Macro Assembler in the example of AVR

AVR, Assembler, macro

Macro - a set of commands and instructions that are grouped together into a single command to automate programming tasks, speed up frequently performed editing and formatting actions.

Macros can also be used to accelerate software development for microcontrollers on Assembler programming language. During compilation macros are replaced by a sequence of commands microcontroller. Such an approach to programming facilitates the perception of the program code, reduce the size of the source, as well as to bring the programming in Assembler in appearance to the programming language C.

Writing Macro

Macro Assembler programming language consists the following components:

1. .macro - Directive start the macro, the macro after the ad in the same line declares the name of the macro;
2. .endm - Directive macro end;
3. @0 - variable;
4. Macro body .

Macro recording for the AVR microcontroller can be as follows:

 ; The macro output to a port or port register
 .macro outi                ; Macro Directive ad the macro name 
        ldi r16, @1         ; Setting the value to the register
        out @0, R16         ; Data output to the port or port register
 .endm                      ; Macro end

The use of this macro is as follows:

 outi DDRB,  0b01111111    ; Initialization Output scan display columns
 outi DDRC,  0b00111111    ; Initialize the output display of scan lines
 outi DDRD,  0b00000001    ; Initialize the output to activate the alarm
 outi PORTD, 0b11111111    ; Initialization PD1 - PD7 in PullUp mode

In this case, the macro is called by its name outi . In the first place @0 is the register DDRB , and the second one is the variable @ 0b01111111 , which is recorded in the register. That is in fact the use of the macro looks like this:

outi @0, @1, @2 ... @N

In the process of compiling software code preprocessor performs macro substitution data and its encapsulation in the code. The macro in this case is similar to the function or subroutine that is called rcall team (or other) with the difference that the subroutine is called rcall team may be present in the firmware of the microcontroller in a single copy, while a macro is simply a convenient insertion of code into the source and therefore can lead to a significant increase in the size of the microcontroller firmware. Whereas application subroutine does not cause such an increase in firmware, but their application must solve other problems, such as limited range rcall command call in the memory of the microcontroller and respectively creating islets transition from one microcontroller memory to another, which may lead to complication of the program code.