Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!ames!killer!elg
From: elg@killer.DALLAS.TX.US (Eric Green)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Message-ID: <4929@killer.DALLAS.TX.US>
Date: 27 Jul 88 07:57:04 GMT
References: <1988Jul22.164129.5495@utzoo.uucp> <4912@killer.DALLAS.TX.US> <1988Jul26.024039.28579@utzoo.uucp>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 39

In message <1988Jul26.024039.28579@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) says:
>In article <4912@killer.DALLAS.TX.US> elg@killer.DALLAS.TX.US (Eric Green) writes:
>>> ... A good implementation of BitBlt,
>>> whether in software *or* hardware, will run the memory flat out, leaving
>>> nothing for "servicing other processes".  
>>... the bandwidth of modern 100ns 
>>memories is much greather than a 8mhz 68000 can take advantage of...
>
>Suggesting that one should either buy a faster processor or spend less
>money on the memory.  There are sillier things than putting 100ns RAMs
>on an 8MHz 68000, yes, but I'd have to think for a moment to come up
>with specific examples.

The slowest 256kbit DRAM chips that I have ever seen are 150ns. I
believe that the Amiga uses the 120ns parts. The difference in price
is on the order of cents. 

>More generally, I will repeat -- more explicitly -- something I pointed
>out before:  the fair comparison is not to the same system without BitBlt
>hardware, but to a system where the same effort and funding (custom chips
>are *not* cheap to design) have been invested in making the main CPU faster
>instead.

The original design goal, as I understand it, was to produce a
low-cost consumer machine with high-performance graphics. Which
basically means using an off-the-shelf microprocessor with adequate
development tools (the Amiga OS was cross-compiled on Sun workstations
using the Greenhills compiler). The machine was based around 256kbit
DRAM's, which means that the memory bandwidth is going to be greater
than can be used up by an 8mhz 68000. To go faster, they would have
needed a faster 68000. Which in 1985 would have probably added $200 to
the cost of the machine (after going through two levels of markups).
That would have made it miss the $1500 price criteria (as vs. the
Blit, which, I understand, had 1 megabyte of memory and a price tag of
$10,000).

--
Eric Lee Green    ..!{ames,decwrl,mit-eddie,osu-cis}!killer!elg
          Snail Mail P.O. Box 92191 Lafayette, LA 70509              
       MISFORTUNE, n. The kind of fortune that never misses.