o  This directive cannot appear in a psect with the NOEXE and
       NOMIX attributes.

    o  Although this directive is accepted by the assembler in a
       psect with the MIX attribute, it has no effect in these psects
       since no code optimizations are in affect for MIX psects.

    o  .BEGIN_EXACT must be paired with a matching .END_EXACT to
       close the exact instruction block.

    o  .BEGIN_EXACT and .END_EXACT instruction blocks can be
       nested. The outermost level of the .BEGIN_EXACT and matching
       .END_EXACT directives delimit the actual exact instruction
       block from which code optimizations are suppressed. Nesting
       .BEGIN_EXACT and .END_EXACT instruction blocks can be useful
       in macro definitions where the macro expansion requires an
       exact instruction sequence. Nested .BEGIN_EXACT and .END_EXACT
       instruction blocks allow a macro to be invoked both from
       within and without the exact instruction block.

    o  .BEGIN_EXACT does not affect automatic alignment. Automatic
       alignment is enabled with the .ENABLE ALIGN_CODE and .ENABLE
       ALIGN_DATA directives or with the /ALIGN=(CODE,DATA) command-
       line qualifier.