>From: Joe Gwinn <joegwinn@xxxxxxxxxxx>
>
>>What are 16 bit apps? Apple users have never had to deal with such things --
>>all Mac programs were always 32 bit.
>
>Yes and no. While the 68000 family of CPUs had 32-bit addressing from the
>start, the memory busses were 16 bit, so moving a 32-bit word (address or
>data) took two bus cycles. Many programs used 16-bit data items, to to reduce
>memory demand.
I guess my point was that whether any given program used 16 bit quantities or
32 bit quantities was completely transparent to the end user. The 80x86 of the
day had a segmented architecture, which required messy mode changes between 16
bit and 32 bit ops, but on the 680x0, 16/32 bit ops were completely regular --
they just had to be on an even 16 bit boundary.
There is no reason a so-called "16 bit" Mac app would not run on the 64 bit G5,
correct?
>The arrival of universal hardware floating-point is changing this, as FP is
>far easier on the programmers.
But still relatively expensive, compared to integer math!
>I did have a lot of fun when computer salesmen from major vendors tried to
>convince me that my code would run faster on their brand new 64-bit computers,
>because such computers could "do twice as much work as a 32-bit computer". I
>replied that the air traffic control code of interest here was originally
>16-bit code, and still was, whatever the word size of the new platforms, so
>those new 64-bit words would only be 1/4 full.
Ah, but have you checked out the G5 architecture white paper? It does extensive
pipelining and branch prediction, and can work on multiple instructions at a
time, even if the original programmer had no intention of multi-processing.
If your memory bandwidth is four times bigger, it should run even 16 bit
programs approaching four times faster.
>Actually, with modern computers, the memory system speed is more important
>than the CPU speed.
Exactly! And the new Mac G5 has a 128 bit memory bus that can hit 6.4 gigabytes
per second! That's nearly four times as fast as the nearest desktop competitor.
I'm counting the days... it should be here by the end of next week...
Obligatory Olympus content: I spent the money I was going to spend on an E-1 on
a dual G5 and Cinema Display instead. My five-year-old G4 was starting to feel
a bit slow working on 1.5 GB drum scans... :-)
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