TMS279X (WD279X) FLOPPY DISK FORMATTER / CONTROLLER FAMILY

23

types III commands

read address

Upon receipt of the Read Address command, the head is loaded and the busy status bit is set. The next

encountered ID field is then read in from the disk, and the six data bytes of the ID field are assembled and

transferred to the DR, and a DRQ is generated for each byte. The six bytes of the ID field are shown below:

TRACK

ADDR

SIDE

NUMBER

SECTOR

ADDRESS

SECTOR

LENGTH

CRC

1

CRC

2

1

2

3

4

5

6

Although the CRC characters are transferred to the computer, the 279X checks for validity and the CRC

error status bit is set if there is a CRC error. The track address of the ID field is written into the sector register

so that a comparison can be made by the host. At the end of the operation, an interrupt is generated and the

busy status is reset.

read track

Upon receipt of the Read Track command, the head is loaded, and the busy status bit is set. Reading starts

with the leading edge of the first encountered index pulse and continues until the next index pulse. All gap,

header, and data bytes are assembled and transferred to the data register and DRQ's are generated for

each byte. The accumulation of bytes is synchronized to each address mark encountered. An interrupt is

generated at the completion of the command.

This command has several characteristics which make it suitable for diagnostic purposes. They are: no CRC

checking is performed; gap information is included in the data stream; the internal side compare is not

performed; and the address mark detector is on for the duration of the command. Because the AM detector

is always on, write splices or noise may cause the chip to look for an AM if an address mark does not appear

on schedule with the lost data status flag being set.

The ID AM, ID field, ID CRC bytes, DAM, data and data CRC bytes for each sector will be correct. The gap

write track formatting the disk

(Refer to section on Type III commands for flowcharts.)

Formatting the disk is a relatively simple task when operating programmed I/0 or when operating under DMA

with a large amount of memory. Data and gap information must be provided at the computer interface.

Formatting the disk is accomplished by positioning the R/W head over the desired track number and issuing

the Write Track command.

Upon receipt of the Write Track command, the head is loaded and the busy status bit is set. Writing starts

with the leading edge of the first encountered index pulse and continues until the next index pulse, at which

time the interrupt is activated. The data request is activated immediately upon receiving the command, but

writing will not start until after the first byte has been loaded into the data register. If the DR has not been

loaded by the time the index pulse is encountered, the operation is terminated making the device not busy,

the lost data status bit is set, and the interrupt is activated. If a byte is not present in the DR when needed, a

byte of zeroes is substituted.

This sequence continues from one index mark to the next index mark. Normally, whatever data pattern

appears in the data register is written on the disk with a normal clock pattern. However, if the 279X detects a

data pattern of F5 thru FE in the data register, this is interpreted as data address marks with missing clocks

or CRC generation.

The CRC generator is initialized when any data byte from F8 to FE is about to be transferred from the DR to

the DSR or by receipt of F5 in MFM. An F7 pattern will generate two CRC characters in FM or MFM. As a

consequence, the patterns F5 thru FE must not appear in the gaps, data fields, or ID fields. Also, CRC's

must be generated by an F7 pattern.

Disks may be formatted in IBM 3740 or System 34 formats with sector lengths of 128, 256, 512, or 1024

bytes.