Alto
Hardware Manual
Section
6:
Disk and Controller
44
controller idles.
At
the beginning
of
each
sector, status information, including the number
of
the current
sector,
is
stored in
KBLK
+
1.
. This can be used
by
the Alto program to sense the readiness
of
the disk
and
to
schedule disk transfers,
for
example. When the disk controller begins executing a command, it
stores the disk address
of
that command in
KBLK
+
2.
This information
is
later used
by
the disk
controller
to
decide whether
seek
operations or
disk
switches are necessary. It can
be
used
by
the Alto
program
for
scheduling
disk
arm
motion.
If
the
Alto
program stores an illegal disk address (like -1) in
this word, the
disk
controller
will
perform a
seek
at the beginning'
of
the next disk operation. (This is
useful, for example, when a
disk
driver
wants
to
force
a restore operation.) The disk controller also
communicates
with
the
Alto
program
by
interrupts
(see
Section
3.2).
At the beginning
of
each sector
interrupts are initiated on the channels specified by the bits in
KBLK
+
3.
KBLK
(521B):
Pointer
to
first disk command
block.
KBLK
+ 1
(522B):
Status at beginning
of
current sector.
KBLK+2
(523B):
Disk
address
of
most-recently started disk command.
KBLK+3
(524B):
Sector interrupt bit
mask.
A disk command block
is
a ten-word block
of
memory
which
describes a disk transfer operation to the
disk controller, and which
is
also
used
by
the controller to record the status
of
that operation. The first
word
is
a pointer to the next
disk
command block in this chain. A 0 means that this
is
the last disk
command block in the chain. When the command
is
complete, the disk controller stores its status in the
second
word.
The third word contains the command itself, telling the disk controller what
to
do.
The
fourth word contains a pointer
to
the block
of
memory from/to which the header block
will
be
transferred. The fifth word contains a similar pointer for the label block. The sixth word contains a
similar pointer for the data block.
The seventh and eighth
words
of
the
disk
command block control the initiation
of
interrupts when the
command block
is
finished.
If
the command terminates without error, interrupts are initiated on the
channels specified by the bits in
DCB+6.
However,
if
the command
term..inates
with an error, the bits
in
DCB
+ 7 are used instead.
The ninth word
is
unused
by
the disk controller, and may be used by the Alto program
to
facilitate
chained disk operations. The tenth word contains the disk address at which the current operation
is
to
take place.
DCB:
DCB+l:
DCB+2:
DCB+3:
DCB+4:
DCB+5:
DCB+6:
DCB+7:
DCB+8:
DCB+9:
Pointer
to
next command block.
Status.
Command.
Header block pointer.
Label block pointer.
Data pointer.
Command complete no-error interrupt bit
mask~
Command complete error interrupt bit
mask.
Currently unused.
Disk address.
A disk address word A contains the
following
fields:
FIELD
RANGE
A[O-3]
O-13B
A[4-12]
0-625B
(Model
44)
0-312B
(Model
31)
A[13]
0-1
A[14]
0-1
SIGNIFICANCE
Sector number.
Cylinder number.
Head number.
Disk number (see also
C[15]).
0
is
removable pack
on
Model
44.
1
is
optional second Model
31
drive.
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