Re: Apple Drops Backward Compatibility for Next MacOS

[nathan@visi.com (Nathan Mates)]

In article <553ih7$1o6i@r02n01.cac.psu.edu>,
Eric Bennett <ericb@pobox.com> wrote:
>In article <28OCT199613435699@vax2.concordia.ca>
>spec@vax2.concordia.ca (Mitchell Spector) writes:
>> By the same token, it bugs me when PC users
>> claim the 8088 PC running at 4.77 MHz, was 4 times faster than a 1.024 MHz
>> 6502 Apple II+. :P (It's not!)

>Of course not.  A 4.77 MHz 8088 is exactly (within significant figure
>limits) 4.66 times faster than a 1.024 MHz 6502.

>  ;-)

   Smiley noted, but for those just joining this thread and not
remembering what was said a few months ago on comp.sys.apple2, here's
the reason why a 4.77Mhz 8088 is fairly close in speed to a 1.024Mhz
6502:

1) The 8088 takes _4_ (four) cycles to read a byte from memory
2) The 6502 takes _1_ (one) cycle to read a byte from memory

   Thus, to read in an identical number of bytes, the 8088 is only
1.16 times faster. However, with the average instruction length being
shorter on a 6502 vs 8088, the 6502 on average can move _more_ data
around in the same amount of time.

   This is due to the electrical properties of the 6502-- it's
designed around the 1 bus cycle per byte arrangement, while the
8088/8086, 68000 and similar chips were designed around 4 bus cycles
per byte. They _NEED_ faster Mhz just to be in the same ballpark of
speed; an IBM PC with a 1Mhz 8088 would be laughed off the market.

   [The 6502 also has some mini instruction pipelining which'll shave
one cycle off a number of instructions, (you get bitten on the 1 byte/
2 cycle instructions though) which is _far_ less important than the
4:1 ratio of memory access speeds.]

   Just for the record, I think it'd be fun to see a hand-optimized
standard benchmark on both systems or some other measurement of the
same program. [Given that word processors can be written well or
crappily, using diffent programs on both ends yields only a subjective
impression.]

Nathan Mates

--
<*> Nathan Mates http://www.visi.com/~nathan/      <*>
# What are the facts? Again and again and again-- what are the _facts_?
# Shun wishful thinking, avoid opinion, care not what the neighbors
# think-- what are the facts, and to how many decimal places?  -R.A. Heinlein