PC87338 datasheet(112/221 Pages) NSC

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3.6.16 The SCAN EQUAL,

the SCAN LOW OR EQUAL and

the SCAN HIGH OR EQUAL

Commands

The scan commands compare data read from the disk

with data sent from the microprocessor. This compar-

ison produces a match for each scan command, as

follows, and as shown in Table 55:

• SCAN EQUAL - Disk data equals microproces-

sor data.

• SCAN LOW OR EQUAL - Disk data is less than

or equal to microprocessor data.

• SCAN HIGH OR EQUAL - Disk data is greater

than or equal to microprocessor data.

Command Phase

SCAN EQUAL

SCAN LOW OR EQUAL

SCAN HIGH OR EQUAL

First through Eighth Command Phase Bytes -

All Scan Commands

See READ DATA command for a description of the

first eight command phase bytes.

Ninth Command Phase Byte, Sector Step Size

During execution, the value of this byte is added to the

current sector number to determine the next sector to

read.

Execution Phase

The most significant bytes of each sector are com-

pared first. If wildcard mode is enabled in bit 4 of the

fourth command phase byte in the MODE command

(page 102), an FF (hex) from either the disk or the mi-

croprocessor always causes a match.

After each sector is read, if there is no match, the next

sector is read. The next sector is the current sector

number plus the Sector Step Size specified in the

ninth command phase byte.

The scan operation continues until the condition is

met, the End of Track (EOT) is reached or the Termi-

nal Count (TC) signal becomes active.

Read error conditions during scan commands are the

same as read error conditions during the execution

phase of the READ DATA command. See page 106.

If the Skip Control (SK) bit is set to 1, sectors with de-

leted data marks are ignored.

If all sectors read are skipped, the command termi-

nates with bit 3 of ST2 set to 1, i.e., disk data equals

microprocessor data.

7654

3210

MFM

0001

IPS

DS1

DS0

Track Number

Head Number

Sector Number

Bytes-Per-Sector Code

End of Track (EOT) Sector Number

Bytes Between Sectors - Gap 3

Sector Step Size

7654

3210

MFM

1001

IPS

DS1

DS0

Track Number

Head Number

Sector Number

Bytes-Per-Sector Code

End of Track (EOT) Sector Number

Bytes Between Sectors - Gap 3

Sector Step Size

7654

3210

MFM

1101

IPS

DS1

DS0

Track Number

Head Number

Sector Number

Bytes-Per-Sector Code

End of Track (EOT) Sector Number

Bytes Between Sectors - Gap 3

Sector Step Size

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