AN669 データシートの表示（PDF） - Microchip Technology

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AN669

Embedding Assembly Routines into C Language Using a Floating Point Routine as an Example

Microchip Technology 

AN669

USING 32-BIT FLOATING POINT

MULTIPLY

Using the 32-bit floating point multiply supplied with

AN575 in a C program is straightforward. First, copy the

entire routine from the file fpm32.a16 (from AN575).

Then, create a function with the same name as the

assembly routine.

Lets take a well known formula:

A = πr2

Let,

π = 3.141592654

r = 12.34567898 meters

Find A:

We need to convert the previous decimal numbers to

Microchip 32-bit floating point. Use fpm32 (from

AN575), to solve the equation. We will use MPLAB-C

and use our C function named fpm32(). The main

routine is listed in Example 6.

AN575 comes with a handy utility called fprep.exe.

This Microchip file is a DOS executable. When running

fprep, you can enter in a decimal number and it dis-

plays the hexadecimal floating point number. Table 1

shows the numbers in our example and their equivalent

floating point formats.

TABLE 1: PICmicro™ 32-BIT FLOATING POINT REPRESENTATIONS OF OUR EXAMPLE

Decimal Number

π = 3.141592654

r = 12.34567898 meters

A = 478.8283246 m2 -- fprep.exe calculated result

A = 478.8283246 m2 -- PIC16C74A measured result

using MPLAB 3.12 and PICMASTER 16J probe

Microchip Floating Point Equivalent

EXP

B0 (MSB)

B1

B2 (LSB)

0x80

0x82

0x87

0x49

0x45

0x6F

0x0F

0x87

0x6A

0xDB

0xE7

0x07

0x87

0x6F

0x6A

0x07

EXAMPLE 6: MAIN ROUTINE TO TEST OUT OUR NEW 32-BIT FLOAT MULTIPLY IN C

#include "16c74a.h"

#include "math16c.c"

#include "fpm32.inc"

void main (void)

{

AEXP = 0X80;

AARGB0 = 0X49;

AARGB1 = 0X0F;

AARGB2 = 0XDB;

BEXP = 0X82;

BARGB0 = 0X45;

BARGB1 = 0X87;

BARGB2 = 0XE7;

// Notice that fpm32 is located in page 0

// Thus, all GOTOs reside in the same page.

// PI = 3.141592654

// r = 12.34567898

fpm32();

BEXP = 0X82;

BARGB0 = 0X45;

BARGB1 = 0X87;

BARGB2 = 0XE7;

fpm32();

while(1);

}

// AARG = PI * r

// you must reload r into BARG since

// fpm32() destroys BARG.

// r = 12.34567898

// AARG = (PI*r)*r

DS00669A-page 4

© 1997 Microchip Technology Inc.

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