Article: 753 of comp.sys.cdc
From: Paul.C.Smith@cdc.com (Paul C. Smith)
Newsgroups: alt.folklore.computers,comp.sys.cdc
Subject: Re: Comparative Speed of CDC 6600 to Current Personal Computers
Date: Mon, 15 Feb 1999 14:56:58 -0600
Organization: Control Data Systems, Inc.
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In article <7a1s7t$nq6$1@nntp1.u.washington.edu>,
dpeschel@u.washington.edu (Derek Peschel) wrote:

)In article
<A16B805DE5A33EDC.F93EAA115A312CF2.B0A9D715D2EED69F@library-proxy.airnews.net>,
)Chuck Maurer <cmaurer@dhc.net> wrote:
)
)>Yes, there was an A0.  I just found my old Compass book!  It says:
)
)One COMPASS book was dedicated to (some of) the registers.  That is, the
)dedication in the front of the book credited the registers.  Am I thinking
)of the one you have?

Ha!  I still have a copy of that book in my bookshelf; more than that, 
I've actually *used* the book within the last 30 days...  :-)

The book is "Assembly Language Programming for the Control Data 6000
and Cyber Series", by Ralph Grishman (1974; Algorithmics, Inc.).
The dedication reads as follows:

           "This book is dedicated to A6 & A7,
            without which none of the results
            in this book could have been saved."

The book is a textbook (as opposed to a reference manual); I inherited
my copy from a programmer who left the group some years ago.

On the subject of A0 and X0:  These registers were not used to load and
store from central memory because they were designed to be used when 
accessing *extended* memory.  

Extended memory on the 6000 (originally ECS, Extended Core Storage; 
later ESM, Extended Semiconductor Memory) was a piece of hardware 
external to the mainframe; in fact, it could be shared among up to 
four mainframes.  Extended memory on the 6000 could only be accessed 
via block transfers to and from central memory.  Before performing a 
block transfer instruction, the program had to set the central memory 
address into A0, and the extended memory address into X0.  The direction 
of the transfer was determined by the opcode, and the length of the 
transfer was specified by Bj+K.

Of course, since most programs did not access extended memory, A0 and X0
were most commonly used as scratch registers, or as temporary storage.

Paul C. Smith
NOS Customer Support
Control Data Systems, Inc.