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register variable           Definition          register variable




register is a C  storage class.  A register declaration tells the
compiler to try to keep the  defined local data item in a machine
register.   Under the  COHERENT C  compiler, the  iinntt ffoooo  can be
declared to be a register variable with the following statement:


        register int foo;


COHERENT places  the first two  register variables declared  in a
function into  registers SI  and DI if  the variable type  is ap-
propriate, i.e., int or SMALL-model pointer.  Subsequent register
declarations are  ignored, because no registers  are left to hold
them.   Note  because  of  this  fact,  declaring more  than  two
register variables may slow processing rather than speed it.

By definition of the  C language, registers have no addresses, so
you cannot  pass the address of register  variable as an argument
to a function.  For  example, the following code will generate an
error message when compiled:


        register int i;
          . . .
        dosomething(&i);        /* WRONG */


This rule applies whether or not the variable is actually kept in
a register.

Placing  heavily-used local  variables into  registers  often im-
proves  performance,   but  in  some   cases  declaring  register
variables can degrade performance somewhat.

***** See Also *****

auto, definitions, extern, static, storage class



















COHERENT Lexicon                                           Page 1

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