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18.6 Fast access to absolute addresses

Q: The "farptr" functions are too slow for my application which MUST have direct access to a memory-mapped device under DPMI. How can I have this in DJGPP? My entire optimized graphics library is at stake if I can't! :-(

A: The following so-called Fat DS, or "nearptr" method was suggested by Junaid A. Walker (he also posted a program which uses this technique to access the video RAM; you can look it up by searching the mailing list archives). But first, a word of warning: the method I'm about to describe effectively disables memory protection, and so might do all kinds of damage if used by a program with a wild pointer. It is depressingly easy, e.g., to overwrite parts of DOS code or data with "Fat DS" on. Or, as Stephen Turnbull has put it when he read the description of this trick:

Surgeon General's WARNING: The description below uses the "Fat DS hack", a steroid derivative which gives your program great strength, a thick neck, baldness, and is known to be closely linked with the Alzheimer's disease.

In addition to the above warning, experience shows that many programs which use the safer "farptr" functions do not sacrifice performance at all. So, with the exception of a small number of programs, "nearptr" is really a convenience trick: it allows you to treat memory-mapped devices with usual C pointers, rather than with function calls. Therefore, I would generally advise against using "nearptr" due to speed considerations, unless your program absolutely needs the last percent of speed.

Having said that, here is the trick: you change the limit of the segment descriptor stored in DS to 0xffffffff (i.e., -1), using library function __djgpp_nearptr_enable. After that, you have access to all the memory which is currently mapped in. This works due to 32-bit wrap-around in the linear address space to access memory at, say, linear address 0xa0000 (which belongs to the VGA), or any other address on your memory-mapped device, by adding the value of the global variable __djgpp_conventional_base to the target address. __djgpp_conventional_base is the negated base address of the DS selector that you program is using to access its data. By adding the value of __djgpp_conventional_base, you effectively subtract the DS base address, which makes the result zero-based, exactly what you need to access absolute addresses.

You should know up front that this trick won't work with every DPMI host. Linux's DOSEmu and Windows/NT won't allow you to set such a huge limit on the memory segment, because these operating systems take memory protection seriously; in these cases __djgpp_nearptr_enable will return zero--a sign of a failure. CWSDPMI, QDPMI, Windows 3.X and Windows 9X all allow this technique (OS/2 Warp seems to allow it too, at least as of version 8.200), but some events break this scheme even for those DPMI hosts which will allow it. A call to malloc or any other library function which calls sbrk might sometimes change the base address of the DS selector and break this method unless the base address is recomputed after sbrk call. (The "nearptr" functions support this recomputation by providing you with the __djgpp_conventional_base variable, but it is your responsibility to recompute the pointers using it.) The same change can happen when you call system, and as a result of some other events external to the executing code thread, like multitasking or debugger execution.

You should also know that the __djgpp_nearptr_enable function in DJGPP v2.0 didn't verify that the limit was properly set. So if the DPMI server would fail the call silently, the function won't detect it and will not return a failure indication. DJGPP v2.01 corrects this omission by always verifying that the DPMI host has honored the request, and returns a failure indication if it hasn't.

If you are aware of these limitations, and don't need your code to run under all DPMI hosts, it might be the fix to your problems.

Confused about how exactly should you go about using this technique in your program? Look at the docs of the "nearptr" functions in the Info file (node __djgpp_nearptr_enable).

Another possibility is to use the DPMI function 0x508 (a wrapper function __dpmi_map_device_in_memory_block is available in the DJGPP library) that can map any range of physical memory addresses into a block that you allocate. Note that this is a DPMI 1.0 functionality which is not supported by most DPMI 0.9 hosts (CWSDPMI does support it). There is a convenience helper function __djgpp_map_physical_memory in the DJGPP C library that you can use to call these services.

If you need a nearptr-style access to a certain region of memory which is above the base address of the DS selector, you can enlarge the limit of the DS selector just enough to cover the highest address you need to access. To this end, use the library function __dpmi_set_segment_limit like this (thanks to Eric Rudd for posting this code):

 unsigned long new_limit;

 if (__dpmi_set_segment_limit (_my_ds (), new_limit) == 0)
     if (__dpmi_get_segment_limit (_my_ds ()) != new_limit)
       /* The DPMI host ignored the call.  Fail.  */
         __dpmi_set_segment_limit (__djgpp_ds_alias, new_limit);
         __dpmi_set_segment_limit (_my_cs (), new_limit);
         _crt0_startup_flags |= _CRT0_FLAG_NEARPTR;
   /* The call failed.  */

Remember that new_limit should have all its lower 12 bits set, otherwise the above snippet will not work!