> On Oct 23, 2018, at 7:08 PM, Noel Chiappa via cctalk <cctalk@classiccmp.org>
> wrote:
>
>> From: Ben Bfranchuk
>
>> I just can't find a clean simple design yet. ...
>> The PDP 11 is nice machine, but I am looking for simpler designs
>> where 16K words is a valid memory size for a OS and small single user
>> software.
>
> There was a recent discussion about code density (I forget whether here, or
> on TUHS), and someone mentioned this paper:
>
> http://web.eece.maine.edu/~vweaver/papers/iccd09/iccd09_density.pdf
>
> which shows that for a combo of benchmarks, the PDP-11 had the densest code
> out of all the ones they looked at. (They didn't look at the PDP-8, but I
> suspect that since it's a single-address design, it's almost ceertainly not
> as dense.)
Interesting. There are lots of single address machines; it isn't all that
obvious they would be less effective.
It also depends on other instruction set features. Some years ago I learned
the architecture of the Dutch Electrologica X1 and X8 machines. They are
single address, with multiple registers (not many though). But they gain a lot
of efficiency by allowing almost all instructions to optionally set a condition
flag, and almost all instructions to be executed conditionally on that flag.
So a lot of code full of branches becomes much shorter. The fact that the
condition flag setting itself is a choice (unlike the setting of condition
codes) helps a lot. For example:
if (x >= 0) { foo (); x += 2; }
else x -= 3;
translates to just 5 instructions:
a=x,p
y,sub(foo)
y,a+2
n,a-3
x=a
since the condition flag is (normally, though it's a choice) preserved by
function call/return.
Pretty efficient.
paul
