3/1/2024 0 Comments Arduino assembly languageNo interrupts will be executed after the CLI instruction, or even if the interrupt occurs simultaneously with the CLI instruction. CLI, which is the mnemonic for “ CLear global Interrupt flag”, clears the Global Interrupt flag (I) in the Status Register ( SREG). These two instructions enable or disable global interrupts. However, two operand-less instructions are very useful. SLEEP instructs the Arduino to go into a low power sleep mode. WDR resets the Watchdog timer and is rarely used in the Arduino. ICALL, IJMP, RET and RETI are advanced instruction dealing with program flow, and we will also cover them at a later time. Beside the 16 SREG specific instructions, the other instructions are NOP, BREAK, SLEEP, WDR, ICALL/IJMP, RET/RETI. We will cover the Status Register in greater detail later. So what can we do with the basic inline asm statement besides insert NOPs? There are 24 assembly instructions without operands, 16 of which deal with clearing and setting bits inside the Status Register ( SREG). To take advantage of more advanced features and processing, or any of the ‘%’ operators, one must use the “extended asm” (more on the Extended Assembler later). Basic assembly is limited to just pure instructions. An operand is the part of a computer instruction which specifies what data is to be manipulated or operated on. You may be surprised to learn that the Basic asm statement uses only assembly instructions without operands. This is the reason why you can’t use macro definitions, like so: The compiler pre-processor does not perform macro expansion within strings. And since GCC does not parse the Assembler instructions, it has no visibility of any symbols it may reference. This can lead to unexpected duplicate symbol errors during compilation if your assembly code defines symbols or labels. Under certain circumstances, GCC may duplicate (or remove duplicates of) your assembly code when optimizing. Jumps from asm to C labels are only supported under extended asm. GCC does not know about these jumps, and therefore cannot take account of them when deciding how to optimize. GCC’s optimizers can move asm statements relative to other code, including across jumps.Īsm statements may not perform jumps into other asm statements. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction asm statement. The avr-as assembler requires a single instruction per line, hence the need for the linefeed.ĭo not expect a sequence of multiple asm statements to remain perfectly consecutive after compilation. The compiler takes each assembly string, strips off the quotation characters and passes the code verbatim onto the assembler. The tab sequence is added after the linefeed simply to format the output. Sometimes you will see the line terminated with “ \n\t“. Each assembler instruction is enclosed inside quotations and terminated with the escape sequence for the linefeed character, “ \n“. The assembly code is encased in parenthesis preceded by the compiler keyword asm or _asm_. The general form of an inline assembler statement is: NOP might seem worthless, however it does come in handy in several ways. NOP does nothing except take up program memory and waist processor time. This statement simply inserts a NOP into your program. NOP is the opcode, or instruction for “ No Operation”. Here is the most basic example of an inline assembler statement: Multiple assembler instructions can be placed together in a single asm string. GCC does not parse the assembler instructions and does not know what they mean or even whether they are valid. The string can contain any instructions recognized by the assembler, including directives, but we will not discuss assembler directives here. An Arduino Inline Assembly Tutorial, like this was long overdue!Īn inline assembly statement is a string of characters which specifies assembler code. Hopefully this series of tutorials will help alleviate many of the discouraging troubles I encountered while teaching myself inline assembly coding. Trust me, any neophyte needs to spend hours searching and studying piece-meal examples while possessing overwhelming patience in order to crack the code. The main focus of existing documentation is the “Cookbook”, which dates from 2002. Besides the cryptic syntax and the high level of understanding the semi-official documentation assumes, there exists very little information about GCC inline assembler coding. Learning inline assembly language on the Arduino AVR 8-bit platform is a daunting task for many (at least it was for me).
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