From: lkoop@TIGGER.STCLOUD.MSUS.EDU (LaMonte Koop)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Chip RAM extremely slow with GVP A530
Date: 28 Jan 93 03:46:39 GMT
Reply-To: lkoop@TIGGER.STCLOUD.MSUS.EDU
Organization: SCS GP/Engineering Cluster

In article <1993Jan27.194717.13182@news.cs.indiana.edu>, "Stefan G. Berg" <sgberg@avocado.ucs.indiana.edu> writes:
>lkoop@TIGGER.STCLOUD.MSUS.EDU (LaMonte Koop) writes:
>
>>In article <93024.073149PATROL@ACI.CVUT.CS>, Pavel Troller <PATROL@ACI.CVUT.CS> writes:
>>The 68020, 68030, and 68040 always prefetch instructions using 32-bit read
>>cycles.  Since the A500 motherboard bus is a 16-bit port to the 68030 on your
>>accelerator, essentially 2 memory read cycles must be accomplished for every
>>instruction prefetch done.  Right off the bat this gives you a 2x performance
>>hit against a 68030 doing prefetches from a full 32-bit bus port.  If you add
>>to this the fact that, as you indicated, the reads are done asynchronously to
>>a bus running a ~7.15 MHz, and the fact that you also have to deal with bus
>>contention from the custom chip DMA on the CHIP RAM bus, you can easily see 
>>where performance drops into the ground in the case with the instruction
>>cache disabled.  Performance figures less than a system running the stock
>>68000 in this situation are not unexpected.
>
>That makes me wonder if this has to do with my serial port problems. I have a
>68030-33 without any 32 bit RAM (I wait to install it until I get the fan I
>ordered), but with 1MB Chip and 8MB Fast. Using my Supra modem at full speed
>works really great since I have the 030. The system doesn't slow down a whole
>lot even at full speed and my cps rate has gone up considerably. However! When
>I write something to a PC disk with Cross DOS or my module player in the
>background switches it's modules I have a pretty good chance of getting
>transmission errors. Could it be that the 030 is being really wasteful with
>the Chip RAM bus and even though the CPU isn't slowed down I get problem with
>my serial port due to an overloaded chip bus? Oh... interlace and high color
>screens are also a pretty bad idea when doing file transmissions. A friend
>of mine with a Supra modem, but "only" an AdSpeed (14MHZ 68000) doesn't seem
>to have any of these problems although my computer outperforms his in terms of
>benchmarking software.

   Well, this sounds like a a symptom of the effects of running off of a 
16-bit ported memory arrangement alone.  It looks like you are getting a lot
of your problems from the latency encountered by the 68030 whenever it has
to hit the CHIP RAM bus.  That, coupled with the fact that a lot of the
conditions you mention cause the contents of the instruction/data caches to
be replaced can result in some of the problems associated with the 68030 
running bus cycles from a slower, asynchronous 16-bit ported resource.
    Word of warning: many 'module players' are not real kind to the system in
terms of multitasking, and will exibit the problem you mention in most 
setups.

>Let me know if there is some truth to this. How could I fix the problem? I
>guess the 32 Bit Fast RAM won't help in this case. It might even make it worse,
>because access to Chip RAM will be more intense (less time will be spend in
>executing programs in the faster 32 Bit RAM). BTW, my KS ROM is mapped in the
>512KB SRAM I have on my accelerator board.

    This paragraph doesn't make any sense.  Adding a 32-bit ported memory
resource to your system will of course help things along in terms of
performance by reducing the latency in memory cycles.  Accesses to the CHIP
BUS will of course remain the same as before, but by having a 32-bit ported
memory area, code/data fetches from that memory will be significantly faster,
reducing the overall execution time.

                 ----------------------------------------
           LaMonte Koop -- SCSU Electrical/Computer Engineering
 Internet: lkoop@tigger.stcloud.msus.edu -OR- f00012@kanga.stcloud.msus.edu
              "You mean you want MORE lights on this thing???"
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