FPSLIC - Application Notes - Atmel Corporation

Mixing C and Assembly Code with IAR

Embedded Workbench™ for FPSLIC™

Features

•Passing Variables between C and

Assembly Code Functions

•Calling Assembly Code Functions

from C

•Calling C Functions from

Assembly Code

•Writing Interrupt Functions in

Assembly Code

•Accessing Global Variables in

Assembly Code

Introduction

This application note describes how to

use C to control the program flow and

main program and assembly modules to

control time-critical I/O functions.

The application note also describes how

to set up and use the IAR C-compiler for

the FPSLIC controller in projects includ-

ing both C and assembly code. By mix-

ing C and assembly, designers can

combine the powerful C language

instructions with the effective hardware-

near assembly code instructions.

Table 1. The Pluses and Minuses of C and Assembly

Assembly C

+ Full control of resource usage

+ Compact/fast code in small applications

- Inefficient code in larger applications

- Cryptic code

- Hard to maintain

- Non-portable

+ Efficient code in larger applications

+ Structured code

+ Easy to maintain

+ Portable

- Limited control of resource usage

- Larger/slower code in small applications

Mixing C and

Assembly

Application

Note

Rev. 1976A–11/00

FPSLIC

Passing Variables between C and

Assembly Code Functions

When the IAR C-compiler is used for the FPSLIC, the reg-

ister file is segmented as shown in Figure 1.

Scratch registers are not preserved across function calls.

Local registers are preserved across function calls. The Y

SRAM. The scratch registers are used for passing parame-

ters and return values between functions.

When a function is called, the parameters to be passed to

the function are placed in the register file (registers

R16 - R23). When a function is returning a value, this value

is placed in the register file (registers R16 - R19), depend-

ing on the size of the parameters and the returned value.

Table 2 shows example placement of parameters when

calling a function.

Figure 1. Segments in the Register File

For a complete reference of the supported data types and

corresponding sizes, see the AVR® IAR Compiler

Reference Guide from IAR Systems, Data Representation

section.

Example C function call:

int get_port(unsigned char temp, int num)

When calling this C function, the 1-byte parameter temp is

placed in R16 and the 2-byte parameter num is placed in

R20:R21. The function returns a 2-byte value, which is

placed in R16:R17 after return from the function.

If a function is called with more than two parameters, the

first two parameters are passed to the function as shown

above. The remaining parameters are passed to the func-

tion on the data stack. If a function is called with a struct or

union as a parameter, a pointer to the structure is passed

on to the function on the data stack.

If a function needs to use any local registers, it first pushes

the registers on the data stack. Then the return value from

the function is placed at addresses R16 - R19, depending

on the size of the returned value.

Scratch Register R0 - R3

Local Register R4 - R15

Scratch Register R16 - R23

Local Register R24 - R27

Data Stack Pointers (Y) R28 - R29

Scratch Register R30 - R31

Table 2. Placement and Parameters of C Functions

Function Parameter 1 Registers Parameter 2 Registers

func (char, char) R16 R20

func (char, int) R16 R20, R21

func (int, long) R16, R17 R20, R21, R22, R23

func (long, long) R16, R17, R18, R19 R20, R21, R22, R23

FPSLIC

Example 1

Calling Assembly Code Functions from a C Program – with No Parameters and No

Return Value

Example C Code for Calling Assembly Code Function

#include "ioat94k.h"

extern void get_port(void);/* Function prototype for asm function */

void main(void)

{

DDRD = 0x00; /* Initialization of the I/O ports */

DDRE = 0xFF;

while(1) /* Infinite loop */

{

get_port(); /* Call the assembler function */

}

The Called Assembly Code Function

NAME get_port

#include "ioat94k.h" ; The #include file must be within the module

PUBLIC get_port ; Declare symbols to be exported to C function

RSEG CODE ; This code is relocatable, RSEG

get_port; ; Label, start execution here

in R16,PIND ; Read in the pind value

swap R16 ; Swap the upper and lower nibble

out PORTE,R16 ; Output the data to the port register

ret ; Return to the main function

END

FPSLIC

Calling Assembly Code Functions from a C Function – Passing Parameters and

Returning Values

This example C function is calling an assembler function.

The 1-byte mask is passed as a parameter to the assem-

bly function; mask is placed in R16 before the function call.

The assembly function is returning a value in R16 to the C

variable value.

