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)...
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17.
Addressing. The smallest addressable unit is a sector (64 words).
A block of" contiguous sectors (90 maximum) may be accessed without rein-·
itialization of the Direct Memory Access (DMA) channel or the Disc Controller.
A block must not overlap track origin (sector 000).
If
the block of words
transferred via DMA is not modulo 64 words, the last word of the block will
be copied into the remainder of the last sector. Though it is possible to
r~ad
a block of any length,
it
is not possible to write a block containing only
one word, that is, a block of length 1 modulo 64. Any transfer that attempts
to put one word in a sector is not possible. For example, write transfers of
the following lengths are not possible: 1 word
=
0
X
64
+
1; 65 words::; 1
X
64
+
1; 129 words
=
2
x64 +
1.
IS.
Parity. Parity (17th bit) is generated during write and checked during
read. The results of the check are available via the Disc Controller status.
19.
Sector Access. The Disc Controller may be interrogated for disc·
status, which includes the rotational position (sector under head) of the disc.
The indicated sector, or following ones, may then be accessed on the same
revolution with the probability/delay as indicated in Figure 2.
If
the sector
flag is set, the current sector cannot be accessed on the same revolution. ,
SECTORN
~.
1.0
S- - --- - - -- --- - --- -- -- -- _...
.
SEC TOR N+f --_ .....
.
.....
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...
.8
. 6
.4
:t
0
12606-9001
...........
'4"
.....
... ...
.....
...
... ...
.....
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...
380 400
APPROXIMATEMICROS£CONOS OELAYFROM
ADDRESS DETECTION TO INIT/ATIONOF TRANSFER
Figure 2. Sector Access Probability/Delay Time
780
6
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