LXR linux-old/Documentation/memory.txt

   1There are several classic problems related to memory on Linux
   2systems.
   3
   4        1) There are some buggy motherboards which cannot properly 
   5           deal with the memory above 16MB.  Consider exchanging
   6           your motherboard.
   7
   8        2) You cannot do DMA on the ISA bus to addresses above
   9           16M.  Most device drivers under Linux allow the use
  10           of bounce buffers which work around this problem.  Drivers
  11           that don't use bounce buffers will be unstable with
  12           more than 16M installed.  Drivers that use bounce buffers
  13           will be OK, but may have slightly higher overhead.
  14        
  15        3) There are some motherboards that will not cache above
  16           a certain quantity of memory.  If you have one of these
  17           motherboards, your system will be SLOWER, not faster
  18           as you add more memory.  Consider exchanging your 
  19           motherboard.
  20
  21All of these problems can be addressed with the "mem=XXXM" boot option
  22(where XXX is the size of RAM to use in megabytes).  
  23It can also tell Linux to use less memory than is actually installed.
  24
  25See the documentation of your boot loader (LILO, loadlin, etc.) about
  26how to pass options to the kernel.
  27
  28There are other memory problems which Linux cannot deal with.  Random
  29corruption of memory is usually a sign of serious hardware trouble.
  30Try:
  31
  32        * Reducing memory settings in the BIOS to the most conservative 
  33          timings.
  34
  35        * Adding a cooling fan.
  36
  37        * Not overclocking your CPU.
  38
  39        * Having the memory tested in a memory tester or exchanged
  40          with the vendor.
  41        
  42        * Exchanging your CPU, cache, or motherboard for one that works.
  43
  44        * Disabling the cache from the BIOS.
  45
  46        * Try passing the "mem=4M" option to the kernel to limit
  47          Linux to using a very small amount of memory.
  48
  49
  50Other tricks:
  51
  52        * Try passing the "no-387" option to the kernel to ignore
  53          a buggy FPU.
  54
  55        * Try passing the "no-hlt" option to disable the potentially
  56          buggy HLT instruction in your CPU.
  57
  58        * Passing for example the "endbase=0x9F000" option to the kernel,
  59          you'll _force_ the kernel to not touch the memory between 0x9F000
  60          and 1Mbyte. As default the kernel reads the endbase limit from
  61          the BIOS. So you need to specify this option only if the BIOS
  62          does not provide the right information to the kernel (or if you
  63          don't have a BIOS at all :). You can discover the endbase value
  64          of your running kernel with this command `dmesg | grep endbase`.
  65