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There is a different directory for each value of "$asmarch". If it contain
files MakeLVL[01].SH, these are included in the general Makefile. It
contains rules for creating headers and object files in the build directory.

The list of object files is given by "$kernel" (default: "mp mpinl")

* Level 0 symbols, in object file mpinl.o, are :
  overflow hiremainder
  addll addllx subll subllx mulll addmul
  divll
  bfffo
If no assembler implementation is available the definitions are in
none/ (addll.h, mulll.h, bfffo.h, divll.h). Otherwise, in $asmarch/asm0.h

If MakeLVL0.SH is not present, the file asm0.h is treated by
config/genkernel, creating parilvl0.h. The script expands the
following tokens [ to be placed in a C comment ] as follows:

  ^ASM  mod1 mod2    // asm0.h implements modules mod1, mod2
  ^NOASM mod1 mod2   // asm0.h does not implement modules mod1, mod2

where mod1, mod2 etc. are modules among addll, mulll, bfffo, divll.
In effect, this includes portable code from kernel/none/<module>.h
to supplement whatever is provided by asm0.h.
CAVEAT: if bfffo and divll are both mentioned, they must appear in this
order (divll depends on bfffo).

* Inline Level 1 symbols, in mpinl.o: the definitions are in
none/level1.h. Inlining may or may not be possible, but a symbol
must be defined in mpinl.o in any case.

* Non-Inline Level 1 symbols, defined in mp.o: the definitions are in
various *.c files in none/ and gmp/ concatenated into $objdir/mp.c.