DJGPP FAQ -- Accessing absolute address above 1MB

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18.7 Accessing absolute address above 1MB

Q: How can I access memory-mapped peripheral devices (or any other absolute address) above 1 MByte mark?

A: You should use DPMI functions to allocate an LDT descriptor, and map it to an absolute physical address. This maps the physical address of the memory on the device to a linear address, and returns that linear address to you. Here are the DPMI calls that you will have to use:

map selector to physical address (Int 31h/AX=0800h);
allocate an LDT descriptor (Int 31h/AX=0);
set segment base address (Int 31h/AX=7);
set segment limit (Int 31h/AX=8).

All of these DPMI calls have __dpmi__XXX wrappers in the DJGPP library. Here's a somewhat schematic example:


      #include <dpmi.h>
      .
      .
      __dpmi_meminfo mi;
      int selector;
      .
      .
      /* Map the physical device address to linear memory.  */
      mi.address = physical_address;
      mi.size    = physical_address_size;
      __dpmi_physical_address_mapping (&mi);
      /* Now mi.address holds the linear address.  */
      .
      .
      /* Allocate an LDT descriptor and set it up to span the entire
         device on-board memory.  */
      selector = __dpmi_allocate_ldt_descriptor (1);
      __dpmi_set_segment_base_address (selector, mi.address);
      __dpmi_set_segment_limit (selector, mi.size - 1);

Note that the segment limit should be one less than the size. Also, segments over 1MB in length must be a multiple of 4KB, otherwise the DPMI server might fail the call, or silently change the limit. You can then use the functions from the sys/farptr.h header file to access that device. See accessing memory-mapped devices, for more details about accessing memory-mapped devices given their linear address.

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Copyright ⌐ 1998 by Eli Zaretskii	Updated Sep 1998

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