On 6/6/2011 10:40, Orchid XP v8 wrote:
> On 06/06/2011 04:59 PM, Darren New wrote:
>> On 6/3/2011 7:45, Invisible wrote:
>>> (By contrast, a *real* 32-bit chip like the Motorola 68000 has
>>> registers A0
>>> through A7 and D0 through D7, and when you do an operation,
>>
>> Which is fine if you're not trying to be assembly-language compatible.
>
> That's amusing. The 68000 is a 16-bit processor, which is
> forwards-compatible with the 68020.

But that's because it has the same machine code. I'm talking about the 8086 
being assembler-language compatible with the 8080.

>>> In short, they kept kludging more and more stuff in. Having a stack-based
>>> FPU register file is a stupid, stupid idea.
>>
>> Not when your FPU is a separate chip from your CPU.
>
> In what way does having a bizarre machine model help here?

First, it's not bizarre; it's pretty much how many (for example) VMs define 
their machine language. It's called a zero-address machine. Second, it's 
because the op codes don't need to have register numbers in them, so they 
can be smaller and hence faster to transfer. Most mathematics involving FP 
that you are actually willing to pay extra to speed up wind up being larger 
expressions, I'd wager. Plus, the intermediate registers were 80 bits.

> Yeah, but every OS will have to support the old arrangement forever more,
> every VM product will have to support it forever more, and every processor
> design will have to support it forever more.

That's not all our software. That's a pretty tiny part of a context switch.

>>> Aliasing the MMX registers to the FPU registers was stupid,
>> No, it saved chip space.
>
> It's quite clear that the design motivation behind this was not chip space
> but OS support.

True, in this case. But why would you say "the old OS works with new 
software to take advantage of this feature" is stupid?

> This is precisely why Haskell's unofficial motto is "avoid success at all
> costs". (I.e., once you are successful, you have to *care* about backwards
> compatibility.)

That's why I mentioned Haskell. Unfortunately, real-world companies building 
billion-dollar semiconductor fabs don't get to actively avoid success.

> I keep hoping that some day somebody will come up with a chip design that
> runs crappy old 16-bit MS-DOS stuff under software emulation, but runs real
> Big Boy software that people might give a damn about on a platform with a
> modern, coherant design.

They do. Intel chips are RISCs interpreting IA32 instructions. :-)


-- 
Darren New, San Diego CA, USA (PST)
   "Coding without comments is like
    driving without turn signals."

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