Hardware and memory dumps

By default, memory dump files are generally sharable between architectures of different types; specifically, on machines of different endianness (see section `Byte order' in GNU C Library). (This is a feature of the Web2c implementation, and is not true of all TeX implementations.) If you specify `--disable-dump-share' to configure, however, memory dumps will be endian-dependent.

The reason to do this is speed. To achieve endian-independence, the reading of memory dumps on LittleEndian architectures, such as PC's and DEC architectures, is somewhat slowed (all the multibyte values have to be swapped). Usually, this is not noticeable, and the advantage of being able to share memory dumps across all platforms at a site far outweighs the speed loss. But if you're installing Web2c for use on LittleEndian machines only, perhaps on a PC being used only by you, you may wish to get maximum speed.

TeXnically, even without `--disable-dump-share', sharing of `.fmt' files cannot be guaranteed to work. Floating-point values are always written in native format, and hence will generally not be readable across platforms. Fortunately, TeX uses floating point only to represent glue ratios, and all common formats (plain, LaTeX, AMSTeX, ...) do not do any glue setting at `.fmt'-creation time. Metafont and MetaPost do not use floating point in any dumped value at all.

Incidentally, different memory dump files will never compare equal byte-for-byte, because the program always dumps the current date and time. So don't be alarmed by just a few bytes difference.

If you don't know what endianness your machine is, and you're curious, here is a little C program to tell you. (The configure script contains a similar program.) This is from the book C: A Reference Manual, by Samuel P. Harbison and Guy L. Steele Jr. (see section References).

main ()
{
  /* Are we little or big endian?  From Harbison&Steele.  */
  union
  {
    long l;
    char c[sizeof (long)];
  } u;
  u.l = 1;
  if (u.c[0] == 1)
    printf ("LittleEndian\n");
  else if (u.c[sizeof (long) - 1] == 1)
    printf ("BigEndian\n");
  else
    printf ("unknownEndian");

  exit (u.c[sizeof (long) - 1] == 1);
}

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