NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/sys/mac/hardware/17073

Internet Message Format | 1992-09-15 | 2.5 KB

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From: [e-mail] (Martin Minow)
Newsgroups: comp.sys.mac.hardware
Subject: Re: ***** HELP ***** MACINTOSH W/ CACHE ****
Message-ID: <[e-mail]>
Date: 15 Sep 92 16:34:18 GMT
References: <[e-mail]>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 42

In article <[e-mail]> [e-mail]
(Baldwin Ng) writes:
>What is "CACHE" or "CACHE CARD"?
>How will a Macintosh computer's performance differ if
>there is a CACHE CARD?

A cache or cache card is a hardware extension to the processor that allows
the cpu and memory to run at different speeds. There are several types
of caches. Take an instruction cache for example. Suppose the cpu wants
to execute the instruction at memory address 123. The cache hardware
first looks in its internal (very fast) memory for that memory value.
If it's present, it delivers the value to the processor. If not, it reads
main memory locations 123, 124, 125, and 126 [for example], delivers
the value at 123 to the processor and remembers (i.e., caches) the others.
Since it is very likely that the next instruction the processor will execute
is at location 124, and the value of 124 will still be in the cache, the
processor + cache combination eliminates a comparitively slow access
to main memory.

Caches can operate similarly for data, and for memory writes. Writing
data is a bit tricky, however, since the processor may write word 123
then immediately read word 123. Here, the cache must deliver the latest
value of that word, and not an out-of-date value that might be left
in the cache.

Caches gain some performance by knowing that instruction accesses are
supposed to be read-only, so it can bypass write-checks for instructions.
This is a problem for programs that dynamically modify the instruction
stream. If a program must do this, the program must explicitly inform
the cache by executing a "cache flush" instruction. (There are similar
problems for concurrent I/O operations.)

Caches may be built into the cpu hardware or available as add-on boards.
There is a noticable performance improvement for cpu-bound tasks (I recently
added a cache board to my Mac-IIci), but your experience will depend
on the kind of programs you run.

Of course, the above explanation has simplified matters considerably; you
should read a computer architecture textbook for more information.

Martin Minow
[e-mail]