Alto
Hardware Manual
Section
6:
Disk and Controller
43
6.0
DISK
AND CONTROLLER'
The
disk controller is designed to accommodate one
of
a variety
of
DIABLO
disk drives, including
models
31
and
44.
Each
drive accommodates one
or
two disks. Each disk
has
two heads,
one
per
side.
Information is recorded
on
each
disk in a 12-sector
format
on
each
of
up
to
406
(depending
on
the
disk
model) radial track positions. Thus, each disk contains
up
to
9744
recording positions (2
heads
x 12
sectors x 406 track positions). Figure
7 tabulates various useful information
about
the
performance
of
the
disk drives. .
DEVICE
DIABLO
31
DIABLO
44
Number
of
drives/Alto
1
or
2
1
Number
of
packs
1
removable
1
removable
1
fixed
Number
of
cylinders
203
406
Tracks/cylinder/pack
2 2
Sectors
per
track
12
12
Words
per
sector
2
header
2
header
8
label
8
label
256
data
256
data
Data
words/track
3072
3072
Sectors/pack
4872
9744
Rotation
time
40
25
ms
Seek
time
(approx.)
15
+
8.6*sqrt(dt)
8 +
3*sqrt(
dt)
ms*
min-avg-max
15-70-135
8-30-68
ms
Average
access
to
1
megabyte
80
32
(using
both
packs)
ms
Transfer
rate:
.
peaklavg
1.6/1.22
2.5/1.9
MHz
peaklavg
10.2/13
6.7/8.
Its/word
per
sector
3.3
2.1
ms
for
full
display
.460
.266
sec
for
64k
memory
1.03
.6
sec
whole
drive
19.3
44(both
packs)
sec
*
The
notation
dt
stands
for
the
number
of
tracks
traveled
during
the
seek.
Figure 7
The
disk controller records three independent data blocks in each sector.
The
first is two words long,
and
is intended
to
include
the
address
of
the sector.
This
block is called
the
Header block.
The
second
block is eight words long,
and
is called the Label block.
The
third
block is 256 words long,
and
is
the
Data block.
Each
block may
be
independently read, written,
or
checked,
except
that writing, once begun,
must continue until the end
of
the
sector.
When
a block is checked, information on the disk is
compared
word for
word
with a specified block
of
main memory.
During
checking, a main memory word containing 0
has
special significance.
When
this
word is encountered,
the
matching word read from
the
disk is stored
in
its place
and
does
not
take
part
in the check. This feature permits a combination
of
reading
and
checking
to
occur
in
the
same block.
(It
also has the drawback
of
making
it
impossible to use
the
disk controller
to
check for words containing
o
on
the
disk.)
The
Alto program communicates with the disk controller via a four-word block
of
main
memory
beginning
at
location
KBLK
(521B).
The
first word is
interpreted
as a
pointer
to
a
chain
of
disk
command
blocks.
If
it
contains
0,
the
disk controller will
remain
idle. Otherwise,
the
disk controller will
commence execution
of
the
command
contained in the first disk
command
block.
When
a
command
is
completed successfully, the disk controller stores in
KBLK
a pointer to
the
next
command
in
the
chain
and
the
cycle repeats.
If
a
command
terminates
in
error, a 0 is immediately stored
in
KBLK
and
the
disk
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