AVR Arduino assembly coding

Its possible to intergrate assembly code in your C C program quite easy

Before we begin

In this tutorial we do only use 1 byte large variable - aka char or unsigned char
All registers are 8 bit (1 byte) registers so this is easier
So this is only first step…
just for now

step 1 - start Arduino IDE

Launch your Arduino IDE, select UNO ( or MEGA 2560) as your hardware
Open a new sketch with no code in it
Tight click the three dots near upper right corner
Click on New Tab ( or ctrl+shift+n)
Name it like myasm.S NB capital S is import
1. meaning assembly file
Name your whole sketch: File ->Save As -> myasm01 01 is short for the first
Connect an UNO or MEGA to your PC

step 2 - hello world

Some code - compile and test it - so we are in the game

 void setup() {
  Serial.begin(115200);
  Serial.println("Hello asm world");
}

void loop() {
}

Lets go assembly code

step 3 - call an assembly function from C

In the C file (ino file) we will call a function named test - test is coded in assembly

The function in C looks like

void test(void);

We must must declare it as an external function in our ino file. This means that the last process in making the program -linking - have to find it somewhere else.

test.ino

extern "C" {
  void test(void); // tell compiler it shall find test for us
}

void setup() {
  Serial.begin(115200);
  Serial.println("Hello asm world");

  test();  // call

  Serial.println("we came back - we did survive");
}

void loop() {
}

In the assembly file we just code it:

 ;Arduino sketch .S subroutine written in AVR assembly code

#include "avr/io.h"

; we tell the world we have something called test
.global test

 ; text is the segment where the code shall be
.text

 ; the funtion - we just RETurn from from the functin (ret command)
 ; test:  is a label or entry point
 test:
  ret  ; return from function

 ; datz all

step 4 - declare variable in assembly and C and use it in C and assembly land

C code - file surname is ino

extern "C" {
  void test(void); // tell compiler it shall find test for us
  extern volatile char xxx;  // xxx is declared in assembly land
}

void setup() {
  Serial.begin(115200);
  Serial.println("Hello asm world");

  test();  // call

  Serial.println("we came back - we did survive");

  xxx = 'a';  // letter a
  Serial.println(xxx);
}

void loop() {
}

assembly part

 ;Arduino sketch .S subroutine written in AVR assembly code

#include "avr/io.h"

; we tell the world we have something called test
.global test
.global xxx


;------ data -----
; data segment is our RAM /SRAM where variable lives

 .data
xxx: .BYTE 1


;-------code or text---------------
 ; text is the segment where the code shall be

.text

 ; the function entry point - we just RETurn from from the functin (ret command)
 test:
  ret   ; return from function

step 5 - increment the variable in Asm from an asm function - called increment

We will write a function in assembly land which incremt

a variable - xxx -in assembly land
a variable - yyy - which is in C land

extern "C" {
  void test(void);  // tell compiler it shall find test for us
  void increment(void);
  extern volatile char xxx;  // xxx is declared in assembly land
}

char yyy;  // in C a variable is public pr default

static char zzz;  // only accessable in this file

void setup() {
  Serial.begin(115200);
  Serial.println("Hello asm world");

  test();  // call

  Serial.println("we came back - we did survive");

  xxx = 'a';  // letter a
  yyy = 'A';
  Serial.println(xxx);
  Serial.println(yyy);
  increment();          // increment xxx by 1
  Serial.println(xxx);  // should b b
  Serial.println(yyy);
}

void loop() {
}

The assembly part

  ;Arduino sketch .S subroutine written in AVR assembly code

#include "avr/io.h"

; we tell the world we have something called test
; if you do not label them global they are secret outside of S file
.global test
.global increment
.global xxx


;------ data -----
; data segment is our RAM /SRAM where variable lives

 .data
xxx: .BYTE 1


;-------code or text---------------
 ; text is the segment where the code shall be

.text

 ;--------- test
 ; the function - we just RETurn from from the function (ret command)
 test:
  ret

;------ increment
increment:
push r16 ; so we dont destroy what may be in register 16

  lds r16,xxx ; load xxx in register 16 (8 bit register)
  inc r16     ; increment register
  sts xxx, r16  ; store register r16 to varable xxx

