Arduino Inline Assembly Tutorial (Interrupts)


Pardon The Interruption

The previous tutorial covered the basics of writing inline functions. A close relative of the function is the Interrupt Service Routine (ISR), which is the topic here. Portions of this tutorial may pertain to functions as well.

As a warning, this tutorial assumes an understanding of the basic concepts of interrupts in general, and specifically interrupt handlers on the arduino (AVR μC). Hopefully, you have already written a few arduino interrupts in C, using the internal arduino functionality. If not, you may want to study some of the links given in the reference section of this tutorial before continuing.

The Deck is Stacked

Basic knowledge of the stack is essential to understanding functions and interrupt handlers. The basic purpose of the stack is to support function calls and interrupts. Whenever a program makes a function call or whenever an interrupt occurs, the stack is used to store critical information which will be restored upon completion of the function or interrupt. Additional information on the stack can be found here and here.

First and primary, during a function call or interrupt, the hardware places the return address on the stack. The saving and restoration of the return address is accomplished transparently by the CALL and RET instructions. It is not necessary to perform any special instruction(s) to make this occur.

Second, if any “call-saved” registers will be “clobbered” inside the function, these registers are “pushed” onto the stack. In the case of an interrupt service routine, all of the registers used inside the ISR (and always the temporary and zero registers, r0 and r1) get pushed onto the stack. Additionally, during an ISR the SREG is saved and restored.

Finally, if the compiler deems it necessary, space is reserved for any local variables on the stack. Many times the compiler will place local variables into specific registers, and therefore doesn’t use the stack for temporary storage.

Here is an example of how the compiler uses the stack to store local variables inside of a function. This is sometimes referred to as “setting up a stack frame.” We will reserve 16 bytes for a character array (note: unrelated code has been removed for the purpose of clarity). The compiler performs all of this stack manipulation for us behind the scenes, so-to-speak:

void example(void) {
  char buffer[16]; //space will be reserved on the stack
  //do something here. . .

Result in this machine code:

  PUSH r28          ;save registers on stack 
  PUSH r29 
  IN   r28, SPL     ;get stack pointer    
  IN   r29, SPH   
  SBIW r28, 16      ;reserve 16 bytes space on stack
                    ;the stack grows downward, hence the subtraction
  OUT  SPH, r29     ;update new stack pointer
  OUT  SPL, r28 
;do something here. . .
  ADIW r28, 16      ;remove the 16 bytes from the stack
  OUT  SPH, r29     ;restore stack pointer
  OUT  SPL, r28 
  POP  r29          ;restore registers from stack
  POP  r28 

Upon return from the interrupt or function, all the preserved values are restored, or “popped” from the stack. Obviously, during the pro and epilogue code, the order of the push and pop instructions is very critical.

Interrupt Before and After

Below, I wrote a very basic interrupt routine that simply increments a byte so we can examine the prologue and epilogue code generated by the compiler:

//here is an example ISR coded in C:
volatile uint8_t a;
ISR(INT0_vect) {
//this is the generated assembly code:
0000027F 1f.92                PUSH r1       ;save r1 register
00000280 0f.92                PUSH r0       ;save r0 register
00000281 0f.b6                IN r0, SREG   ;get status register
00000282 0f.92                PUSH r0       ;save sreg 
00000283 11.24                CLR r1        
00000284 8f.93                PUSH r24      ;save r24 register
;increment byte (a) here
00000285 80.91.c3.01          LDS r24, (a) 
00000287 8f.5f                SUBI r24, 0xFF     
00000288 80.93.c3.01          STS (a), r24 
0000028A 8f.91                POP r24       ;restore r24 register
0000028B 0f.90                POP r0        ;restore status register
0000028C                OUT SREG, r0
0000028D 0f.90                POP r0        ;restore r0 register
0000028E 1f.90                POP r1        ;restore r1 register
0000028F 18.95                RETI          ;return from interrupt

As you can see, the meat of the ISR is only 10 bytes long. However, together the prologue and epilogue add another 24 bytes, for a total of 34. It might be possible to save a few bytes and program cycles by tightly writing your own ISR pro and epilogue. GCC has a provision which allows writing your own pro and epilogues, which will be covered later.

