I detect here a hint of confusion: The choice of 64-bit memory
addressing is independent of the choice of size of individual data
objects and independent of the width of internal hardware data paths
either within the processor or to/from memory or I/O channels. The only
thing to which 64-bit addressing is directly related is the total amount
of storage that may be addressed.
Even before z-architecture and the introduction of 64-bit addressing
there were z-architecture-predecessor machines with only 31-bit
addressing that employed 128-bit in parallel or possibly even wider
internal data transfers between real memory and high-speed cache in
order to achieve the desired performance. The frequency of access to
real memory and number of bytes transferred per access to and from the
processor and to and from high speed cache is mainly a cost-performance
hardware design issue for a specific hardware model and not tied to the
number of bits in a memory address.
I would expect the fetch time for the same sequence of instructions on
the same model processor to be independent of run-time address mode.
Joel C. Ewing
On 01/22/2014 01:57 AM, Itschak Mugzach wrote:
> 64 bit addressing execution is faster if less access to real memory is
> required to fetch the next instruction. This is what quadword promise,
> is'It? the "performance gain" is also depend on the logic of the program
> (if commands sequenced well with less brunch instructions).
>
> ITschak
>
>
> On Wed, Jan 22, 2014 at 5:30 AM, Jim Mulder <[email protected]> wrote:
>
>>> One caveat to that statement is as follows, from the POps:
>>> "The performance of CDSG on some models may
>>> be significantly slower than that of CSG. WHEN
>>> QUADWORD CONSISTENCY IS NOT REQUIRED BY THE PROGRAM,
>>> alternate code sequences should be used."
>>> (my caps)
>>
>> CDSG was implemented in millicode on the z900, z800, z990, z890,
>> and z9 machines. It was moved to hardware on the z10 and later
>> machines.
>>
>> Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY
>>
--
Joel C. Ewing, Bentonville, AR [email protected]
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