Reproducing old machines with newer technology

[Chuck Guzis]

On 07/14/2015 10:55 AM, Noel Chiappa wrote:

> I guess I don't know the 6600 that well (I have the book, and have skimmed it
> in the past). What are the novel features in the 6600 that were widely
> adopted by other machines? (I listed the Atlas because of paging, and the 801
> because of RISC.)

There are more than a few folks who call the CDC 6000 one of the first 
RISC machines.  In particular, the 6600 with its instruction scheduling 
and reservation control, multiple functional units, instruction cache, 
etc. was quite noteworthy.   You could easily recognize the earmarks of 
a very different machine of the time.   The issues that arose when 
programming later RISC CPUs made me feel that I was back in familiar 
territory.

3-address architecture, 60 bit ones complement words, with separate sets 
of registers for addresses and indexing.

A typical beginning programmer's problem was to write a CPU loop to move 
non-overlapping fields of words in the shortest amount of time (ECS not 
available).  The best solution usually involved two words per iteration, 
with one instruction issue per cycle; bottom of the loop load (to 
overlap the branch) and top-of-the-loop store.

Another challenge was to write a routine that could load and store the 
values of all registers from and to memory.  Not as simple as it sounds.

For me, one distinguishing feature is that there is no condition code 
register.  You could branch on a register value being signed or zero, or 
compare and branch on the values of two index registers.  Thus, the 
result of an operation was covered by the regular register reservation 
rules.

The I/O processors (PPUs) had access to all of CPU memory and 
essentially were their own world, using a 12-bit instruction set derived 
from the 160A.  One could also say that the PPUs were only one machine 
with memory and registers being time-multiplexed to simulate 10 machines.

--Chuck