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Chapter 5. Assembler Macros |
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Defining a Macro
Macros must be defined in the program before they can be used. A macro definition begins with the MACRO directive which declares the name of the macro as well as the formal parameters. The macro definition must be terminated with the ENDM directive. The text between the MACRO and ENDM directives is called the macro body.
Example
WAIT |
MACRO |
X |
; macro definition |
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REPT |
X |
; generate X NOP instructions |
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NOP |
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ENDM |
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; end REPT |
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ENDM |
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; end MACRO |
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WAIT is the name of the macro and X is the only formal |
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parameter. |
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In addition to the ENDM directive, the EXITM directive can be used to |
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immediately terminate a macro expansion. When an EXITM directive is |
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detected, the macro processor stops expanding the current macro and resumes |
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processing after the next ENDM directive. The EXITM directive is useful in |
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Exampleconditional statements. |
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WAIT |
MACRO |
X |
; macro definition |
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IF NUL X |
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; make sure X has a value |
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EXITM |
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; if not then exit |
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ENDIF |
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REPT |
X |
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NOP |
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ENDM |
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; end REPT |
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ENDM |
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; end MACRO |
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Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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Parameters
Up to 16 parameters can be passed to a macro in the invocation line. Formal parameter names must be defined using the MACRO directive.
Example
MNAME MACRO P1,P2,P3,P4,P5,P6,P7,P8,P9,P10,P11,P12,P13,P14,P15,P16
defines a macro with 16 parameters. Parameters must be separated by commas in both the macro definition and invocation. The invocation line for the above macro would appear as follows:
MNAME A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P
where A, B, C, … O, P are parameters that correspond to the format parameter
names P1, P2, P3, … P15, P16.
Null parameters can be passed to a macro. Null parameters have the value NULL and can be tested for using the NUL operator described later in this chapter. If a parameter is omitted from the parameter list in the macro invocation, that parameter is assigned a value of NULL.
Example |
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MNAME A,,C,,E,,G,,I,,K,,M,,O, |
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P2, P4, P6, P8, P10, P12, P14, and |
P16 will all be assigned the value NULL |
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when the macro is invoked. You should note that there are no spaces between the comma separators in the above invocation line. A space has an ASCII value of 20h and is not equivalent to a NULL.
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Chapter 5. Assembler Macros |
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Labels
You can use labels within a macro definition. By default, labels used in a macro are global and if the macro is used more than once in a module, Ax51 will generate an error.
Example
LOC OBJ |
LINE |
SOURCE |
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1 |
GLABEL |
MACRO |
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2 |
LOOP: |
NOP |
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3 |
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JMP |
LOOP |
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ENDM |
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6 |
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7 |
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GLABEL |
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0000 00 |
8+1 |
LOOP: |
NOP |
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0001 80FD |
9+1 |
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JMP |
LOOP |
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GLABEL |
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11+1 |
LOOP: |
NOP |
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***_________________________^
***ERROR #9, LINE #11, ATTEMPT TO DEFINE AN ALREADY DEFINED LABEL
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0003 80FB |
12+1 |
JMP |
LOOP |
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14 |
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15 |
END |
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Labels used in a macro should be local labels. Local labels are visible only |
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within the macro and will not generate errors if the macro is used multiple times |
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in one source file. You can define a label (or any symbol) used in a macro to be local with the LOCAL directive. Up to 16 local symbols may be defined using the LOCAL directive.
NOTE
LOCAL must be in the next line after the MACRO definition.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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Example
CLRMEM |
MACRO |
ADDR, LEN |
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LOCAL |
LOOP |
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MOV |
R7, #LEN |
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MOV |
R0, #ADDR |
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MOV |
A, #0 |
LOOP: |
MOV |
@R0, A |
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INC |
R0 |
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DJNZ |
R7, LOOP |
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ENDM |
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In this example, the label LOOP |
is local because it is defined with the LOCAL |
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directive. Any symbol that is not defined using the LOCAL directive will be a global symbol.
Ax51 generates an internal symbol for local symbols defined in a macro. The internal symbol has the form ??0000 and is incremented each time the macro is invoked. Therefore, local labels used in a macro are unique and will not generate errors.
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