semi-homemade micro - test-drb@ccmp.vtda.org

14 Nov 2005

...

Subject: Re: semi-homemade micro
   From: Jim Battle <frustum at pacbell.net>
   Date: Mon, 14 Nov 2005 13:33:01 -0600
     To: General Discussion: On-Topic and Off-Topic Posts <cctalk at
classiccmp.org>

woodelf wrote:

....
  Having got a new PC ... 2 GHZ???  who knows the
real speed.
 I have tried Spare time gizmos - pdp 8 emulator  ( A minor bug
 -- with windows  how do you get the bell to sound ?).
 I was running some sort of diagnostics and had the RTC displayed
 and for about 10 minutes of real time, the clock advanced a hour.
 That must be at least 6x faster than the real thing on this computer. 
If a 2 GHz (give or take) x86 CPU emulates a pdp 8 at 6x, it means either the 
code is inefficient, or the code contains a speed regulator that doesn't work 
properly. 
It's inefficient, there was no goal to be efficient only useful and interesting.

...
 The original PDP-8 took 10 clocks at 1 MHz to execute
one instruction, a 2 GHz 
CPU has 20 million cycles to interpret one instruction.  So at 6x realtime, the 
program is using 3.2 million cycles to interpret one instruction. 
An 8e series was around 1.5uS core cycle time and some instructions took a
few cycles.

However it's timing at the macro level is easy. FETCH, execute as needed.
So to fetch a 12bit word and decide one of 8 major actions is pretty light
on code to do.  The next step execute is tempered by addressing but thats
a minor calculation.  OPR instructions you need more decisions and they
have a distinct sequence.  Then the IOT, again it's all decided by the 
device but for codes like 6000Q-6007Q and a few others the path is already 
set.  EMA adds overhead in all cases.

...
 Most likely speed and efficiency weren't goals of
the emulator, so I bring this 
up not to discredit the program's author but rather to say: don't use that data 
point as anything but a lower limit on what kind of horsepower it would take to 
use a micro to emulate a PDP-8.  I imagine an AVR device at 20-40 MHz should be 
able to emulate a PDP-8 at real time. 
That may be possible. Keep in mind this type of emulation is almost like 
building a microcode sequencer.  

An alternate approach is a more hardware (ACC, Link, MQ, MAR, IR ALU
and shifter) and use a micro like 8048(or whatever) to controls the loads 
and all and not execute arithmetic logical or register ops in the micro.
Speed there could exceed the 6100/6120 series cmos parts as the micro
is decoding instructions and controling events rather than calculating.
the differnce is your approaching the real thing hardware wise but 
leaving out a lot of control logic (sequential or microded hardware).

Allison