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I
CHAPTER ZERO
Access Time - Finding The Right Sector
The process of finding the correct track is called "seeking".
It
takes a small
amount of time forthe head to jump from track to track as it seeks the correct one
and this is called the "track step-time". The drive used in the H89 has a guaran-
teed track step-time of
30
milliseconds. "Test" will explain how to determine
your drive's optimum time.
Another factor that detemines system speed is called "rotational latency". Since
the diskette rotates at
300
rpm, the system may have to wait as long as
200
milliseconds for the desired sector to rotate under the head after it finds the
correct track. This latency period averages
100
milliseconds for a random disk
access.
A little arithmetic, then, shows that up to
500
milliseconds will be required to
access a random sector. This depends on the track step-time of the particular
drive; and it is based on calculations which show that one-third of the tracks will
have to be skipped over, on the average, for a random disk access. The average
drive will require about
225
milliseconds for every access of a random sector. If
many sectors are accessed and if they are close to each other, the average access
time will be much lower.
As soon as the system finds the correct sector, it reads or writes the data, one bit at
a time (serially) at a rate of
16,000
bytes per second. The data is transferred
between the drive and the controller. The controller converts the serial data into
a parallel format, and communicates it by means of an 1/0 port to the CPU.
As an example of its quick access and high performance, the H89 system can load
Extended Benton Harbor BASIC into memory in about two seconds. The Heath
cassette system, operating at
1200
baud, requires about two minutes to load from
the beginning of the tape.
The Software System
The sector, because it is the smallest amount of data that the system can transfer
between the disk drive and memory in a single access, is the basic unit of the
diskette. The disk system is "hard-sectored". This means that the size and
location of each sector is determined by the disk controller board rather than by
the operating system. This is actually where HDOS comes into play.
It
must
communicate with hardware devices, such as a disk drive controller. On the
other side of the interface, it must also communicate with you, the "user" or
"operator", by means of programmed instructions or console commands.
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