PC87310 datasheet(31/52 Pages) NSC

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40 Functional Description (Continued)

447 Command Descriptions

READ DATA

The Read Data op-code is written to the data register fol-

lowed by 8 bytes as specified in the Command Description

Table After the last byte is written the controller starts

looking for the correct sector header Once the sector is

found the controller sends the data to the mP After one

sector is finished the Sector Number is incremented by one

and this new sector is searched for If MT (Multi-Track) is

set both sides of one track can be read Starting on side

zero the sectors are read until the sector number specified

by End of Track Sector Number is reached Then side one

is read starting with sector number one

In DMA mode the Read Data command continues to read

until the TC pin is set This means that the DMA controller

should be programmed to transfer the correct number of

bytes TC could be controlled by the mP and be asserted

when enough bytes are received An alternative to these

methods of stopping the Read Data command is to program

the End of Track Sector Number to be the last sector num-

ber that needs to be read The controller will stop reading

the disk with an error indicating that it tried to access a

sector number beyond the end of the track

The Number of Data Bytes per Sector parameter is defined

in Table 4-15 If this is set to zero then the Data Length

parameter determines the number of bytes that the control-

ler transfers to the mP If the data length specified is smaller

than 128 the controller still reads the entire 128 byte sector

and checks the CRC though only the number of bytes spec-

ified by the Data Length parameter are transferred to the

P Data Length should not be set to zero If the Number of

Bytes per Sector parameter is not zero the Data Length

parameter has no meaning and should be set to FF (hex)

If the Implied Seek Mode is enabled by both the Mode com-

mand and the IPS bit in this command a Seek will be per-

formed to the track number specified in the Command

Phase The controller will also wait the Head Settle time if

the implied seek is enabled

After all these conditions are met the controller searches

for the specified sector by comparing the track number

head number sector number and number bytessector giv-

en in the Command Phase with the appropriate bytes read

off the disk in the Address Fields

If the correct sector is found but there is a CRC error in the

Address Field bit 5 of ST1 (CRC Error) is set and an abnor-

mal termination is indicated If the correct sector is not

found bit 2 of ST1 (No Data) is set and an abnormal termi-

nation is indicated In addition to this if any Address Field

track number is FF bit 1 of ST2 (Bad Track) is set or if any

Address Field track number is different from that specified in

the Command Phase bit 4 of ST2 (Wrong Track) is set

After finding the correct sector the controller reads that

Data Field If a Deleted Data Mark is found and the SK bit is

set the sector is not read bit 6 of ST2 (Control Mark) is set

and the next sector is searched for If a deleted data mark is

found and the SK bit is not set the sector is read bit 6 of

ST2 (Control Mark) is set and the read terminates with a

normal termination If a CRC error is detected in the Data

Field bit 5 is set in both ST1 and ST2 (CRC Error) and an

abnormal termination is indicated

If no problems occur in the read command the read will

continue from one sector to the next in logical order (not

physical order) until either TC is set or an error occurs

If a disk has not been inserted into the disk drive there are

many opportunities for the controller to appear to hang up It

does this if it is waiting for a certain number of disk revolu-

tions for something If this occurs the controller can be

forced to abort the command by writing a byte to the Data

for the current command that is being executed The soft-

ware should then execute the Result Phase activity (ie for

the Read Data Command it should read 7 status bytes)

TABLE 4-15 Sector Size Selection

BytesSector

Number

Code

of Bytes in

Data Field

128

256

512

1024

2048

4096

8192

An interrupt will be generated when the Execution Phase of

the Read Data command terminates The values that will be

read back in the Result Phase are shown in Table 4-16 If an

error occurs the result bytes will indicate the sector being

read when the error occurred

READ DELETED DATA

This command is the same as the Read Data command

except for its treatment of a Deleted Data Mark If a Deleted

Data Mark is read the sector is read normally If a Regular

Data Mark is found and the SK bit is set the sector is not

read bit 6 of ST2 (Control Mark) is set and the next sector

is searched for If a Regular Data Mark is found and the SK

bit is not set the sector is read bit 6 of ST2 (Control Mark)

is set and the read terminates with a normal termination

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