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78
Assembly Language Fundamentals
At execution time for a module with EXT instruction, all of the symbols listed in it must be either
a utility name or be contained in an ENT or SUB (described in Chapter 6) of another module. It
is not necessary that the module be in source form; it may already be an object module
assembled from a source module which contained the symbol as an ENT or SUB.
Other Absolute Elements
There are additional absolute elements which may be used in expressions. These are
"machine addresses", short-precision numbers, and full-precision numbers.
A machine address is one of the following -
• An assembler pre-defined symbol.
• A symbol associated with an EQU or SET pseudo-instruction whose expression is
evaluated as a machine address (Le., it contains a pre-defined symbol or another EQU-
associated symbol whose expression contains a pre-defined symbol).
For the most part, machine addresses can be used just like absolutes. However, they remain
defined from assembly to assembly. By defining a machine address in one module (with an
EQU or SET),
it
then oecomes available to you with the same value in other modules which you
assemble.
For example, if you were to assemble a module containing -
then RIOO is a machine address following the above rules, just as if the assembler had pre-
defined it. If you don't do any SCRATCH or GET statements in the meantime, then the next
assembly you do would also have this symbol available without ever having to define it.
When full-precision numbers (like - 2.5, 3E3, 3.141592) and short-precision numbers (like
IS, - 2.55,3.141595, 3E3S) are used in expressions, they become the entire expression. This
is because these numbers are only intended as simple data-generating devices in literals and in
DAT pseudO-instructions. Explicitly, the rules for using full- and short-precision numbers are -
• They may only appear alone in an expression,
i.
e., they may not be in combination with
other elements.
• They may only appear in literals and in DAT pseudo-instructions.
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