M
AN669
Embedding Assembly Routines into C Language Using a Floating Point
Routine as an Example
EXAMPLE 1:
Authors:
Rick Evans
Richard Fischer
Microchip Technology, Inc.
INTRODUCTION
With the advent of MPLAB-C, the Microchip C-compiler, many PICmicro users need to embed existing
assembly language routines and/or Microchip application notes into C. This application note explains how to
embed an assembly language program into MPLAB-C,
version 1.10, and the issues therein. For example,
embedding interrupt save and restore must be done
using assembly language. Also, critical timing routines
may require assembly. The 32-bit floating point multiply
routine from AN575 is used to illustrate this process.
The remaining 32-bit floating point math routines are
embedded into individual C functions and are included
in the file accompanying this application note.
PROCEDURE
For this example, we’ll use a PIC16C74A with 4K
Program Memory, and 192 bytes of RAM.
Embedding assembly routines
In order to embed an assembly language routine in C
code place the #asm and #endasm directives around
the assembly routine. Furthermore, if this is a subroutine, as is the case with the floating point multiply, then
embed the assembly code within a C function declaration. The #asm construct is illustrated in Example 1 with
an excerpt from the 32-bit floating point routine.
1997 Microchip Technology Inc.
#ASM, #ENDASM
CONSTRUCT
void fpm32(void)
{
#asm
FPM32
MOVF
BTFSS
MOVF
BTFSC
GOTO
AEXP,W
_Z
BEXP,W
_Z
RES032M
M32BNE0
MOVF
XORWF
MOVWF
MOVF
ADDWF
MOVLW
AARGB0,W
BARGB0,W
SIGN
BEXP,W
EXP, F
EXPBIAS-1
;test for zero
;arguements
;save sign
;in SIGN
;...etc.
#endasm
}
Locating the Routine in Program Memory,
GOTOs and CALLs
There are two 2K word pages of program memory in
the PIC16C74A. Program memory 000h to 7FFh is
page 0, 800h to FFFh is page 1. By making fpm32() a
C function, MPLAB-C initializes the appropriate page
bit in the PCLATH register before the subroutine call is
made. (See data sheet for more on PCLATH).
A potential problem could arise, however, if the new C
function, fpm32(), crosses the page boundary
(7FFh,800h). MPLAB-C does not insert code into the
assembly code to initialize the page bits (remember
MPLAB-C does take care of paging for function calls).
That means it is up to the programmer to either; 1) add
assembly language to initialize PCLATH appropriately,
or 2) move the entire #asm function within a single
page. Option 1 involves more work. The programmer
must first compile the C code, then analyze the listing
file to see if the assembly function crossed a page
boundary. Finally, add the appropriate assembly language to initialize PCLATH then re-compile. This solution is not desirable since every time new C code is
added to or deleted from the program, the routine,
fpm32() can potentially move across the page boundary. Option 2 is the simplest solution - to locate the C
function in a single page.
DS00669A-page 1
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"
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 necessary assembly language code needed to call fpm32()
or any other C function. The C function is called correctly, but once within the C function, the raw embedded assembly language might have GOTOs or CALLs
that cross over the page boundary and cause
problems.
The listing file generated is shown in Example 3. Notice
the statement GOTO MTUN32 at address 0x7FC. However, 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 assembly 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.
EXAMPLE 3:
FPM32 FORCED TO ADDRESS 0x7F0 TO SHOW CROSSING FROM PAGE 0 TO
PAGE 1
void fpm32 (void)
{
#asm
07F0
07F1
07F2
07F3
07F4
0838
1D03
0839
1903
284E
07F5
07F6
07F7
07F8
07F9
0826
0633
00AE
0839
07B8
.
. some code here
.
FPM32
MOVF
BTFSS
MOVF
BTFSC
GOTO
AEXP,W
_Z
BEXP,W
_Z
RES032M
MOVF
XORWF
MOVWF
MOVF
ADDWF
AARGB0,W
BARGB0,W
SIGN
BEXP,W
EXP, F
07FA 307E
07FB 1C03
07FC 2801
MOVLW
BTFSS
GOTO
EXPBIAS-1
_C
MTUN32
07FD
07FE
07FF
0800
SUBWF
BTFSC
GOTO
GOTO
EXP,F
_C
SETFOV32M
MOK32
SUBWF
BTFSS
GOTO
EXP,F
_C
SETFUN32M
M32BNE0
02B8
1803
2843
2804
0801 02B8
0802 1C03
0803 2854
MTUN32
;test for zero arguments
;save sign in SIGN
;****** WON’T WORK !
;set multiply overflow flag
;****** IN PAGE 1 !
.
. some more code here
.
#endasm
}
1997 Microchip Technology Inc.
DS00669A-page 2
AN669
Assembly Language Variables, Include Files,
etc.
For the floating point math routines of AN575, there is
one include file which contains important constant and
register declarations: math16.inc. This file of declarations is rather extensive, however, it is straightforward
to convert it to C. Example 4 shows a segment of the
math16.inc requiring some attention for the conversion.
EXAMPLE 4:
B0
B1
B2
B3
B4
B5
B6
B7
MSB
LSB
equ
equ
equ
equ
equ
equ
equ
equ
equ
equ
.
. etc.
.
