[Simh] Non-2^n architectures - Re: PIP10 on PDP-8 SIM

[Toby Thain]

On 19/03/13 9:43 AM, Johnny Billquist wrote:
> On 2013-03-19 14:30, Armistead, Jason wrote:
>> I had trouble with Timothe’s link to the USPTO, but found this same
>> patent in PDF form at
>>
>> http://www.brouhaha.com/~eric/retrocomputing/dec/dectape/3387293.pdf
>>
>> As a relative newbie who started my serious journey into computing with
>> an Apple ][ I’ve never fully understood DEC’s fascination with word
>> lengths that weren’t multiples of 2 ...
>
> It wasn't just DEC. Back in the day, most everyone used various word
> lengths that wasn't a power of two. I can't really make many comments on
> why other word lengths were more popular.

12, 15, 18, 36, 60 ...

24 is still common in DSPs (and maybe 48 and 56?) - and even word 
addressing is still used.

 >  I've seen mentioned that
> floating point formats was pretty nice to do with something like 60 or
> 72 bits. Reason being that you had large enough exponents for useful
> things, and enough precision for most calculations.
> So a word length that related to this made sense.
>
> Number of bits being a power of two started with IBM in the 60s, and
> became common with the PDP-11 in the 70s. (Or so I'd like to think.)



And, not insignificantly, the rise of 8-bit microprocessors.

(Though octal was still standard notation for addresses and many 
constants on PDP-11, hence C's octal literals.)

--Toby

>
> Johnny
>