From - Fri Sep 26 07:30:23 1997
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From: jeh@cmkrnl.com (Jamie Hanrahan)
Newsgroups: comp.os.ms-windows.programmer.nt.kernel-mode
Subject: Re: What are size limits on MmMapIoSpace?
Message-ID: <1997Sep25.174329.8226@cmkrnl>
Date: 25 Sep 97 17:43:29 PST
References: <342abb06.25938234@9.2.249.42>
Organization: Kernel Mode Systems, San Diego, CA
Lines: 31

In article <342abb06.25938234@9.2.249.42>, false@watson.ibm.com (Bill Anzick) writes:
> I am trying to map a memory on a PCI bus adapter [...]  When I
> then attempt to use MmMapIoSpace to get a virtual address for each
> area, the 256 byte area returns a valid address, but the second area
> with is 32Meg is size returns a zero.
> The manual says that a zero is returned if "space for mapping the
> range is insufficient."  Since this is outside of system memory, what
> space are they talking about? Since this is virtual system space, what
> are the limitations?

Spare page table entries that describe system virtual address space 
are required.  The system preallocates a number of these, but not 
many, for implementing MmMapIoSpace, MmGetSystemAddressForMdl, and 
similar functions.  New free PTEs for mapping system space can't be 
extended on the fly, for reasons I'm not clear on. 

Under the registry key \System\CurrentControlSet\Control\Session
Manager\Memory Management, you will find a value called SystemPages. 
Try setting this to its current value (which is probably zero) plus
8192.  (8192 being the number of virtual pages you're creating). 

Note, in the past many of us assumed that these values would be taken 
verbatim by the system.  It is now my understanding that the system 
ADDS these values to its default calculations.  

	--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
        Internet: jeh@cmkrnl.com (JH645)  CompuServe: 74140,2055  
drivers, internals, networks, applications, and training for VMS and Windows NT
NT driver FAQ, links, and other information:  http://www.cmkrnl.com/

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