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ST-506 MFM and RLL web servbers
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Academy of Art University American largely privately accredited art university Academy of maths University Academy of science Unive... -
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Saturday, October 17, 2015
wifi network - CAD/CAM
The situation is so complex that apart from specialized number crunching applications such as statistics and engineering which clearly need one, it is possible to give only very broad guidelines about when a co-processor might be useful or not. To decide about the ‘sometimes’ applications in the following table you need to look carefully at the way that they actually work. For example, the DTP package Ventura makes specific mention of the fact that it doesn’t need. Or indeed make any use of a numeric co-processor.
Academy of Art University
American largely privately accredited art university
Academy of maths Universitys
Academy of science Universality s
Online degrees
Education of degrees
Higher education it degrees
Higher education Human recourse degree
Application Co-processor needed?Word processing No
Database No
Spreadsheet Sometimes
Graphics Sometimes
CAD/CAM Yes
DTP Sometimes
Data link layer ?
One of the
difficult things about trying to quantify the effect of adding a numeric
co-processor is that not only do different applications use numeric
co-processors to different extents, but a single applications use numeric
co-processors to different extents, but a single application will vary in its
use of a co-precessor depending on the job it is tackling. For example, it is
generally held that spreadsheet programs are heavy co-processor users but it
isn’t difficult to find examples of spreadsheets that do virtually no floating
point arithmetic! For example, a spreadsheet that has been constructed to act
as a database is very unlikely to perform any arithmetic worth installing a
co-processor for.
To find out the
likely impact of a numeric co-processor, estimate how much time your program
spends in floating point calculation. Even in situations where the application
appears to do nothing but arithmetic the estimation can be much more difficult
than you might expect. For example. Suppose you have a spreadsheet consisting
of numeric data and formulae and it takes 30 minutes to recalculate. A simple
minded approach would suggest that if you add a mumeric co-processor that does
floating point calculations five times faster than software the calculation
would only take six minutes. If you try this out you will usually be
disappointed. The reason is that the 30 minutes to do the calculation isn’t
occupied solely by floating point calculations-data has to found and results
returned to the correct locations. In all probability no more than 75% of the
30 minutes is spent in performing floating point calculations and this is the
only portion of the calculation time that can be reduced by the addition of a
co-processor. In this case the calculation time would go down to only 12
minutes instead of the promised six.
Saturday, October 3, 2015
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The most common solution to this problem is to include a special SCSI BIOS ROM on the controller itself. This will usually contain the software that identifies the device connected to the SCSI interface as a disk drive and will even boot the system from the drive. In this case the usual system BIOS has no part to play in handling the disk drive and the usual setup procedure should be used to set the drive type to “none”. If an ST-506, IDE or ESDI drive is used at the same time as a SCSI drive then it is usual for it not to be used to boot the system. The SCSI BIOS usually scans the SCSI bus when the machine is first switched on by sending message asking each device ID in turn to identify itself. There is usually a limit on the number of hard disks supported typically two. If you need more then you have to make use of additional software drivers. The devices are scanned in order of ID number, 0 to 6 and the first device to identify itself as a hard disk is used to boot the system. After this SCSI hard disk behave like normal hard disks but low level formatting and advanced diagnostic programs generally don’t work unless you obtain a version specifically written for SCSI drives.
ST-506 MFM and RLL web servbers
Although the ST-506
drive is no longer the first choice foe
even moderate capacity single-user
systems there are a great many in use and
it still represents a well tried
standard. Indeed , real problem
with the ST-506 is that is interface
with the controller is so well defined
that there is little scope for
improvement. If you where to try to
increase the data transfer rate
say then the result wouldn’t be an ST-506 drive and it wouldn’t work with any
of the many ST-506 disk controllers!
There are two slightly
different types of ST-506 drivers – MFM
and RLL. Indeed these two designation are often used as alternative names for
ST-506 drivers. The difference between
the two types is to do with the
way that
data is coded onto the disk as magnetic
pulses or flux reversals.MFM
(Modified Frequency Modulation) uses an average of 75 flux reversals per bit
whereas RLL (Run Length Limited) coding uses an average of 25 flux reversals.
This means that an RLL drive/controller
combination will store more data in the same space. Strangely enough this increase
in storage capacity isn’t of much interest to and end user (it is to a
disk drive manufacturer however!) The reason being that if I offer you two
drivers that store 100 Mbytes, does it really
matter how this is achieved?
What is really important about RLL drivers it that they offer a potentially faster data
transfer rate to MFM drivers. (There is
a subsidiary issue of reliability its often
said that RLL drivers are less
reliable than MFM drivers because they
store data at a greater recorded-bit density. In practice this difference seems
not to matter as long as the drive is
indeed an RLL certified drive and
not an MFM drive that has been
pr1essed into service as a cheap way of gaining 50% more storage.)
The reason that RLL drives are faster is that a
cylinder represents are amount of data
that can be read in one
revolution of the drive, i.e the amount
of data that can be read in roughly 16ms
in the case of most PC hard
disks. Thus the larger the capacity of a
cylinder or track the faster the
potential data transfer and RLL
coding crams more data into each track.
The maximum data transfer rates
achievable with MFM and RLL are 0.6 and 0.9 MByte/s respectively.
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