AN3350 datasheet(65/185 Pages) STMICROELECTRONICS

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AN3350

Working with the assembler

Doc ID 018512 Rev 2

65/185

10.4.3

Symbol directives

During execution all symbols are stored in a single table, the symbol table. Symbols must be

defined exactly once by placing them as labels, giving explicit values, using the .equ and

.def

directives, or declaring them external through use of the .extern directive.

10.4.4

The current location

The current location, explicitly referenced by the predefine symbol '.', continuously points

to the address of instruction or data storage. Current location units reflect those of the actual

memory section (usually one or four bytes) wherein they will be located. For absolute

sections, the location is the actual address. For relative sections, the location is calculated

as though the section begins at address zero.

Example:

jmp.+3

The above example transfers control to the third instruction appearing after the current one

('.').

Caution:

Explicit use of the current location symbol is not considered safe.

10.4.5

Change of flow

Some instructions contain direct jumps to other points in the code. The target of the jump

can be specified as any legal expression. However, to create code that is less prone to error

and easier to maintain, it is better to make this expression a label. When using the option –

Wall

, etpu_bins will warn if this convention is violated, as illustrated in Example 4.

Example 4 Using labels in code

.extern There

.equ Here,0x7686

Start:

jmp 0x3075

; Warning - a constant

jmp Start

; OK

Table 7.

Symbol directives

Directive

Description

.equ symbol, value

Define symbol as an absolute symbol, and set its

value to value. value must be a constant expression

(all referenced symbols must be resolved).

.global symbol

Declare symbol as a global symbol whose value is

available to other modules outside the current

compilation unit. symbol must be defined in the

same compilation unit.

.extern symbol

Declare symbol as a symbol defined outside the

current compilation unit, whose value is available in

the current compilation unit.

.weak symbol

Declare symbol as a global weak symbol.

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