> From: Andres Djordjalian <adjordj@ALEPH.FI.UBA.AR>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: 17C5X (was thanks... 0402)
> Date: Thursday, 22 May 1997 11:45
> Peter F. Klammer wrote:
>
> > MICROCHIP, ARE YOU LISTENING!!?
> > ``The salient features of Extended Indirect Mode are:
> > 1. Full upward compatibility with existing PIC17C4X binary code.
> > 2. Single-cycle access to contiguous linearly-addressable RAM via
> > indirect addressing.
> > 3. Single-cycle any-bank-to-any-bank RAM copy (MOV INDR0,INDR1).
> > 4. A bank-selectable 224-byte RAM context (1Ch through FFh in any
bank).''
>
> But the implementation of this seems (to me, just an observer) far from
> trivial at the circuit-level. You would need a new bus for the FSRs'
> higher bits, the FSRs would need added circuitry apart from doubling
their
> size, and you would have to add a new MUX before the register address
> latch.

It *is* an upper-end processor, and yet doesn't approach the number of
transistors Pentiums have. (thank ghu!)  A little more complexity in the
FSR circuitry would be A Good Thing.

>
> If I was microchip I think I'd rather use my resources to do diverse
> faster and cheaper micros with more memory and peripherals, instead of
> doing all this work just to save some programmers' work and perhaps make
a
> certain kind of code run a little faster. After all, isn't that the way
> they're products are positioned? The simplicity of the scheme (from a
> hardware point of view) might be one of various things that allow the
high
> speed and low price we expect from a PIC.
>

This is the kind of thinking that made segmented addressing so popular with
ppl programming the 80x86 - NOT!!!  The FSR is the logical way to use
pointers in C, and having to bank-switch pointers plays hell with
efficiency.  Anyone remember the HUGE pointer type in PC type C?  How
inefficient it was?

consider this dumb code fragment.

#define BYTE unsigned char

#ifdef BLOAT
#define MAXARRAY 400
#else
#define MAXARRAY 40
#endif

main()
{       BYTE source_array[MAXARRAY];
        BYTE dest_array[MAXARRAY];
        BYTE *source_ptr, *dest_ptr;
        BYTE idx;
        source_ptr = source_array;
        dest_ptr = dest_array;
        for(idx = 0; idx < MAXARRAY; idx++)
                *source_ptr++ = *dest_ptr++;
}

Doesn't do much, but think about the extra code the compiler emits to
generate bankswitching when BLOAT is defined...  - and how many times you
would use pointers in C - and the slowdown...


MikeS
<mikesmith_oz@relaymail.net>