Orthogonality and contrivedness

[Allison]

>
>Subject: Re: Orthogonality and contrivedness
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Sun, 04 Jan 2009 11:57:30 -0800
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>On 4 Jan 2009 at 13:40, Tim Shoppa wrote:
>
> Try programming an 1802 for a while. You'll know you're really into
>> it when it seems "contrived" that all those other processors can
>> only use a single of their registers as a program counter :-).
>> Twisting my mind to switch to 1802 mode and back is a interesting
>> experience.
>
>Well, at least the PC on the 430 is a regular register, operable by 
>any applicable instruction of the set.  Doing a decimal add to the PC 
>must certainly yield interesting results!
>
>I'd always considered the 1802 to be in a unique position in the 
>70's, in that it was a (comparatively) low-power CMOS design.  No 
>doubt this was aided somewhat by its simple architecture.  Was 1802 
>RCA's one and only veture in the microprocessor world?

In the 70s CMOS was mostly RCAs game and calling card.  They never 
got the density very high till mid 70s.

There was 6100 (aka PDP-8 in cmos) and the 1800/1801 then the 1802
and 1804 and 1805  The 1800/01 was the base of the family and took 
two chips to complee the processor.  The 1802 was the first CPU from
RCA that took only one chip and the 04/05 added minor improvements and
brought rom on the chip.  

If memeory serves RCA also had a mini that has a similar archetecture.

The odditiy of the cosmac is once you program with it enough it's 
PHI SEX and GLO.  Seriously it's fairly efficient once you get used 
to it.  If it were made with current processes, the number of clocks
per cycle dropped it would likely still have staying power.

>> The smaller PIC's make perfect sense once you realize they're
>> Harvard architecture. Bigger PIC's, I never really grokked.
>
>The Harvard architecture really falls down in uCs because of lack of 
>space to store read-only constants.  Small PICs have to resort to all 
>sorts of oddball tricks to accommodate this for things such as lookup 
>tables.  (Use a "return from subroutine with immediate value" 
>instruction).  Upper PICs include instructions to access program 
>memory and AFAIK, all AVRs have them.  Which doesn't make them 
>strictly Harvard architecture anymore.  Had the ROM area included a 
>space in data memory for constant storage, it might have done the 
>trick.  Yes, there's EEPROM, but it has a different purpose and is 
>not easy to use.

Most all of the Harvard machines have a way to load a constant or
acccess a table in rom.  Started with the TMS1000.

>The 430 drops the charade and adopts a Von Neumann architecture, 
>relying on the read-only nature of ROM to enforce the separation 
>between code and data.
>
>> To me it's perfectly obvious that the MSP430 is PDP-11 like, and
>> in some ways even more orthogonal than the -11. The CP1600 was
>> substantially less orthogonal, more Nova-like with some
>> of the registers obviously intended for index use.

Never tried that one.

>
>How many Nova programmers would have killed for 16 registers?  To me, 
>what distinguishes the CP1600 was its inefficient use of instruction 
>memory (Yes, I know about the 10-bit ROM, but still...)
>

It was aimed at a rom based systems and back then rom was A)bulky, 
B)expensive silicon.

ALlison

>Cheers,
>Chuck