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Chapter 7. Invocation and Controls |
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Controls for Conditional Assembly
The controls for conditional assembly are General controls—they may be specified any number of times in the body of a source file. Conditional assembly may be used to implement different program versions or different memory models with one source file. You may use conditional assembly to maintain one source module that satisfies several applications.
Conditional text blocks are enclosed by IF, ELSEIF, ELSE and ENDIF.
The SET and RESET controls may be used in the invocation line of the assembler to set and reset conditions tested by the IF and ELSEIF controls.
The remaining instructions for conditional assembly are only allowed within the source file and cannot be part of the assembler invocation line.
IF blocks may be nested a maximum of 10 levels deep. If a block is not translated, conditional blocks nested within it are also skipped.
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Conditional Assembly Controls
Conditional assembly controls allow you to write x51 assembly programs with sections that can be included or excluded from the assembly based on the value of a constant expression. Blocks that are conditionally assembled are enclosed by IF, ELSEIF, ELSE, and ENDIF control statements.
The conditional control statements IF, ELSE, ELSEIF, and ENDIF may be specified only in the source program. They are not allowed on the invocation line. Additionally, these controls may be specified both with and without the leading dollar sign ($).
When prefixed with a dollar sign, the conditional control statements may only access symbols defined by the SET and RESET controls.
When specified without a dollar sign, the conditional control statements may access all symbols except those defined by the SET and RESET controls. These control statements may access parameters in a macro definition.
The following table lists the conditional assembly control statements.
Directive |
Page |
Description |
IF |
233 |
Translate block if condition is true |
ELSE |
235 |
Translate block if the condition of a previous IF is false. |
ELSEIF |
234 |
Translate block if condition is true and a previous IF or ELSEIF is |
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false. |
ENDIF |
236 |
Marks end of a block. |
RESET |
232 |
Set symbols checked by IF or ELSEIF to false. |
SET |
231 |
Set symbols checked by IF or ELSEIF to true or to a specified |
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value. |
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Chapter 7. Invocation and Controls |
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Predefined Constants (A251 only)
The A251 macro assembler provides you with predefined constants to use in conditional $IF / $ELSEIF controls for more portable assembler modules. The following table lists and describes each one.
Constant |
Description |
_ _INTR4_ _ |
Set to 1 to when A251 assumes 4 byte interrupt frames. If the A251 control |
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INTR2 is used, the _ _INTR4_ _ symbol is not defined. |
_ _MODBIN_ _ |
Set to 1 if the binary mode of 251 CPU is used. If the source mode is |
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specified with the MODSRC control, the _ _MODBIN_ _ symbol is not |
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defined. |
_ _MODSRC_ _ |
Set to 1 if the source mode of 251 CPU is specified with the MODSRC |
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control. If the binary mode of the 251 CPU is used the _ _MODSRC_ _ |
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symbol is not defined. |
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SET
Abbreviation: |
None. |
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Arguments: |
A list of symbols with optional value assignments separated |
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by commas and enclosed within parentheses. For example: |
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SET (symbol = number , symbol = number … ) |
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Default: |
None. |
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Control Class: |
General |
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µVision2 Control: |
Options – Ax51 – Set. |
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Description: |
The SET control assigns numeric values to the specified |
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symbols. Symbols that do not include an explicit value |
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assignment are assigned the value 0FFFFh. Symbols that |
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are specified with an equal sign (‘=’) and a numeric value |
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are assigned the specified value. |
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These symbols can be used in IF and ELSEIF control |
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statements for conditional assembly. They are only used for |
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conditional assembly. These symbols are administered |
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separately and do not interfere with other symbols. |
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Example: |
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A251 SAMPLE.A51 SET(DEBUG1=1, DEBUG2=0, DEBUG3=1) |
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$SET (TESTCODE = 0) |
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$SET (DEBUGCODE, PRINTCODE) |
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232 Chapter 7. Invocation and Controls
RESET
Abbreviation: |
None. |
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Arguments: |
A list of symbols separated by commas and enclosed within |
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parentheses. For example: |
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RESET (symbol , symbol … ) |
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Default: |
None |
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Control Class: |
General |
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µVision2 Control: |
Options – Ax51 – Reset. |
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Description: |
The RESET control assigns a value of 0000h to the |
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specified symbols. These symbols may then be used in IF |
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and ELSEIF control statements for conditional assembly. |
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These symbols are only used for conditional assembly. |
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They are administered separately and do not interfere with |
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other symbols. |
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Example: |
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A251 SAMPLE.A51 RESET(DEBUG1, DEBUG2, DEBUG3) |
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$RESET (TESTCODE) |
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$RESET (DEBUGCODE, PRINTCODE) |
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IF
Abbreviation: |
None |
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Arguments: |
A numeric expression |
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Default: |
None |
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Control Class: |
General |
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µVision2 Control: |
This control cannot be specified on the command line. |
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Description: |
The IF control begins an IF–ELSE–ENDIF construct that |
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is used for conditional assembly. The specified numeric |
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expression is evaluated and, if it is non–zero (TRUE), the IF |
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block is assembled. If the expression is zero (FALSE), the |
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IF block is not assembled and subsequent blocks of the IF |
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construct are evaluated. |
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IF blocks can be terminated by an ELSE, ELSEIF, or |
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ENDIF control statement. |
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Example: |
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$IF (DEBUG_VAR = 3) |
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. |
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. |
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. |
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Version_3: |
MOV DPTR, #TABLE_3 |
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. |
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$ ENDIF |
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. |
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ELSE
Abbreviation: |
None. |
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Arguments: |
None. |
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Default: |
None. |
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Control Class: |
General |
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µVision2 Control: |
This control cannot be specified on the command line. |
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Description: |
The ELSE control is used to introduce an alternative |
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program block after an IF or ELSEIF control. The ELSE |
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block is only assembled if previous IF and ELSEIF |
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conditions in the IF–ELSE–ENDIF construct were all |
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FALSE. ELSE blocks are terminated with an ENDIF |
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control. |
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Example: |
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. |
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. |
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$IF (DEBUG) |
; TRUE when DEBUG <> 1 |
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. |
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. |
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$ELSEIF (TEST) |
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. |
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$ELSE
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$ENDIF
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236 Chapter 7. Invocation and Controls
ENDIF
Abbreviation: |
None |
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Arguments: |
None |
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Default: |
None |
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Control Class: |
General |
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µVision2 Control: |
This control cannot be specified on the command line. |
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Description: |
The ENDIF control terminates an IF–ELSE–ENDIF |
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construct. When the Ax51 assembler encounters an ENDIF |
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control statement, it concludes processing the IF block and |
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resumes assembly at the point of the IF block. Since IF |
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blocks may be nested, this may involve continuing in |
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another IF block. The ENDIF control must be preceded by |
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an IF, ELSEIF, or ELSE control block. |
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Example: |
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. |
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$IF TEST |
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$ENDIF |
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