ACT apricot - Page 119

FLOPPY DISK INTERFACE
Each of the three commands contain an Update SSO bit.
This bit is used to select the disk side for the formatting
operations and affects the logic state of the Side Select
Output (SSO), supplied to the disk drive. When U is set to
logic low, SSO is updated to logic low (side 0). When U is set
to logic high, SSO is updated to logic high (side 1).
Track Format. The track format used on the MicroFloppy
disks is illustrated at the end of the chapter.
Read Address. On receipt of the Read Address command,
the FDC searches the disk for an lD field. The first lD field
encountered is then read from the disk. The FDC transfers
the six lD field bytes detailed below, from the disk to the
Data Register one byte a time. Every time a data byte is
stored in the Data Register, the FDC generates a DMA
request to the lOP. The lD field track number is also
transferred
to the Sector Register.
Byte
Description
1
Track Number
2
Side
Number
3
Sector Number
4
Sector
Length (02H)
5
CRC 1
6
CRC2
If the FDC fails to locate an lD field within five
revolutions of the disk, the command is aborted and the
Record Not Found bit in the Status Register set. If the lD
field contains an incorrect CRC character, this is also
recorded in the Status Register.
Failure of
the lOP to read the contents of the Data
Register, before the register is overwritten with the next
byte of data from the disk, causes the FDC to set the Lost
Data bit in the Status Register.
Read Track. The Read Track command allows a complete
track of information (Gaps, Headers and Data bytes) to be