ACT apricot - Page 121

Number
of bytes
required
80
12
3
1
50
12
3
1
1
1
1
1
1
22
12
3
1
512
1
84
598**
FLOPPY DISK INTERFACE
Track Image
Byte Description
Value
(Hex)
4E
Gap
00
Sync
FE
Control Bytes PREAMBLE
FC
Index Mark
4E
Gap
00
Sync
F5
Control Bytes
FE
ID Address Mark
00 to 45* Track Number
00 or 01 Side Number
01 to 09 Sector Number
SECTOR
02
F7
4E
00
F5
FB
00
F7
4E
4E
Sector Length (Repeated 9
CRe character command times
Gap incrementing
Sync the Sector
Control Bytes Number each
Data Address
Mark
time)
Data Bytes
eRe character command
Gap
Gap
POSTAMBLE
* For 70 track disks.
** Nominal figure, write operation
continues until terminated by the
INTRQ output, on detecting the
Index
Pulse.
Some of the bytes supplied to the Data Register are not
written onto the disk, but act as control bytes to the FDC.
These are the three bytes F5H, F6H and F7H. F5H
commands the FDC to perform two functions; write a
unique byte pattern corresponding to Al with a missing
clock transition, onto the disk; initialize the CRC generator
circuitry. F6H commands the FDC to write a unique pattern
corresponding to C2H with a missing clock transition onto
the disk. These patterns enable the FDC to interpret the
byte following the three byte pattern as an address mark,
during decoding operations. The byte F7H commands the