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Re: RFA: distinguish between pointers and addresses


Jim Blandy wrote:
> 
> > > + For example, the Mitsubishi D10V is a 16-bit processor that uses 32-bit
> > > + instructions.@footnote{Some D10V instructions are actually pairs of
> > > + 16-bit instructions, but you can't jump into the middle of a pair, so
> > > + they're effectively 32-bit instructions, for the sake of this
> > > + discussion.}  If the D10V used ordinary byte addresses to refer to code
> > > + locations, then the processor would only be able to address 64kb of
> > > + instructions.  However, since instructions must be aligned on a
> > > + four-byte boundary, the low two bits of any valid instruction address
> > > + are always zero --- byte addresses waste two bits in every address.  So
> > > + instead of byte addresses, the D10V uses word addresses --- byte
> > > + addresses shifted right two bits --- to refer to code.  Thus, the D10V
> > > + can use 16-bit words to address 256kb of code space.
> >
> > Perhaphs ``the Mitsubishi D10V is a 16-bit VLIW processor that has a
> > 32-bit instruction word''.
> > (The 32 bit instruction can be issued as a single operation or two sub
> > operations processed sequentially or in parallel but that sort of detail
> > is distracting :-)
> 
> I've changed it to:
> 
>     For example, the Mitsubishi D10V is a 16-bit VLIW processor whose
>     instructions are 32 bits long@footnote{Some D10V instructions are
>     actually pairs of 16-bit sub-instructions.  However, since you can't
>     jump into the middle of such a pair, code addresses can only refer to
>     full 32 bit instructions, which is what matters in this explanation.}.
> 
> Is that better?

Yes thanks - its nice to be able to refer to VLIW and not need to
mention IA-64 :-)

	Andrew

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