© 1997 Microchip Technology Inc.

DS00669A-page 2

AN669

To illustrate, lets force fpm32() to cross the page

boundary. A pragma directive is required to locate a

routine (Example 2).

EXAMPLE 2:

FORCING FPM32 TO CROSS

THE PAGE BOUNDARY

#pragma memory ROM [MAXROM-0x7F0] @ 0x7F0;

#include "fpm32.inc"

The listing file generated is shown in Example 3. Notice

the statement GOTO MTUN32 at address 0x7FC. How-

ever, the routine MTUN32 is located at address 0x801.

Remember, with the PIC16C74A the GOTO instruction

only has an eleven bit address range. With the GOTO

MTUN32

example, one more bit of address is needed to

branch to 0x801 from 0x7FC. The extra bit of address

is located in the PCLATH register. That means assem-

bly code would have to be inserted into the floating

point routines to initialize PCLATH before each GOTO.

Since this solution is not desirable, the best approach

is to locate the floating point subroutine in a single

page. For example, change the pragma directive in

Example 2 to locate the routine at 0x800.

It is important to note that when fpm32() is called as a

C function, the page bit in PCLATH is updated by

MPLAB-C. In other words MPLAB-C adds the neces-

sary assembly language code needed to call fpm32()

or any other C function. The C function is called cor-

rectly, but once within the C function, the raw embed-

ded assembly language might have GOTOs or CALLs

that cross over the page boundary and cause

problems.

EXAMPLE 3:

FPM32 FORCED TO ADDRESS 0x7F0 TO SHOW CROSSING FROM PAGE 0 TO

PAGE 1

void fpm32 (void)

{

#asm

.

. some code here

.

07F0 0838

FPM32

MOVF

AEXP,W

;test for zero arguments

07F1 1D03

BTFSS

_Z

07F2 0839

MOVF

BEXP,W

07F3 1903

BTFSC

_Z

07F4 284E

GOTO

RES032M

07F5 0826

M32BNE0

MOVF

AARGB0,W

07F6 0633

XORWF

BARGB0,W

07F7 00AE

MOVWF

SIGN

;save sign in SIGN

07F8 0839

MOVF

BEXP,W

07F9 07B8

ADDWF

EXP, F

07FA 307E

MOVLW

EXPBIAS-1

07FB 1C03

BTFSS

_C

07FC 2801

GOTO

MTUN32

;****** WON’T WORK !

07FD 02B8

SUBWF

EXP,F

07FE 1803

BTFSC

_C

07FF 2843

GOTO

SETFOV32M

;set multiply overflow flag

0800 2804

GOTO

MOK32

0801 02B8

MTUN32

SUBWF

EXP,F

;****** IN PAGE 1 !

0802 1C03

BTFSS

_C

0803 2854

GOTO

SETFUN32M

.

. some more code here

.

#endasm

}