How to use inline assembly

• version

  1.0.1

• scope

  Example.

  This code is provided as example code for a user to base their code on.

• description

  How to use inline assembly

• boards

  Unless otherwise specified, this example runs on the SliceKIT Core Board, but can easily be run on any XMOS device by using a different XN file.

The asm statement can be used to embed assembly code inside a C or XC function. The basic syntax for doing this is:

asm("assembly template" : output operands : input operands)

The following example calls the add assembly instruction to add 5 to the value y, storing the result in x:

int x, y = 5;

asm("add %0, %1, %2" : "=r"(x) : "r"(y), "r"(5));

Each operand is described by an operand constraint string followed by an expression in parentheses. The “r” in the operand constraint string indicates that the operand must be located in a register. The “=” indicates that the operand is written. Each output operand must have “=” in its constraint. Operands can be referred to in the assembler template using an escape sequence of the form %num where num is the operand number.

If the assembly code overwrites specific registers, this can be described by using a third colon after the input operands, followed by the names of the clobbered registers as a comma-separated list of strings:

asm("get r11, id; mov %0, r11" : "=r"(x) : /* no inputs */ : "r11");

If an asm statement has output operands, the compiler assumes the statement has no side effects apart from writing to the output operands. This may result in the compiler removing the asm statement if the output operands are are unused. To mark an asm statement as having side effects use volatile:

asm volatile("in %0, res[%1]" : "=r"(x) : "r"(p));

If the asm statement accesses memory, add “memory” to the list of clobbered registers:

asm volatile("stw %0, dp[a]" : /* no outputs */ : "r"(x) : "memory");

This prevents the compiler caching memory values in registers around the asm statement.