Data Transfer And Diskette Simultaneity; Basic Track Format - Honeywell LEVEL 6 Operation Manual

DATA TRANSFER AND DISKETTE SIMUL­
TANEITY
Diskettes attached to an MDC are software
addressable via channel numbers. Each individual
diskette has two channel numbers assigned, differ­
ing from each other only in the low order bit
position called the direction bit. When a data
transfer I/O command is issued to a bidirectional
device such as the diskette, the direction bit of
the channel number will specify whether the
transfer of data is input or output. For all other
commands the direction bit is ignored by hard­
ware (see Figure 5-4).
CHANNEL NUMBER
fr----------~A~------------,
I I
I
: MDC PORT NUMBER
~
1\
L
I I I I I I I II
v
DIRECTION BIT
SWITCH SELECTABLE MDC O-READ (INPUT)
IDENTIFIER l-WRITE (OUTPUT)
Figure 5-4. Channel Assignments
A basic diskette subsystem consists of a single
adapter which supports up to two diskette de­
vices. To the software, the two diskettes appear
completely independent of one another in that a
command will be accepted by a channel that is
INDEX
GAP 1
GAP2
00
100 OOIAIOO
~O
00
~
00 00
1----46
BYTES~
f---32 BYTES'"
not busy. This is because the MDC circuitry is
shared simultaneously by both diskettes. The
MDC91 0 I controller will accept commands for
one diskette while the other diskette is in the
process of transferring data. However, the transfer
of data on one diskette must be completed before
data can be accepted and processed by the
remaining diskette. All other operations are ex­
ecuted concurrently. If an MDC is configured
with less than four peripherals, it will respond to
channel numbers associated only with the devices
installed.
All data transfers are executed in the DMA
mode and are normally in word mode but byte
mode transfers can occur if associated with the
first and/or last memory cycle of a particular data
transfer provided that the main memory buffer
begins or ends on an odd byte boundary.
BASIC TRACK FORMAT
Each track on the diskette contains up to 26
equal length sectors of 128 bytes each. There are
77 tracks numbered from· zero (0) through 76
yielding a total formatted capacity of 256,256
bytes. The data encoding scheme is double
frequency recording and each field is preceded by
an AM (Address Mark) and followed by a two
byte EDC (Error Detection Code). The basic track
format is shown in Figure 5-5.
INDEX
GAP4
0001
000
~
VARIABLE-----..j
...... . . _ - - - - - - - - - - - - - - - - - - - - - - SECTORS l-M 1M::; 26)
-------------------------------1 ... _
SECTOR
ID
GAP3
EDC
00
00
GAP2
00
00
2 BYTES
r--
32 BYTES
----J
C = CLOCK BIT
D ~ DATA BIT
C D C D C D C D C D C D C D C D
INDEX ADDRESS MARK (AM4)
SECTOR I D ADDR ESS MAR K (AM 1)
DATA FIELD ADDRESS MARK (AM2)
DELETED DATA ADD. MARK (AM3)
DISKETTE DEVICE
AT04
Figure 5-5. Basic Track Formats
5-3