  ; now increment yyy in C land
  lds r16,yyy
  inc r16
  sts yyy,r16

; bad line below gives compiler error bq zzz is declared static in the c file == private
; so we cant see it
; lds r16, zzz


  pop r16 ; restore r16
  ret

  ; lds see https://courses.cs.washington.edu/courses/csep567/10wi/resources/ATmegaInstructionSet.pdf page 93
  ; sts see https://courses.cs.washington.edu/courses/csep567/10wi/resources/ATmegaInstructionSet.pdf page 147

The instructions lsd and store

notice that Register 0 to 31 can be used in lds and sts. Prerefable use r16 or higher

step 6 - blink led pin 13

ino

extern "C" {
  void led13on();
  void led13off();
  void set13out();
}


void setup() {
  set13out();
  //start();
}
void loop() {
   //forever();
  led13on();
  delay(200);
  led13off();
  delay(200);
}

asm

; Arduino sketch .S subroutine written in AVR assembly code

#include "avr/io.h"

.global led13on
.global led13off
.global set13out

set13out:
  sbi DDRB-0x20,5
  ret

led13on:
  sbi PORTB-0x20,5
  ret

led13off:
  cbi PORTB-0x20,5
  ret


; sbi see https://courses.cs.washington.edu/courses/csep567/10wi/resources/ATmegaInstructionSet.pdf page 120
; cbi see https://courses.cs.washington.edu/courses/csep567/10wi/resources/ATmegaInstructionSet.pdf page 48

step 7 - blink 13 vrs 2

Has slow and fast code - FYI

ino

extern "C" {
  void led13on();
  void led13onfast();
  void led13off();
  void led13offfast();
  void set13out();
  void set13outfast();
}


void setup() {
  set13out();
  //start();
}
void loop() {
  //forever();
  led13on();
  delay(200);
  led13off();
  delay(200);

  led13onfast();
  delay(200);
  led13offfast();
  delay(200);
}

assembly

; Arduino sketch .S subroutine written in AVR assembly code

#include "avr/io.h"

.global led13on
.global led13onfast

.global led13off
.global led13offfast

.global set13out
.global set13outfast

.text

set13outfast:
  sbi DDRB-0x20,5  ; do only set bit 5 - do not touch others
  ret

set13out:
  push r16
  ;sbi DDRB-0x20,5  ; do only set bit 5 - do not touch others
  ldi r16,  0b00100000
  out DDRB-0x20, r16      ; sets all 8 bite
  pop r16
  ret

led13onfast:
  sbi PORTB-0x20,5
  ret;

led13on:
  push r16
  in r16, PORTB-0x20
  ori r16,0b00100000  ; or immediate mask bit 5 high - rest untouched
  out PORTB- 0x20, r16
  pop r16
  ret


led13offfast:
  cbi PORTB-0x20,5  ; cbi : clear bit no 5
  ret

led13off:
   push r16
  in r16, PORTB-0x20
  andi r16,0b11011111
  out PORTB- 0x20, r16
  ;cbi PORTB-0x20,5
  pop r16
  ret

; ori see https://courses.cs.washington.edu/courses/csep567/10wi/resources/ATmegaInstructionSet.pdf page 107
; andi see https://courses.cs.washington.edu/courses/csep567/10wi/resources/ATmegaInstructionSet.pdf page 20

step 8 - from C call asm fct which calls a C function

ino

extern "C" {
  void asmfct();
  void cfct();
}

volatile int mark = 0;

void cfct() { // is called from asm world
  mark++;
}

void setup() {
  Serial.begin(115200);
  Serial.println(__FILE__);   Serial.println(__DATE__);   Serial.println(__TIME__);
  Serial.println("bef");
  Serial.print("mark ");
  Serial.println(mark);

  asmfct();
  Serial.println("after");
  Serial.print("mark ");
  Serial.println(mark);
}
void loop() {}

asm

; Arduino sketch .S subroutine written in AVR assembly code

#include "avr/io.h"

.text

 .extern cfct

.global asmfct

 asmfct:
   call cfct ;
   ret