3.9 How to configure your system for DJGPP

Q: How do I configure my system to get optimal performance under DJGPP?

A: That depends on the amount of RAM you have installed in your machine. Below are some guidelines to help you.

1. If you have 2 MBytes or less RAM installed:
   • Don't use any memory manager.
   • Use of CWSDPMI as your DPMI host is highly recommended.
   • Remove any TSR and device drivers you don't absolutely need (like SETVER.EXE, HIMEM.SYS etc.) from your CONFIG.SYS and AUTOEXEC.BAT.
   • Do not install disk cache or RAM disk; point your TMPDIR environment variable to a directory on your hard disk. Put a sufficiently large BUFFERS= statement into your CONFIG.SYS (I recommend setting BUFFERS=40,8) to make DOS file operations faster⁸.
   • If you use CWSDPMI as your DPMI host, get the CWSPARAM program (from the csdpmi4b.zip archive) and set the "Minimum application memory desired before 640K paging" parameter to 512K or larger. Depending on how much memory you actually have, you might need to further fine-tune this parameter. This parameter defines the lowest amount of extended memory CWSDPMI will use; if your system doesn't have that much free extended RAM, CWSDPMI will use conventional memory instead, where usually there should be around 600K of free RAM.
   • If you run under Windows, be sure to set the maximum amount of extended memory on your PIF file for the DOS box to a reasonable value.

   With this configuration, GCC will run out of free physical RAM and start paging when compiling almost any C program and all C++ programs. If you are serious about DJGPP development, you need to buy more RAM urgently.

2. If you have 2-4 MBytes of RAM installed:
   • Don't use any memory manager.
   • Remove any TSR and device driver you don't absolutely need (like SETVER.EXE, HIMEM.SYS) from your CONFIG.SYS and AUTOEXEC.BAT.
   • Get a disk cache which works from conventional memory and configure it to 256K size at most, or don't use a cache at all.
   • Do not install a RAM disk; point your TMPDIR environment variable to a directory on your hard disk.
   • If you run under Windows, be sure to set the maximum amount of extended memory on your PIF file for the DOS box to a reasonable value.

   With this configuration, GCC will still run out of free physical RAM and start paging when compiling large C programs and most C++ programs. Plan to buy more RAM as soon as you can.

3. If you have 5-8 MBytes of RAM installed:
   • Use a memory manager such as EMM386 or QEMM386. Try using the FRAME=NONE parameter of the memory manager. This will disable Expanded Memory (EMS) services as far as most programs are concerned; if you must use DJGPP together with any program which needs EMS, try to configure that program to use Extended Memory (XMS) instead.
   • Load DOS, device drivers and TSRs HIGH. This is done by using the DEVICEHIGH= command (instead of DEVICE= in CONFIG.SYS, and by using the LOADHIGH command in AUTOEXEC.BAT.
   • Give your disk cache 1 MByte of RAM. Enable its delayed-write (a.k.a. write-back) feature.
   • Do not install a RAM disk; point your TMPDIR environment variable to a directory on your hard disk.
   • If, after configuring your system as above, you still have more than 2.5 MBytes of free RAM left (4 MBytes, if you plan to program in C++ a lot), enlarge the disk cache size.
   • If you run under Windows, be sure to set the maximum amount of extended memory on your PIF file for the DOS box to a reasonable value.
4. If you have more than 8 MBytes of RAM:
   • Use a memory manager to load DOS, TSRs and device drivers HIGH.
   • Install at least a 2-MByte-large disk cache, configured to use the delayed- write feature. If you have plenty of RAM, you can give your cache as much as 8 MBytes of memory. Here's an example of a line to put into your AUTOEXEC.BAT file that installs an 8-MByte cache for hard disks C:, D:, and F::

     loadhigh c:\dos\smartdrv.exe c+ d+ f+ 8192
     

     (The + character after the drive letter enables the delayed-write (a.k.a. write-back) feature for that drive.) Note that you do not need, and should not install a disk cache if you intend to use DJGPP programs from Windows 9X, because Windows includes its own built-in disk cache (called VCACHE) that is loaded together with the operating system.

   • If you have more than 10 MBytes left, install a RAM disk with a size of at least 1.5 MBytes and point your TMPDIR environment variable to it. If your RAM disk is less than 4 MBytes, GCC might run out of space there for very large source files (e.g., cccp.c file from the GCC source distribution), but this shouldn't happen unless the size of the source file you are compiling approaches 1 MByte. Note that software is available that lets you install a RAM disk even on Windows 9X. (However, I'm told that Microsoft's own RAMDRIVE.SYS only supports long file name on the RAM disk if its size is less than 9MB.)
   • As a general rule of thumb, you should leave at least 8 MBytes of free RAM after installing the disk cache and the RAM disk. 16MB free is even better, especially if you need to run large programs like RHIDE or Emacs, or to compile large source files.

Some people disable the delayed-write feature for safety reasons, to avoid losing files due to system crashes. If you are worried about this, you can usually gain performance without sacrificing safety by enabling delayed-write together with an option that causes the cache to flush the write-behind data before the system returns to the DOS prompt. For a SmartDrv disk cache, this is achieved by specifying /N/F switches instead of /X.

Using a memory manager, such as EMM386 or QEMM, is not required (DJGPP will run without it), but highly recommended, since it has several advantages:

• Memory managers provide an API for allocating extended memory called VCPI (the Virtual Control Program Interface). Using that API allows CWSDPMI to allocate only as much extended memory as is needed, leaving the rest for non-DJGPP programs, in case you invoke them from DJGPP programs. In contrast, without a memory manager, CWSDPMI will allocate all of the available extended memory to itself, leaving none of it to non-DJGPP programs. This consideration is especially important if you use some DJGPP program, like Bash or Emacs, as your primary system interface.
• Without a memory manager, you cannot access UMBs (the Upper Memory Blocks) which give you more DOS memory to work with. In particular, CWSDPMI will load itself into UMBs if they are available.
• Memory managers provide the VDS (Virtual DMA Services) API which allows to write programs that use DMA in protected mode.
• Memory managers support the expanded (EMS) memory, which some older DOS programs still use.

If your memory manager is EMM386, I recommend to put the NOEMS NOVCPI parameters on its command line. This will allow you to use UMBs and up to 128MB of physical memory (if you have that much installed). Without these parameters, many versions of EMM386 limit your physical memory to 32MB.

It is generally not recommended to install DJGPP on a networked drive, since this makes it slower, particularly when linking programs. If you do install DJGPP on a networked drive, you should consult your network administrator to configure the network for maximum performance. For Novell networks, a good place to look for advice is the Novell FAQ (search for a file called nov-faq.htm).