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Alto Hardware Manual
Section 6: Disk and Controller
46
STRT.
Before setting
KBLK,
the Alto program should put a
JMP STRT
instruction in
STRT;
afterward it
should jump to
STRT.
The disk controller transfers data downward, from high to low addresses, so that
when location'
STRT
is changed the reading of the block is complete. (See section 3.4 on the standard
bootstrap loading microcode.)
Another trick is to chain disk reads through their label blocks. That is, the label block for sector n
contains part of the disk command block for reading sector n
+
1,
and so on.
6.1
Disk Controller Implementation
The following walk-through of an average day in the life of the standard disk controller is not intended
for the casual reader, but rather as a roadmap to ease the pain of learning the innermost workings of the
controller. If you really want to benefit from this next section, you should have a copy of the standard
disk controller microcode and logic drawings close at hand.
The disk controller consists of a modest amount of hardware and two microcode tasks (the sector task
and the word task). Communication with the emulator is via the four special main memory words, the
disk command blocks, and the interrupts described earlier. In following few paragraphs the actions of the
standard disk controller microcode are described. Occasionally it may be unclear whether the actor is
microcode or hardware. Referring to microcode listings and/or logic drawings will resolve any such
questions.
The sector task is awakened by a sector signal from the disk. When awakened, it stores the status of the
disk and controller in the special disk status word
(KBLK +
1).
In addition, if this sector signal terminates
a disk command (for example; a data transfer during the previous sector), the status of the disk and
controller are stored in the status word of the disk command block containing the terminated command,
and the command block pointer
(KBLK)
is advanced. If a command was terminated with an error,
KBLK
(DeB
pointer) is set to 0 and
KBLK
+
2
(current disk address) is set to -1. The effect of this is to cause the
disk controller to abandon the current disk command chain and to forget where the disk arm is
positioned.
'
Next, the sector task considers the first command on the disk command block chain (by using
KBLK).
If
there is none, or if the disk unit is not ready to accept a command, the sector task goes to sleep until the
next sector pulse. Otherwise, the sector specified in the new command is verified to be less than 13.
Then, the disk and cylinder specified in the new command are compared with those stored in
KBLK + 2
(current disk address), and then the new disk address is stored in
KBLK+2
and in the disk controller
hardware. Part of the new command is also stored in the hardware. If the comparison is unequal, a seek
is initiated and the sector task goes to sleep until the next sector pulse.
If the comparison was equal, the
SEEKOK
hardware flag is tested. If that is
OK,
then the no-transfer bit
of the disk command (bit
14
of the command word of the current disk command block) is tested to see
whether a data transfer is required. If not, the sector task goes to sleep such that the command will
terminate at the next sector pulse. If a data transfer is required, the specified sector number and the
current disk sector number are compared.
If
unequal, the sector task goes to sleep until the next sector
pulse. If sector numbers are equal, awakening of the word task is enabled and the sector task goes to
sleep such that the command will terminate at the next sector pulse.
The word task awakens when a word has been processed by the disk controller hardware and the word
task has been enabled by the sector task. First, a starting delay is computed, based on whether the
current record is to be read or written.
Second, control is dispatched based on the current record
number. A record length and main memory starting address are computed based on the record number.
In addition, special starting delays are computed for record number O. The disk unit is set into the delay
mode appropriate for the operation (read/write) and the word task goes to sleep the appropriate number
of times;
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