PC97338 datasheet(106/221 Pages) NSC

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Bit 6 - Modified Frequency Modulation (MFM)

This bit indicates the type of the disk drive and the

data transfer rate, and determines the format of

the address marks and the encoding scheme.

0 - FM mode, i.e., single density.

1 - MFM mode, i.e., double density.

Bit 7 - Multi-Track (MT)

This bit controls whether or not the controller con-

tinues to side 1 of the disk after reaching the last

sector of side 0.

0 - Single track. The controller stops at the last

sector of side 0.

1 - Multiple tracks. the controller continues to side

1 after reaching the last sector of side 0.

Second Command Phase Byte

Bits 1,0 - Logical Drive Select (DS1,0)

These bits indicate which logical drive is active.

See “Bits 1,0 - Logical Drive Select (DS1,0)” on

page 96.

00 - Drive 0 is selected. (Default)

01 - Drive 1 is selected.

10 - If four drives are supported, or drives 2 and 0

are exchanged, drive 2 is selected.

11 - If four drives are supported, drive 3 is select-

ed.

Bit 2 - Head (HD)

This bit indicates which side of the Floppy Disk

Drive (FDD) is selected by the head. Its value is

the inverse of the HDSEL disk interface output sig-

nal.See “Bit 2 - Head Select (HD)” on page 96.

0 - HDSEL is not active, i.e., the head of the FDD

selects side 0. (Default)

1 - HDSEL is active, i.e., the head of the FDD se-

lects side 1.

Bit 7 - Implied Seek (IPS)

This bit indicates whether or not an implied seek

should be performed. See also, “Bit 5 - Implied

Seek (IPS)” on page 101.

0 - No implied seek operations. (Default)

1 - The controller performs seek and sense inter-

rupt operations before executing the com-

mand.

Third Command Phase Byte - Track Number

The value in this byte specifies the number of the

track to read.

Fourth Command Phase Byte - Head Number

The value in this byte specifies head to use.

Fifth Command Phase Byte - Sector Number

The value in this byte specifies the sector to read.

Sixth Command Phase Byte -

Bytes-Per-Sector Code

This byte contains a code in hexadecimal format

that indicates the number of bytes in a data field.

Table 44 on page 97 indicates the number of bytes

that corresponds to each code.

Seventh Command Phase Byte -

Bytes Between Sectors - Gap 3

The value in this byte specifies how many bytes

there are between sectors. See “Fifth Command

Phase Byte - Bytes in Gap 3” on page 97.

Eighth Command Phase Byte -

Data Length (Obsolete)

The value in this byte is ignored and must be set

to FF (hex).

Execution Phase

In this phase, data read from the disk drive is trans-

ferred to the system via DMA or non-DMA modes.

See 3.4.2 on page 85.

The controller looks for the track number specified in

the third command phase byte. If implied seeks are

enabled, the controller also performs all operations of

a SENSE INTERRUPT command and of a SEEK

command (without issuing these commands). Then,

the controller waits the head settle time. See bits 3-0

of the fourth command phase byte of the MODE com-

mand on page 102.

The controller then starts the data separator and waits

for the data separator to find the address field of the

next sector. The controller compares the ID informa-

tion (track number, head number, sector number,

bytes-per-sector code) in that address field with the

corresponding information in the command phase

bytes of the READ DATA command.

If the contents of the bytes do not match, then the

controller waits for the data separator to find the ad-

dress field of the next sector. The process is repeated

until a match or an error occurs.

Possible errors, the conditions that may have caused

them and the actions that result are:

• The microprocessor aborted the command by writ-

ing to the FIFO.

If there is no disk in the drive, the controller gets

stuck. The microprocessor must then write a byte

to the FIFO to advance the controller to the result

phase.

• Two pulses of the INDEX signal were detected

since the search began, and no valid ID was

found.

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