back to the AGC, was Re: TTL 7400's Available

[Chuck Guzis]

On 2 Jan 2007 at 20:24, Brent Hilpert wrote:


> Now that was a fun read. I'm still trying to understand his description of
> the core rope ROM though. I'm familiar with core-rope ROM (or at least one
> version of it) from attempting to make a reader to dump the contents of a Wang
> calc microcode ROM, but the AGC version sounds like the
> address-decoding/word-selection is done differently (..need a diagram).

Somewhere, on one of the AGC sites linked to by the guy who's 
building his own, is a diagram of both the ROM and RAM setup.

In the paper, the author says: "I don´t know of any other computer 
that uses 1´s complements ."  He obviously never met Seymour Cray.  
One's complement arithmetic isn't hard to deal with; and it helped 
that Cray designed his ALU as a subtracter rather than an adder, so 
that the only way (using addition and subtraction) to get a negative 
zero was to subtract negative zero from negative zero.  In practice, 
this came up very rarely.  The zero and nonzero conditional branches 
worked on both species of zero at any rate.    There are a few very 
clever bit-twiddling tricks possible on a 1's complement machine that 
don't carry over to 2's complement.

Cheers,
Chuck