24 Sep
2022
24 Sep
'22
12:38 a.m.
On Sep 23, 2022, at 5:49 PM, Chuck Guzis via cctalk
<cctalk(a)classiccmp.org> wrote:
On 9/23/22 13:47, Fred Cisin via cctalk wrote:
Roughly that, though it's more refined than that. It starts with 15 4-bit population
count circuits. Then those counts are summed in a tree consisting of 8 3-bit adders
feeding 4 4-bit adders feeding 2 5-bit adders and finally a single 6-bit adder. There are
latches at each stage, clocked 75 ns apart to deliver the result in 400 ns (including
overhead into and out of the functional unit).
Thornton is nice for a superficial view of the machine, enough for a first impression
that's largely accurate. To see the full details, the "Block Diagram"
manuals, which can be found on Bitsavers, are amazing resources. For example, while
Thornton's famous "barrel" picture is a good conceptual model of the PPU
design, the actual details of how the barrel operates are a whole lot more complex than
that simple picture suggests. The block diagram spells it all out in full.
paul
It was useful as a simple test of whether an
applicant had any
bit-twiddling experience. But, I couldn't think of practical application.
The 6600 implemented it (IIRC) as a tree of 8 bit adders. If you
haven't read Jim Thornton's "Design of a Computer--The CDC 6600", you
owe it to yourself to look it up:
http://archive.computerhistory.org/resources/text/CDC/cdc.6600.thornton.des…