From: "Jan Humme" <humme AT EuroNet DOT NL>
Newsgroups: comp.os.msdos.djgpp
Subject: Why does reading from memory mapped PCI board cause a page fault?
Date: 8 May 1998 17:12:06 GMT
Organization: Imedia Engineering BV
Lines: 64
Message-ID: <6ived6$j7t@news3.euro.net>
NNTP-Posting-Host: p718.asd.euronet.nl
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

I have a memory-mapped PCI board; the memory map is at 0xE5000000.
I try to read from the device using the following program (faq 18.7), but
get a Page fault
as soon as I try to read ("i = _farpeek(......);").

Why?

Jan Humme,
Imedia Engineering BV,
jan DOT humme AT imedia-engineering DOT nl

=================================

#define ADDR 0xe5000000

short ldt;

void map_device()
{
    __dpmi_meminfo mi;

    if((ldt = __dpmi_allocate_ldt_descriptors(1)) < 0)
    {
        printf("LDT allocation error\n");
        exit(1);
    }

    mi.handle = ldt;
    mi.size = 4 * 1024 * 1024;
    mi.address = ADDR;
    if(__dpmi_physical_address_mapping(&mi) < 0
    || __dpmi_lock_linear_region(&mi) < 0
    || __dpmi_set_segment_base_address(ldt, ADDR) < 0
    || __dpmi_set_segment_limit(ldt, 0x10000 - 1) < 0)
    {
        printf("physical mapping error\n");
        exit(1);
    }
}

void main(int argc, char **argv)
{
    int i;

    map_device();

    i = _farpeekl(ldt, 0);
// here comes the page fault

    printf("%08x", i);

    __dpmi_free_ldt_descriptor(ldt);

    exit(0);
}



--
Jan Humme
Imedia Engineering BV
Amsterdam, the Netherlands