NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/sys/amiga/hardware/22020

Internet Message Format | 1992-12-23 | 2.8 KB

Path: sparky!uunet!spool.mu.edu!umn.edu!msus1.msus.edu!msus1.msus.edu!news
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Data/Instruction Cache & BURST modes on 68030? Why/when?
Message-ID: <[e-mail]>
From: [e-mail] (LaMonte Koop)
Date: 23 Dec 92 19:08:38 -0600
Reply-To: [e-mail]
References: <hellerS.724958427@batman>,<[e-mail]>
Organization: SCS GP/Engineering Cluster
Nntp-Posting-Host: tigger.stcloud.msus.edu
Lines: 36

In article <[e-mail]>, [e-mail] (ANTHONY FRANCIS PRESTON) writes:
>You should leave the setting  alone!  A small tutorial is in order!
>SetPatch will setup the system correctly so that the caches and burst
>are correct for the Amiga.

    Well, generally yes...SetPatch will set up the CPU cache system to the best
performance setup for a particular system configuration.  However, due to the
dynamic nature of such things, there are times where it may be necessary to
modify the settings of the CPU caches.  This may be done due to necessity, or
due to curiosity.
    In a normal, properly designed Amiga system, there should not be any time
where the Data cache would be problematic to be enabled.  A performance increase
is generally found with it on, so this is the best configuration to leave it
in (as SetPatch does).  However, there are some systems which require the
68030 Data cache to be disabled.  One such setup is the Ronin Hurricane
'030 accelerators.  These are improperly designed in that they do not do
cache management in hardware and thus can inadvertantly cache CHIP RAM.
Due to the constant DMA there, a quick case of stale cache data arises and
is followed by a good old fashioned system failure.
    Note that this problem is not common, and yes..under most circumstances
the data cache can be ON for any particular system.  (in other words, it is
not a 'dangerous' thing to have on...and is helpful in terms of performance).
    Burst mode on the 68030 is a mixed-blessing thing.  Generally it has been
found that the instruction cache burst mode is helpful to about a 20% degree in
performance, while the data cache burst mode has been found to be mostly
a hindrance.  (Note that this is highly dependent on system data arrangement
and general access tendencies...on the Amiga, it has been found that the
data cache burst mode does not help in many cases)  Hence, Setpatch will set
up Burst mode for the instruction cache to be on, while leaving the data
cache burst mode off.

                 ----------------------------------------
           LaMonte Koop -- SCSU Electrical/Computer Engineering
 Internet: [e-mail] -OR- [e-mail]
              "You mean you want MORE lights on this thing???"
 ---------------------------------------------------------------------------