bits and butts

From: Jim Battle <frustum_at_pacbell.net>
Date: Fri Dec 24 21:54:55 2004

Tom Jennings wrote:

...
> How many computers (automatic, electronic, digital,
> stored-program, etc) used an odd number of bits?
>
> Were any of them not signed-magnitude? Eg. odd number of bits
> 2's complement?
>
> Any of them post-1964? Any LSI/VLSI based?

Post 1964? Why, yes, although I can only just now talk about it since
it is past the 10 year cctalk rule.

I was an early employee of Chromatic Research, a failed 1993-1999 start
up that I believe coined the term "media processor." The MPACT1 and
MPACT2 chips contained a LIW processor that had 9b bytes, and all math
was 2's complement. The datapath was 72b wide, and most operations
could work on eight 9b bytes, four 18b words, or two "36b words"
(although the multipliers copped out an only did a 27bx27b multiply).

Why the 9-bittedness? There were two reasons. First, Chromatic wrongly
foresaw MPEG as being the big draw, and computing ((color component 1) -
(color component 2)) required a 9b delta; 16b could have been used, but
only four pixels could have been processed per clock. We doubled the
peak throughput by adding only 12.5% more datapath resources. The
second reason was that the chip used RAMBUS memories, which were only
sold with 9b channels, and we had no particular need for parity.

Other MPACT oddities:

Each 72b instruction bundle had a two bit field that indicated that the
pair of instructions had to be either executed sequentially, executed in
parallel, executed in parallel "VECLEN1" or "VECLEN2" times in a row
(where VECLEN1 and VECLEN2 were registers containing a repeat count).

The chip had 512 72b registers, and the programming game was to work on
vectors of data in this register while streaming data to/from the RDRAM
memory (Chromatic was one of early adopters of RAMBUS memory).

The multipliers had various rounding modes, including "round random",
where 6b pseudorandom bias was added just below the trucation point, as
some audio filter algorithms have better spectral properties with this
type of rounding.

There weren't separate shift left/shift right instructions, just a
shift. Positive shift counts shifted left, negative shifted right.
This was especially important since for SIMD data, sometimes different
fields had to shift opposite directions in the same instruction.

The chip implemented CGA/EGA/VGA in all of their textmode, planar mode,
packed mode glory via emulation. All VGA register accesses actually got
funnelled into a VGA command FIFO. When the FIFO went non-empty, an
interrupt was taken to the VGA emulation routine; it would parse the VGA
commands and carry out the necessary work on the flat, packed pixel
framebuffer so that the results looked as they should. It seems
laughable now when multiple VGA cores would fit on a pinhead, but back
then in 0.65 um, a VGA core would have been a significant cost.

I guess I'll stop there, although there were a number of other
interesting features.
Received on Fri Dec 24 2004 - 21:54:55 GMT