Experimental Physics and Industrial Control System
Subject: |
Logical Naming of Hardware Addresses |
From: |
Andrew Johnson <[email protected]> |
Date: |
Wed, 25 May 1994 13:09:35 BST |
Hi,
I will be writing a driver for the VMIC 5578 Reflective Memory card (based on
and extending Bob Daly's 5576 driver) and am looking into various questions
about the functionality. One question which I am looking at is how to allocate
space within the Reflective Memory for the records which use this device. My
current solution may have a more general application than just to the Reflective
Memory card, and I present it here for comments.
The existing 5576 driver uses a Page Number + Byte Offset addressing scheme,
which is appropriate given that it only supports Analogue I/O records. My
driver will support other record types as well, which will take up differing
amounts of space in memory. I am considering whether the addressing scheme
could/should be changed or enhanced to avoid the difficulties of having to
calculate the Byte Offset value manually and to ensure that the same addresses
are used in all the IOCs.
The approach I am suggesting is to have a file on the host which is read at
initialisation and contains some form of Logical Name to Physical Address
translation table. The address itself could be explicitly stated, or implicit
based on the position within the file and the previously declared names & types.
The database designer would be able to use a name in place of the address, and
the driver would do a name lookup at record initialisation time to find the
physical address. It should be possible to mix & match names with old-style
addresses as desired.
I can see advantages in using this scheme for devices other than just the
Reflective Memory - the physical configuration data is no longer held in the
database, so it should be possible to separate logical (database) functions from
physical addresses. I plan to include the name resolution in the driver support
code for the 5578 driver. If it could be moved to record support this could be
even more flexible, but (ignoring the amount of work needed in changing the
record types) I'm not sure if it is possible to change the device type in this
way.
Anybody got any comments on this? Would it work with the existing database
tools?
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[email protected] Royal Greenwich Observatory
Tel +44 223-374823 Fax +44 223-374700 Cambridge, UK
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