDC could

position the head on using Relative Seek will be

295 (D), the initial track + 255 (D). The

maximum count that the head can be moved

with a single Relative Seek command is 255

(D).

The internal register, PCN, will overflow as the

cylinder number crosses track 255 and will

contain 39 (D). The resulting PCN value is thus

(RCN + PCN) mod 256. Functionally, the FDC

starts counting from 0 again as the track

number goes above 255 (D). It is the user's

responsibility to compensate FDC functions

(precompensation track number) when

accessing tracks greater than 255. The FDC

does not keep track that it is working in an

"extended track area" (greater than 255). Any

command issued will use the current PCN value

except for the Recalibrate command, which only

looks for the TRACK0 signal. Recalibrate will

return an error if the head is farther than 79 due

to its limitation of issuing a maximum of 80 step

pulses. The user simply needs to issue a second

Recalibrate command. The Seek command and

implied seeks will function correctly within the

44 (D) track (299-255) area of the "extended

track area". It is the user's responsibility not to

issue a new track position that will exceed the

maximum track that is present in the extended

area.

To return to the standard floppy range (0-255) of

tracks, a Relative Seek should be issued to

cross the track 255 boundary.

A Relative Seek can be used instead of the

normal Seek, but the host is required to

calculate the difference between the current

head location and the new (target) head

location. This may require the host to issue a

Read ID command to ensure that the head is

physically on the track that software assumes it

to be. Different FDC commands will return

different cylinder results which may be difficult

to keep track of with software without the Read

ID command.

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