[sdiy] Re: Bottom Ten ICs

[Scott Gravenhorst]

Richard Wentk <richard at skydancer.com> wrote:
>At 17:34 07/10/2004 +0200, Rainer Buchty wrote:
>> >It was architecturally stupid because if you're going to make no
>> >distinction between RAM and registers you might as well design the
>> >architecture to reflect that. Otherwise why bother with registers at
>> >all?
>>
>>Because plain memory-to-memory operations are costly in terms of code
>>size. We're talking 70s here. Memory was worth its weight in gold. You
>>wouldn't want to waste at least 5 bytes per operation (2x 16bit address
>>plus 1 byte opcode) when you can fit it in two (1 byte opcode, 1 byte
>>src/dst operand register).
>
>What's wrong with good old offset addressing, of the sort most 8-bit CPUs 
>had from the days of even the 8080?
>
>And why, if the design was that smart, why did TI's engineers then cripple 
>it with an 8-bit external bus?
>
>The whole thing seems like a classic case of wood for trees. One unusual 
>architectural feature makes no sense on its own. You need everything around 
>it to work too. All the way up to the s/ware support and marketing levels, 
>which is another area where TI crashed and burned.
>
>>Besides, as already mentioned otherwise, the design of the TMS9900
>>allowed for fast context switches. With other architectures you always
>>had to go through a push/pull orgy
>
>Yes, but how often did you use time sharing on a processor designed for use 
>in a home micro?

Truly.  I designed and wrote assembly language code for a RadioShack Color
Computer (6809) to do two task multitasking, even to the point it could
multitask two BASIC programs, each having floppy access.  But yes, it was
slow.  I also built a multitasking OS for a 6800.  Yes, that's 6800, not
68000.  It's doable, but not frightfully useful.

>>Concerning your other posting where you state that you like to have
>>"unused MIPS to play around" may I nitpick that MIPS are nothing.
>
>No, in this case MIPS were a useful metaphor.

Well, while MIPS is meaningless when you compare different processors, it has
significance in this case, call it unused cycles or whatever, it means that
it had capacity to do more work.




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