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Section III
indicates whether reading or writing will take place. The
LIA/B instruction acquires a disc status word from the disc
system. The CLF and SFS instructions are used principally
for equipment diagnostic purposes, and indicate when the
track origin is reached 'or passed. The SFC instruction
checks the state of the SCP flip-flop on the disc command
card; this instruction is used only for equipment trouble-
shooting programming.
3-15.
DATA CHANNEL I/O SELECT CODE.
3-16.
Two instructions use the data channel I/O select
code; these are the STC and CLC instructions. The STC
instruction initiates the transfer of data to or from the disc
after preliminary instructions have established the disc and
core-memory locations to be used. The STC instruction
must come after the OTA/B instruction that furnishes the
control word containing the disc starting address. However,
the time between the OTA/Band STC instructions can be
any time that is convenient. The CLC instruction is used to
abort a disc read or write operation by cutting off data
transfer during the course of the operation.
3-17.
TIMING.
3-18.
Timing characteristics of the disc are as follows (all
figures are approximate because disc speed may vary
slightly from the nominal amount):
a. Speed: 3450 RPM for a disc operating from a 60-Hz
power line, 2880 RPM for a 50-Hz power line.
b. Maximum access time (time for two disc revolu-
tions):
34.8 milliseconds for the 60-Hz disc, 41.6
milliseconds for the 50-Hz disc.
c. Average access time: 17.4 milliseconds for the
60-Hz disc, 20.8 milliseconds for the 50-Hz disc.
d. Word transfer rate to or from the disc:
(1) For the 60-Hz disc, 6.0 microseconds per word,
375 microseconds per sector, 17.4 milliseconds
per 90-sector track (5760 words), 165 words
per millisecond.
(2) For the 50-Hz disc, 7.2 microseconds per word,
450 microseconds per sector, 20.8 milliseconds
per 90-sector track (5760 words), 135 words
per millisecond.
3-19.
The address of the next sector to pass under the
read/write head forms part of the disc status word. By
examining the next-sector address, the program can take
action that will most f!fficiently utilizEl. time. For instance,
if disc data are to be read from two groups of sectors, the
sectors that will be first to pass under the read/write head
can be read first. Another use of the next-sector address is
to determine the next sector or sectors available for writing.
3-20.
The address indicated for the next .sector could
immediately precede an address in which reading or writing
3-2
12606B
is desired. However, the time required for execution of read
or write instructions may make it impossible to access the
desired sector until the following disc revolution. A flag in
the disc status word is set to logic 1 when it is impossible'to
access the next sector in the current disc revolution. When
the flag is logic 0, access to the next sector may be possible,
depending on how many read or write instructions are
required and on how close the sector is to the read/write
head. Since there is no indication of the distance of the
sector from the read/write head, obtaining access to the
sector in the same disc revolution can be expressed only
as
a probability. Figure 3-1 shows this probability. The hori-
zontal axis of the graph is the time from the end of the
LIA/B instruction which acquires the disc status word, to
the end of the STC instruction which initiates a transfer of
data to or from the disc. Included in figure 3-1 is a line
showing the probability of accessing sector N + 1, where N
is the next sector. As the illustration shows, the access
times are dependent on the power line frequency for which
the disc is designed.
~
1.0
w
CJ
.8
CJ
«
U-
.6
0
>
....
.4
:i
iii
.2
«
III
0
0
II:
0
II.
-------~
,
,
355* 375*
425** 450**
',SECTOR N
+
1
,
,
,
, ,
730*
875**
APPROXIMATE MICROSECONDS DELAY FROM
ADDRESS DETECTION TO INITIATION OF TRANSFER
NOTES:
*
For 6q-Hz power line
** For 50-Hz power line
2032-1
Figure 3-1. Probability of Sector Access in Current
Disc Revolution
3-21.
PARITY.
3-22.
Each word written on the disc consists of 16 data
bits and one parity bit. The parity bit is generated when the
word is written, and parity is automa.tically checked each
time the word is read. Occurrence of a parity error is
indicated by a flag in the disc status word.
3-23.
DISC STATUS WORD.
3-24.
The disc status word is a 16-bit word which can be
acquired from the disc system by the LIA/B instruction.
The instruction must use the disc command channel I/O
select code.
3-25.
The format and content of the disc status word
are
shown in table 3-2. The various parts of the word
are
updated at different times.
3-26.
Bits 3 and 1 of the disc status word are reset to
logic 0 by an STC instruction with the disc data channel
I/O select code. Since this instruction starts a transfer of
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