Disk And Controller - Xerox Alto I Hardware Manual

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