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

Mixing Assembly Language and C Modules
: [foo] "=r" (foo)
: "0" (foo), [bar] "r" (bar));
EXAMPLE 9-9: VOLATILE ASM STATEMENTS
You can prevent an asm instruction from being deleted, moved significantly, or
combined, by writing the keyword volatile after the asm. For example:
#define disi(n) \
asm volatile ("disi #%0" \
: /* no outputs */ \
: "i" (n))
In this case, the constraint letter “i” denotes an immediate operand, as required by the
disi instruction.
EXAMPLE 9-10: MAKING CONTROL FLOW CHANGES
There are special precautions that must be taken when making control flow changes
within inline assembly statements.
There is no way, for example, to tell the compiler that an inline asm statement may
result in a change of control flow. The control should enter the asm statement and
always proceed to the next statement.
Good control flow:
asm("call _foo" : /* outputs */
: /* inputs */
: "w0", "w1", "w2", "w3", "w4", "w5",
"w6", "w7");
/* next statement */
This is acceptable because after calling foo, the next statement will be executed. The
code tells the compiler that some registers do not survive this statement; these
represent the registers that will not be preserved by foo.
Bad control flow:
asm("bra OV, error");
/* next statement */
return 0;
asm("error: ");
return 1;
This is unacceptable as the compiler will assume that the next statement, return 0,
is executed when it may not be. In this case, the asm("error: ") and following statements
will be deleted because they are unreachable. See further information regarding
labels in asm statements.
© 2008 Microchip Technology Inc. DS51284H-page 125