>16MB RAM & ISA 24bit DMA??

>16MB RAM & ISA 24bit DMA??

Post by Tania Lewandows » Sat, 18 Jun 1994 14:08:29



Hello!

        Lately I have read a few posts of people who are using >16MB
of RAM in their Linux machines??  Is this with ISA based DMA option
cards installed in the system?  Tell me it true!  Pinch me, I must
be dreaming!  I have an EISA/VESA/ISA bus motherboard.  I am using an AHA-1542
and a SB16 in my machine.  Both are ISA/DMA cards.  I have 32MB of RAM in the
system but use a kernel with the 16MB memory limit enabled.  I thought I
had to do this to prevent DMA i/o to the above cards from wrapping around into
the wrong addresses in RAM if they got passed address information that was
above the lower 16MB in the system.  Is there a workaround in the 1.0 kernel
somehow that dodges this problem and offers me a chance to use all 32MB of
my RAM????

 
 
 

>16MB RAM & ISA 24bit DMA??

Post by Bill Hog » Tue, 21 Jun 1994 06:05:25


: Hello!

:       Lately I have read a few posts of people who are using >16MB
: of RAM in their Linux machines??  Is this with ISA based DMA option
: cards installed in the system?  Tell me it true!  Pinch me, I must
: be dreaming!  I have an EISA/VESA/ISA bus motherboard.  I am using an AHA-1542
: and a SB16 in my machine.  Both are ISA/DMA cards.  I have 32MB of RAM in the
: system but use a kernel with the 16MB memory limit enabled.  I thought I
: had to do this to prevent DMA i/o to the above cards from wrapping around into
: the wrong addresses in RAM if they got passed address information that was
: above the lower 16MB in the system.  Is there a workaround in the 1.0 kernel
: somehow that dodges this problem and offers me a chance to use all 32MB of
: my RAM????

   Not a work-around exactly.

   Double-buffering has been available in the Linux kernel since at least
0.99.14 (when I started, six months ago).

   I *believe* that the point of the >16MB RAM question is essentially to
find out whether the extra buffer required to do the double-buffering
required to handle SCS<->ISA<->RAM transfers in cases where the memory
address involved is > 24bits has to be allocated or not.

   But if I had an EISA machine, I sure as shoot would be saving my
pennies for an EISA SCSI controller!

  Bill
--
  Bill Hogan


 
 
 

>16MB RAM & ISA 24bit DMA??

Post by Rogier Wol » Thu, 23 Jun 1994 04:10:44


:    I *believe* that the point of the >16MB RAM question is essentially to
: find out whether the extra buffer required to do the double-buffering
: required to handle SCS<->ISA<->RAM transfers in cases where the memory
: address involved is > 24bits has to be allocated or not.

This is not true.

The >16Mb question is there because some motherboards (like mine) don't
cache well above 16Mb.

The symptom is that a kernel compile with 16Mb limit (no X) takes less
time than that same compile in a 32Mb configuration.
The difference with proper caching should be neglegible.

Hardware background: In direct mapped caches with n-bit wide tags and cs
byte cachelines, you need at least M/(cs * 2^n) words tagmemory. Here M
is the amount of memory.

With 32k byte tagmemory chips you can fit 7bit tags and 1 valid bit in
the tagmemory. This leads to max 32k * 4 * 128 = 16M of memory.

Some motherboards allow installation of a second bank of tagmemory.
Others allow a larger tagram chip. In the first case you hit the limit again
at 32Mb, in the other case you'd get a 128kbyte chip (there are no 64kbyte
srams) and you would hit the limit at 64Mb.

                                Roger.

--
 * As a protest against the recent bunch proposed anti-cryptography        *
 * laws, this message has been doubly encrypted using the rot13 algorithm. *

 
 
 

>16MB RAM & ISA 24bit DMA??

Post by Bill Hog » Thu, 23 Jun 1994 18:47:36



: :    I *believe* that the point of the >16MB RAM question is essentially to
: : find out whether the extra buffer required to do the double-buffering
: : required to handle SCS<->ISA<->RAM transfers in cases where the memory
: : address involved is > 24bits has to be allocated or not.

: This is not true.

: The >16Mb question is there because some motherboards (like mine) don't
: cache well above 16Mb. ...

:                               Roger.
: --

  Thank you very much.

  I am certainly glad to have this information.

  I got side-tracked in my reply.

  What I know is that I have been using a ISA SCSI controller with > 16MB
RAM since before I started running Linux, and I started running Linux when
it was at version 0.99.14.

  Bill

--
  Bill Hogan

 
 
 

1. EISA/ISA DMA limits for >16MB, is this supported?

I have an EISA motherboard, but all my current controller cards are
ISA.  I expect to add more then 16MB of memory before upgrading the
controller cards to EISA cards.  I did a quick look in both the FAQ and
some of the kernal stuff, but no where did I see anything like the
sysvr4 DMAMAXPAGE parameter that tells sysvr4 to restrict DMA to memory
below 16MB.

Is there a parameter like this in linux?  Can this be done on a per
controller basis, ie EISA disk, but ISA ethernet and serial cards?

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