[TUHS] 68k prototypes & microcode

[Toby Thain]

On 2021-02-04 2:23 a.m., Arno Griffioen wrote:
> On Wed, Feb 03, 2021 at 05:33:56PM -0800, Larry McVoy wrote:
>> I have to admit that I haven't looked at ARM assembler, the M1 is making
>> me rethink that.  Anyone have an opinion on where ARM lies in the pleasant
>> to unpleasant scale?
> 
> 'Different' is what I would call it..
> 
> Years ago I did a bunch of assembly hacking on the original ARM2 used in the 
> Archimedes A3000, which was an amazingly fast CPU for the time.
> 
> The thing that stood out on these CPU's to me, which was wildly different
> to what I was used to (M68K, 6502, Z80/8080, VAX), was the fact that 
> many instructions were (somewhat) composeable.
> 
> Aka. you can/could add varuous logical operations like AND, OR, etc. 'into' an 
> instruction like a load or store and it would take the same number of clock 
> cycles to execute it all in 1 go. 

That is immediately reminiscent of DG Nova, PDP-8 (and to a tiny extent,
PowerPC).

> 
> That was great for doing data manipulation at very high rates for the time
> compared to the common CISC CPU's as you did not need to go through multiple 
> fetch and modify cycles.
> 
> Reminiscent of some VLIW setups, but still more 'human readable' :)
> 
> The original ARM1/2/3 design did have some oddities like status bits being 
> encoded in the top of the (23) address bits, which meant that later versions of
> the original design had to do some memory tricks to use a bigger address
> space and keep compatilibity to the original code.
> 
> I suspect the current common ARM revisions since the move to the StrongARM 
> (ARM4) architecture, when DEC got involved and ARM became a standalone chip 
> design firm, have long fixed those oddities.
> 
> Probably still retains the way in which it encodes it's instructions to make 
> a lot of common logic operations while shuffling data more efficient though..

ARM MCUs also have the "bit manipulation engine" for a similar goal, I
think.

--Toby

> 
> Having said that.. (and bringing it more back to TUHS instead of COFF ;) )
> 
> The ARM2 and ARM3 based machines could already run UNIX with Acorn selling 
> RISC iX for a short time, which was a 4.3BSD port done in the late 80's 
> and early 90's.
> 
> Very few of those were ever used/sold though as the Acorn Archimedes series 
> of machines were quite a bit more expensive than more widespread CISC machines.
> Most were found in the UK and often in universities and the like.
> 
> 								Bye, Arno.
>