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

16-Bit C Compiler User’s Guide
TABLE 9-1: CONSTRAINT LETTERS SUPPORTED BY THE COMPILER
Letter Constraint
a Claims WREG
b Divide support register W1
c Multiply support register W2
d General purpose data registers W1 - W14
e Non-divide support registers W2 - W14
g Any register, memory or immediate integer operand is allowed, except for
registers that are not general registers.
i An immediate integer operand (one with constant value) is allowed. This
includes symbolic constants whose values will be known only at assembly time.
r A register operand is allowed provided that it is in a general register.
v AWB register W13
w Accumulator register A - B
x x prefetch registers W8 - W9
y y prefetch registers W10 - W11
z MAC prefetch registers W4 - W7
0, 1, … , An operand that matches the specified operand number is allowed. If a digit is
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used together with letters within the same alternative, the digit should come last.
By default, %n represents the first register for the operand (n). To access the
second, third, or fourth register, use a modifier letter.
T A near or far data operand.
U A near data operand.
TABLE 9-2: CONSTRAINT MODIFIERS SUPPORTED BY THE COMPILER
Letter Constraint
= Means that this operand is write-only for this instruction: the previous value is
discarded and replaced by output data.
+ Means that this operand is both read and written by the instruction.
& Means that this operand is an earlyclobber operand, which is modified
before the instruction is finished using the input operands. Therefore, this
operand may not lie in a register that is used as an input operand or as part of
any memory address.
d Second register for operand number n, i.e., %dn..
q Fourth register for operand number n, i.e., %qn..
t Third register for operand number n, i.e., %tn..
EXAMPLE 9-3: PASSING C VARIABLES
This example demonstrates how to use the swap instruction (which the compiler does
not generally use):
asm ("swap %0" : "+r"(var));
Here var is the C expression for the operand, which is both an input and an output
operand. The operand is constrained to be of type r, which denotes a register operand.
The + in +r indicates that the operand is both an input and output operand.
Each operand is described by an operand-constraint string followed by the C expression
in parentheses. A colon separates the assembler template from the first output
operand, and another separates the last output operand from the first input, if any.
Commas separate output operands and separate inputs.
DS51284H-page 122 © 2008 Microchip Technology Inc.