PC87307 datasheet(90/218 Pages) NSC

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TABLE 5-10. Typical Values for PC Compatible Diskette Media

a. Gap 2 is speciﬁed in the command phase of read, write, scan, and verify commands. Although this is the rec-

ommended value, the FDC ignores this byte in read, write, scan and verify commands.

b. Gap 3 is the suggested value for the programmable GAP3 that is used in the FORMAT TRACK command and

is illustrated in Figure 5-20.

c. The 2.88 MB diskette media is a barium ferrite media intended for use in perpendicular recording drives at the

data rate of up to 1 Mbps.

Media Type

Bytes in Data

Field (decimal)

Bytes-Per-Sector

Code (hex)

End of Track (EOT)

Sector # (hex)

Bytes in Gap 2 a

(hex)

Bytes in Gap 3 b

(hex)

360 KB

512

1.2 MB

512

720 KB

512

1.44 MB

512

2.88 MBc

512

First Command Phase Byte, Opcode

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.

Second Command Phase Byte

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

These bits indicate which logical drive is active. They re-

flect the values of bits 1,0 of the Digital Output Register

(DOR) described on page 72 and of result phase status

registers 0 and 3 (ST0 and ST3) described on pages 81

and 83, respectively.

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 selected.

Bit 2 - Head Select (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 signal.

This bit reflects the value of bit 3 of Status Register A

(SRA) described on page 70 and bit 2 of result phase

status registers 0 and 3 (ST0 and ST3) described on

pages 81 and 83, respectively.

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

lects side 0. (Default)

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

side 1.

Third Command Phase Byte -Bytes-Per-Sector Code

This byte contains a code in hexadecimal format that in-

dicates the number of bytes in a data field.

Table 5-11 shows the number of bytes in a data field for

each code.

TABLE 5-11. Bytes per Sector Codes

Fourth Command Phase Byte - Sectors Per Track

The value in this byte specifies how many sectors there

are in the track.

Fifth Command Phase Byte - Bytes in Gap 3

The number of bytes in gap 3 is programmable. The

number to program for Gap 3 depends on the data

transfer rate and the type of the disk drive. Table 5-12

shows some typical values to use for Gap 3.

Figure 5-20 illustrates the track format for each of the

formats recognized by the FORMAT TRACK command.

Sixth Command Phase Byte - Data Pattern

This byte contains the contents of the data field.

Execution Phase

The system transfers four ID bytes (track number, head

number, sector number and bytes-per-sector code) per sec-

tor to the Floppy Disk Controller (FDC) in either a DMA or a

non-DMA mode. Section 5.4.2 on page 78 describes these

modes.

The entire track is formatted. The data block in the data field

of each sector is filled with the data pattern byte.

Only the first three status bytes in this phase are significant.

Bytes-Per-Sector Code (hex)

Bytes in Data Field

128

256

512

1024

2048

4096

8192

16384

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