Microchip Technology XC8 Standard Compiler (Workstation) SW006021-1 SW006021-1 User Manual

 2012 Microchip Technology Inc.

DS52053B-page 181

The assembly generated from C source code by the compiler will use certain registers 
in the PIC MCU register set. Most importantly, the compiler assumes that nothing other 
than code it generates can alter the contents of these registers. So if compiler-gener-
ated assembly code loads a register with a value and no subsequent code requires this 
register, the compiler will assume that the contents of the register are still valid later in 
the output sequence.

If any of the applicable registers listed are used by interrupt code, they will be saved 
and restored when an interrupt occurs, either in hardware or software. See 
Section 5.9.3 “Context Switching”.

The registers that are special and which are used by the compiler are listed in Table 5-9

The state of these registers must never be changed directly by C code, or by any 
assembly code inline with C code. The following example shows a C statement and 
inline assembly that violates these rules and changes the ZERO bit in the STATUS reg-
ister.

#include <xc.h>

void getInput(void)

{

ZERO = 0x1;  // do not write using C code

c = read();

#asm

#include <caspic.h>

bcf ZERO_bit  ; do not write using inline assembly code

#endasm

process(c);

}

MPLAB XC8 is unable to interpret the register usage of inline assembly code that is 
encountered in C code. Nor does it associate a variable mapped over an SFR to the 
actual register itself. Writing to an SFR register using either of these two methods will 
not flag the register as having changed and may lead to code failure.

All 8-bit devices

W

All 8-bit devices

STATUS

All mid-range devices

PCLATH

All PIC18 devices

PCLATH, PCLATU

Enhanced mid-range and PIC18 devices

BSR

Non-enhanced mid-range devices

FSR

Enhanced mid-range and PIC18 devices

FSR0L, FSR0H, FSR1L, FSR1H

All PIC18 devices

FSR2L, FSR2H

All PIC18 devices

TBLPTRL, TBLPTRH, TBLPTRU, TAB-

LAT

All PIC18 devices

PRODL, PRODH