MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

Mixing Assembly Language and C Modules
EXAMPLE 9-11: USING REQUIRED REGISTERS
Some instructions in the dsPIC DSC instruction set require operands to be in a particular
register or groups of registers. Table 9-1 lists some constraint letters that may be
appropriate to satisfy the constraints of the instruction that you wish to generate.
If the constraints are not sufficient or you wish to nominate particular registers for use
inside asm statements, you may use the register-nominating extensions provided by
the compiler to support you (and reduce the need to mark registers as clobbered) as
the following code snippet shows. This snippet uses a fictitious instruction that has
some odd register requirements:
{ register int in1 asm("w7");
register int in2 asm("w9");
register int out1 asm("w13");
register int out2 asm("w0");
}
in1 = some_input1;
in2 = some_input2;
__asm__ volatile ("funky_instruction %2,%3,%0; = %1" :
/* outputs */ "=r"(out1), "=r"(out2) :
/* inputs */ "r"(in1), "r"(in2));
/* use out1 and out2 in normal C */
In this example, funky_instruction has one explicit output, out1, and one implicit
output, out2. Both have been placed in the asm template so that the compiler can
track the register usage properly (though the implicit output is in a comment statement).
The input shown is normal. Otherwise, the extended register declarator syntax is used
to nominate particular hard registers which satisfy the constraints of our fictitious
funky_instruction.
EXAMPLE 9-12: HANDLING VALUES LARGER THAN INT
Constraint letters and modifiers may be used to identify various entities with which it is
acceptable to replace a particular operand, such as %0 in:
asm("mov %1, %0" : "r"(foo) : "r"(bar));
This example indicates that the value stored in foo should be moved into bar. The
example code performs this task unless foo or bar are larger than an int.
By default, %0 represents the first register for the operand (0). To access the second,
third, or fourth register, use a modifier letter specified in Table 9-2.
© 2008 Microchip Technology Inc. DS51284H-page 127