Because of a lack of documentation, especially for the AVR version of the compiler, it may take some time to figure out the implementation details by studying the compiler and assembler source code. There are also a few sample programs available in the net. Hopefully this document will help to increase their number. It's assumed, that you are familiar with writing AVR assembler programs, because this is not an AVR assembler programming tutorial.
It's not a C language tutorial either. Note that this document does not cover file written completely in assembler language, refer to avr-libc and assembler programs for this. Permission is granted to copy and distribute verbatim copies of this manual provided that the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
This document describes version 3. There may be some parts, which hadn't been completely understood by the author himself and not all samples had been tested so far. Because the author is German and not familiar with the English language, there are definitely some typos and syntax errors in the text.
As a programmer the author knows, that a wrong documentation sometimes might be worse than none. Anyway, he decided to offer his little knowledge to the public, in the hope to get enough response to improve this document. Feel free to contact the author via e-mail. For the latest release check http: As of 26th of July , this document has been merged into the documentation for avr-libc.
The latest version is now available at http: JavaScript is disabled on your browser. Please enable JavaScript to enjoy all the features of this site. Programmierung in Assembler und C Schaltungen und Anwendungen: Schaltungen und Anwendungen German Edition.
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Permission is granted to copy and distribute modified versions of this manual provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. This document describes version 3. There may be some parts, which hadn't been completely understood by the author himself and not all samples had been tested so far.
Because the author is German and not familiar with the English language, there are definitely some typos and syntax errors in the text. As a programmer the author knows, that a wrong documentation sometimes might be worse than none. Anyway, he decided to offer his little knowledge to the public, in the hope to get enough response to improve this document.
Feel free to contact the author via e-mail. For the latest release check http: You can write assembler instructions in much the same way as you would write assembler programs. However, registers and constants are used in a different way if they refer to expressions of your C program. The connection between registers and C operands is specified in the second and third part of the asm instruction, the list of input and output operands, respectively.
The general form is. In the code section, operands are referenced by a percent sign followed by a single digit. From the above example:. This may still look a little odd now, but the syntax of an operand list will be explained soon. Let us first examine the part of a compiler listing which may have been generated from our example:.
The comments have been added by the compiler to inform the assembler that the included code was not generated by the compilation of C statements, but by inline assembler statements. The compiler could have selected any other register, though. It may not explicitely load or store the value and it may even decide not to include your assembler code at all.
All these decisions are part of the compiler's optimization strategy. For example, if you never use the variable value in the remaining part of the C program, the compiler will most likely remove your code unless you switched off optimization.
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To avoid this, you can add the volatile attribute to the asm statement:. Alternatively, operands can be given names. Thus, the above example could also be written as. The last part of the asm instruction, the clobber list, is mainly used to tell the compiler about modifications done by the assembler code.
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This part may be omitted, all other parts are required, but may be left empty. If your assembler routine won't use any input or output operand, two colons must still follow the assembler code string.
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A good example is a simple statement to disable interrupts:. You can use the same assembler instruction mnemonics as you'd use with any other AVR assembler.
And you can write as many assembler statements into one code string as you like and your flash memory is able to hold. The linefeed and tab characters will make the assembler listing generated by the compiler more readable. It may look a bit odd for the first time, but that's the way the compiler creates it's own assembler code. Register r0 may be freely used by your assembler code and need not be restored at the end of your code.
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Each input and output operand is described by a constraint string followed by a C expression in parantheses. The selection of the proper contraint depends on the range of the constants or registers, which must be acceptable to the AVR instruction they are used with. The C compiler doesn't check any line of your assembler code. But it is able to check the constraint against your C expression.
However, if you specify the wrong constraints, then the compiler may silently pass wrong code to the assembler. And, of course, the assembler will fail with some cryptic output or internal errors. For example, if you specify the constraint "r" and you are using this register with an "ori" instruction in your assembler code, then the compiler may select any register.
This will fail, if the compiler chooses r2 to r It will never choose r0 or r1 , because these are uses for special purposes. That's why the correct constraint in that case is "d". On the other hand, if you use the constraint "M" , the compiler will make sure that you don't pass anything else but an 8-bit value. Later on we will see how to pass multibyte expression results to the assembler code.