#include "ioat94k.h"

char get_port(char mask); /*Function prototype for asm function */

void C_task main(void)

{

DDRE=0xFF

while(1) /* Infinite loop*/

{

char value, temp; /* Decalre local variables*/

temp = 0x0F;

value = get_port(temp); /* Call the assembler function */

if(value==0x01)

{

/* Do something if value is 0x01 */

PORTE=~(PORTE); /* Invert value on Port E */

}

The Called Assembly Code Function

NAME get_port

#include "ioat94k.h" ; The #include file must be within the module

PUBLIC get_port ; Symbols to be exported to C function

RSEG CODE ; This code is relocatable, RSEG

get_port: ; Label, start execution here

in R17,PIND ; Read in the pind value

eor R16,R17 ; XOR value with mask(in R16) from main()

swap R16 ; Swap the upper and lower nibble

rol R16 ; Rotate R16 to the left

brcc ret0 ; Jump if the carry flag is cleared

ldi r16,0x01 ; Load 1 into R16, return value

ret ; Return

ret0: clr R16 ; Load 0 into R16, return value

ret ; Return

END

FPSLIC

Calling C Functions from Assembly Code

Assuming that the assembly function calls the standard C

library routine rand() to get a random number to output

to the port. The rand() routine returns an integer value

(16 bits). This example writes only the lower byte/8 bits to

aport.

NAME get_port

#include "ioat94k.h" ; The #include file must be within the module

EXTERN rand, max_val ; External symbols used in the function

PUBLIC get_port ; Symbols to be exported to C function

RSEG CODE ; This code is relocatable, RSEG

get_port: ; Label, start execution here

clr R16 ; Clear R16

sbis PIND,0 ; Test if PIND0 is 0

rcall rand ; Call RAND() if PIND0 = 0

out PORTE,R16 ; Output random value to PORTE

lds R17,max_val ; Load the global variable max_val

cp R17,R16 ; Check if number higher than max_val

brlt nostore ; Skip if not

sts max_val,R16 ; Store the new number if it is higher

nostore:

ret ; Return

END

FPSLIC

Writing Interrupt Functions in Assembly

Interrupt functions can be written in assembly. Interrupt

functions cannot have any parameters or return any value.

Because an interrupt can occur anywhere in the program

execution, it needs to store all used registers on the stack.

Care must be taken to avoid problems with the interrupt

functions in C when assembler code is placed at the inter-

rupt vector addresses.

Example Code Placed at Interrupt Vector

NAME EXT_INT1

#include "ioat94k.h"

extern c_int1

COMMON INTVEC(1) ; Code in interrupt vector segment

ORG INT1_vect ; Place code at interrupt vector

RJMP c_int1 ; Jump to assembler interrupt function

ENDMOD

;The interrupt vector code performs a jump to the function c_int1:

NAME c_int1

#include "ioat94k.h"

PUBLIC c_int1 ; Symbols to be exported to C function

RSEG CODE ; This code is relocatable, RSEG

c_int1:

st -Y,R16 ; Push used registers on stack

in R16,SREG ; Read status register

st -Y,R16 ; Push Status register

in R16,PIND ; Load in value from port D

com R16 ; Invert it

out PORTE,R16 ; Output inverted value to port E

ld R16,Y+ ; Pop status register

out SREG,R16 ; Store status register

ld R16,Y+ ; Pop Register R16

reti

END

FPSLIC

Accessing Global Variables in Assembly

The main program introduces a global variable called

max_val. To access this variable in assembly, the variable

must be declared as EXTERN max_val. To access the

variable, the assembly function uses LDS (Load Direct

from SRAM) and STS (STore Direct to SRAM) instructions.

#include "ioat94k.h"

char max_val;

void get_port(void); /* Function prototype for assembler function */

void C_task main(void)

{

DDRE = 0xFF; /* Set port E as output */

while(1) /* Infinite loop */

{

get_port(); /* Call assembly code function */

}

NAME get_port

#include "ioat94k.h" ; The #include file must be within the module

EXTERN rand, max_val ; External symbols used in the function

PUBLIC get_port ; Symbols to be exported to C function

RSEG CODE ; This code is relocatable, RSEG

get_port: ; Label, start execution here

clr R16 ; Clear R16

sbis PIND,0 ; Test if PIND0 is 0

rcall rand ; Call RAND() if PIND0 = 0

out PORTE,R16 ; Output random value to PORTE

lds R17,max_val ; Load the global variable max_val

cp R17,R16 ; Check if number higher than max_val

brlt nostore ; Skip if not

sts max_val,R16 ; Store the new number if it is higher

nostore:

ret ; Return

END

References

AVR® IAR Compiler Reference Guide from IAR Systems

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