🚀 go-pugleaf

In article <1729@idacrd.UUCP> desj@ccr-p.ida.org (David desJardins) writes:
>Jan Vorbrueggen <jan@pallas.neuroinformatik.ruhr-uni-bochum.de> writes:
>>So this loop requires 42 ops, ie, on the Alpha 164 bytes.
>>[...]
>>[On the transputer] The whole loop thus requires 91 ops and 131 bytes.
>
>Who cares how many bytes it is?

The cache, the external bus, and the disk.

>                                 It seems like the obvious tendency for
>microprocessors is to go to longer and longer instructions, because with
>modern technologies the overhead of fetching and decoding the
>instructions is rapidly disappearing.  In 1960 or so it might have made
>a big difference whether you could pack the instructions into one byte
>instead of four, but I don't see the argument for it today.

With everything else being equal code density is a big win (for example
dubling the code density is the same as dubling the I-cache size, and
the bus width during I-fetches, as long as everything else remains the
same).  However everything else does not remin the same.  (oh, yeah
it would halve TLB misses due to I-fetch, and it would...)

>Obviously if the length of the instructions were important, the Alpha
>could have been designed with variable-length instructions.  So the
>evidence is that the cost of 4-byte instructions instead of 1-byte
>instructions is quite small, less than the extra complexity of dealing
>with variable-length and unaligned instructions.

No, it's if the Alpha designers had thought the code density was more
important then a simple fast decoder the Alpha would have variable
length instructions.  I am sure the Alpha designers know that code
density is important.

>                                        David desJardins
-- 
           stripes@pix.com              "Security for Unix is like
      Josh_Osborne@Real_World,The          Multitasking for MS-DOS"
      "The dyslexic porgramer"                  - Kevin Lockwood
We all agree on the necessity of compromise.  We just can't agree on
when it's necessary to compromise.       - Larry Wall