AARGB7 equ
AARGB6 equ
AARGB5 equ
AARGB4 equ
AARGB3 equ
AARGB2 equ
AARGB1 equ
AARGB0 equ
AARG
equ
EXAMPLE 5:
#define
#define
#define
#define
#define
#define
#define
#define
#define
#define
unsigned
unsigned
unsigned
unsigned
unsigned
unsigned
unsigned
unsigned
unsigned
MATH16.INC EXCERPT
FROM AN575. ASSEMBLY
LANGUAGE FILE
These Constant and Variable Declarations
Need to be Converted to C Language
Declarations
Example 5 shows the equivalent C constant and variable declarations. The equates in assembly language
create constants. The equivalent C language is a
#define. Moreover, variables are declared in assembly language by equating a variable name to a register
RAM location (i.e. AARGB7 equ 0x20). In C the variables are declared by assigning a type to the variable.
In the listing in Example 5, AARGB7 is declared as an
unsigned integer data type.
0
1
2
3
4
5
6
7
7
0
0x20
0x21
0x22
0x23
0x24
0x25
0x26
0x27
0x27
; most significant
; byte of argument A
THE CONVERTED MATH16C.C FILE. C LANGUAGE FILE
B0
0
B1
1
B2
2
B3
3
B4
4
B5
5
B6
6
B7
7
MSB
7
LSB
0
.
. etc.
.
int AARGB0
int AARGB1
int AARGB2
int AARGB3
int AARGB4
int AARGB5
int AARGB6
int AARGB7
int AARG
@
@
@
@
@
@
@
@
@
1997 Microchip Technology Inc.
ACCB0;
ACCB1;
ACCB2;
ACCB3;
ACCB4;
ACCB5;
ACCB6;
ACCB7;
ACC;
// most significant byte of argument A
// least significant byte of argument A
// most significant byte of argument A
DS00669A-page 3
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:
2
A = πr
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 displays the hexadecimal floating point number. Table 1
shows the numbers in our example and their equivalent
floating point formats.
Let,
π = 3.141592654
r = 12.34567898 meters
Find A:
TABLE 1: PICmicro 32-BIT FLOATING POINT REPRESENTATIONS OF OUR EXAMPLE
Microchip Floating Point Equivalent
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
EXAMPLE 6:
EXP
B0 (MSB)
B1
B2 (LSB)
0x80
0x49
0x0F
0xDB
0x82
0x45
0x87
0xE7
0x87
0x6F
0x6A
0x07
0x87
0x6F
0x6A
0x07
MAIN ROUTINE TO TEST OUT OUR NEW 32-BIT FLOAT MULTIPLY IN C
#include "16c74a.h"
#include "math16c.c"
#include "fpm32.inc"
// Notice that fpm32 is located in page 0
// Thus, all GOTOs reside in the same page.
void main (void)
{
AEXP
= 0X80;
AARGB0 = 0X49;
AARGB1 = 0X0F;
AARGB2 = 0XDB;
BEXP
= 0X82;
BARGB0 = 0X45;
BARGB1 = 0X87;
BARGB2 = 0XE7;
fpm32();
BEXP
= 0X82;
BARGB0 = 0X45;
BARGB1 = 0X87;
BARGB2 = 0XE7;
fpm32();
while(1);
// PI = 3.141592654
// r = 12.34567898
//
//
//
//
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.
AN669
SUMMARY
For this discussion only the 32-bit floating point multiply
is used. However, the same principles of embedded
assembly language routines into C code can be used
with other assembly language routines. A summary list
of a step- by- step process to embed assembly code
into your C code is below:
separate file for each floating point routine. The files
may be included individually into your C code. Table 2
shows a list of all the files and routines included with
this application note.
• Convert assembly register EQU equates to C variable types such as unsigned int.
• Convert constants to #define in C.
• Place the assembly code into a subroutine using
#asm and #endasm
• To avoid paging issues in parts with multiple program memory pages, force the code to an
address where it will not cross a page boundary.
For example:
#pragma memory ROM [MAXROM-0x800] @ 0x800;
• Macros and conditional assembly will have to be
rewritten in actual in-line assembly code. The
MPLAB-C compiler does not support these higher
level assembly options to the same degree as the
assembler, MPASM.
For your convenience, all the 32-bit floating point routines in application note AN575 are provided in a zip file
along with this application note. Each routine has been
separated to work as a stand-alone routine. There is a
TABLE 2: 32-BIT FLOATING POINT C FILES/FUNCTIONS INCLUDED WITH THIS APPLICATION
NOTE
AN575 Original
Assembly Routine/file *
-
Equivalent C
file/function
Purpose
example.c
The example main() routine calculating
the area given the radius. (uses fpm32)
FLO2432
flo2432.inc
24-bit integer to 32-bit floating point conversion
FLO3232
flo3232.inc
32-bit integer to 32-bit floating point conversion
FPD32
fpd32.inc
32-bit floating point divide
FPM32
fpm32.inc
32-bit floating point multiply
FPA32
FPS32
fpsa32.inc
fps32() 32-bit subtract
fpa32() 32-bit add
32-bit floating point add
32-bit floating point subtract
INT3224
int3224.inc
32-bit floating point to 24-bit integer conversion
INT3232
int3232.inc
32-bit floating point to 32-bit integer conversion
NRM3232
nrm3232.inc
32-bit normalization of unnormalized 32-bit
floating point numbers
NRM4032
nrm4032.inc
32-bit normalization of unnormalized 40-bit
floating point numbers
math16.inc
math16c.c
variables and constants need for the floating point functions
* Check Microchip web site and bulletin board for latest code.
1997 Microchip Technology Inc.
DS00669A-page 5
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•
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