UM0144 datasheet(56/92 Pages) STMICROELECTRONICS

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ST7 Assembler Linker

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UM0144

A.16

EQU

A.17

EXTERN

Purpose:

Equate the label to expression.

Format:

label EQU <EXPRESSIONS>

Description:

Most labels created in a program are attached to a source code line that generates

object code, and are used as a target for jumps or memory references. The rest are

labels used as 'constants', used for example, to hold the IO port number for the sys-

tem keyboard: a number that will remain constant throughout the program.

The EQU directive allocates the value, segment type and length to the label field.

The value is derived from the result of the expression, the relativity (absolute or seg-

ment-relative derived from the most recent segment), the length is BYTE, WORD or

LONG, derived from the size default (starts off as WORD and may be changed by

directives BYTES, WORDS or LONGS).

Example:

See Also:

Purpose:

Declare external labels.

Format:

EXTERN

Description:

When your program consists of several modules, some modules need to refer to

labels that are defined in other modules. Since the modules are assembled sepa-

rately, it is not until the link stage that all the necessary label values are going to be

known.

Whenever a label appears in an EXTERN directive, a note is made for the linker to

resolve the reference.

Declaring a label external is a way of telling the assembler not to expect the label to

be defined in this module, although it will be used. Obviously, external labels must

be defined in other modules at link stage, so that all the gaps left by the assembler

can be filled with the right values.

Because the labels declared external are not actually defined, the assembler has no

way of knowing the length, i.e., (byte, word or long) of the label. Therefore, a suffix

must be used on each label in an EXTERN directive declaring its type; if the type is

undefined, the current default label scope (set by BYTES, WORDS, LONGS direc-

tives) is assumed.

Example:

UM0144 Datasheet(PDF) 56 Page - STMicroelectronics

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