From: ian@syacus.acus.oz.au (Ian Joyner)
Subject: Re: Info about A Serie
Organization: ACUS Australian Centre for Unisys Software, Sydney
Date: Fri, 16 Oct 1992 06:34:01 GMT

Thanks to Bob Hale for being a bit more precise about my fairly quick
overview of A Series architecture.

hale@btree.uucp (Bob Hale) writes:

>In article <1992Oct13.015615.24672@syacus.acus.oz.au> ian@syacus.acus.oz.au (Ian Joyner) writes:

>>The architecture is also known as EMode architecture. There are
>>several (compatible) levels of this, alpha, beta and gamma.

>The different Emode levels are not fully compatible.  Three levels
>exist because of the incompatibility.  It is possible to transition
>from alpha to beta, and later from beta to gamma.  You cannot mix
>alpha and gamma, at least not without obtaining "unpredictable
>results".

These levels relate to generations. So an alpha machine is quite old (ten
years). You probably would not migrate a program from an alpha machine
to a gamma machine without recompiling. Otherwise the compatibility
between them is very impressive.

>The MCP (operating system) and many of its supporting libraries are
>written in NEWP which is a language similar to Algol but which
>offers direct access to critical machine resources.  Much of the
>other system software is written in languages which are appropriate
>to the task - e.g., the Algol compiler is written in Algol.

I didn't want to mention NEWP, but since you bring it up. The original
MCP was written in ESPOL (executive systems programming oriented
language). NEWP extends ALGOL even further, and has a very neat
module concept. Different modules will be loaded for different
machines, giving them a kind of object-oriented polymorphism. For
example the PHYSICALIO module is different for the smaller and
larger machines.

>You can do *almost* anything you want in NEWP, but even NEWP has its
>limitations.  For example, you cannot insert a NOP after a STOD if
>you need one.  This is the reason why I wrote an assembler for the
>large systems.

>> The machine architecture is therefore
>>a run time environment for Algol.

>More or less true.

Yes I know it's a simplification :-)

>> The basis of the architecture is
>>a hardware stack.

>Actually, the architecture is based on numerous stacks.  Generally,
>each task will have one code stack and one data stack.  The use of
>libraries will increase the number of stacks associated with a task.
>When I last looked, the machines could handle up to 4096 stacks.

The number of stacks has changed between EMode levels, so it may
again be different in future levels.

>Perhaps we have a different definition of "user program".  A NEWP
>program can easily modify a machine instruction word.  This is not
>to say that I recommend that you do this, but it is possible.

Let me qualify it as user 'written' program. Users don't often
write programs in NEWP. Since NEWP allows you to do more than the
average ALGOL program, many sites don't leave NEWP compilers around
for people to play with.

>A good introduction to the Burroughs architecture can be found in
>the book "Computer System Architecture [Organization?]" by Organick.
>Things have changed a little since he wrote the book but the principles
>are still the same.  The greatest difference is that memory accesses
>are now done indirectly and automatically by the hardware using a
>concept called Actual Segment Descriptors.  This overcomes the
>1 megaword memory size limitation.

Glad you mentioned Organick. There is also another book: "Computer
Architecture: A Structured Approach" by R.W. Doran (Academic Press 1979).
Both these books are quite old, but still quite relevant as a philosophical
and conceptual background.

>It's a very interesting and powerful architecture.  Too bad that it
>didn't capture a larger market share.

Don't say didn't, past tense. The A Series line is still alive and
kicking, and is capturing an ever increasing market share, in times
when the economic situation is bad, and the mainframe is supposed to
be dead. But then A Series is not really a mainframe. It is a computer,
some models of which happen to be in the mainframe power range, and
some of which happen to be in the desktop range (micro A). Who needs
desktop, workstation, mini, mainframe distictions anyway. That is
quite an artificial notion, made up to easily segment the market ;-)


-- 
Ian Joyner  ACUS (Australian Centre for Unisys Software)    ian@syacus.acus.oz
"Where is the man with all the great directions?...You can't imagine it,
 how hard it is to grow, Can you imagine the order of the universe?" ABWH
Disclaimer:Opinions and comments are personal.