We Interrupt This Program to Blink

It is now time to write an interrupt handler, or ISR in inline assembler. I can’t think of a better example than to adapt the basic Blink sketch to use the Timer #1 Overflow interrupt. Please note, because this code alters the Timer #1 registers, it will render any use of the arduino Timer #1 as nonfunctional (i.e. analogWrite pins 9 & 10, the Servo Library, etc.).

Handle It

The first order of business is to write the interrupt handler for the Timer #1 Overflow. This is the routine that is called when the Timer #1 counter (TCNT1) rolls over from 0xffff to zero. Our the ISR is very basic, and as always, it should be kept as short as possible. Inside the handler we perform two functions:

  • Reset the counter (TCNT1) allowing the next overflow to reoccur at 1 second intervals.
  • Toggle the LED.

An ISR can be coded using inline assembler just as in a “C Stub Function”, relying upon the compiler to insert the necessary prologue and epilogue code. I suggest you use this stub technique at first before graduating to writing the entire “naked” ISR. Here is a stub version of our ISR:

#define TCNT_BASE   0x0bdc
#define TCNT_BASE_H (((TCNT_BASE)>>8)&0xff)
#define TCNT_BASE_L ((TCNT_BASE)&0xff)

ISR(TIMER1_OVF_vect) {
  asm (
    //reload TCNT1 counter for 1sec interrupt
    "ldi r24, %3           \n"
    "st  Z+, r24           \n" //TCNT1L
    "ldi r24, %4           \n"
    "st  Z, r24            \n" //TCNT1H
    //toggle LED
    "in   __tmp_reg__, %0  \n" //read port
    "ldi  r24, %1          \n" //LED bit mask
    "eor  __tmp_reg__, r24 \n" //toggle LED bit
    "out  %0, __tmp_reg__  \n" //write port
    : : "I" (_SFR_IO_ADDR(PORTB)), "I" (_BV(PORTB5)),
    "z" (_SFR_MEM_ADDR(TCNT1)), "M" (TCNT_BASE_L), "M" (TCNT_BASE_H) : "r24"

Having said all that, the boilerplate code the compiler inserts is not always the most efficient, and many times inadequate. For these reasons, and for the academic exercise, we will also select the “ISR_NAKED” attribute when defining the ISR. This gives us full control over all of the code inside the ISR. Full control is a good thing:


Eleven instructions encompass the prologue and epilogue, which is more than the code required for the main purpose of the interrupt. Notice inside the handler, we utilize 3 registers, r24, r30 and r31. This means we need to preserve the content of these registers since the interrupt could be triggered at any time, even precisely when these registers may be in use. Additionally we need to preserve the status register (SREG). The SREG holds critical information on the state of the program when the interrupt fired. Neglecting to reserve any of this information would probably cause the program to crash.

Don’t forget to include the terminating RETI instruction also. By comparison, this ISR_NAKED version is 10 bytes shorter than the “Stub” version:

#include "k328p.h"

#define TCNT_BASE   0x0bdc
#define TCNT_BASE_H (((TCNT_BASE)>>8)&0xff)
#define TCNT_BASE_L ((TCNT_BASE)&0xff)

  asm (
    "push r31           \n" //save r30, r31 contents
    "push r30           \n"
    "push r24           \n"
    //preserve SREG
    "in   r24, __SREG__ \n"
    "push r24           \n"

    //reload TCNT1 counter for 1sec interrupt
    "clr r31            \n"
    "ldi r30, %2        \n"
    "ldi r24, %3        \n"
    "st  Z+, r24        \n" //TCNT1L
    "ldi r24, %4        \n"
    "st  Z, r24         \n" //TCNT1H
    //toggle LED
    "in   r30, %0       \n" //read port
    "ldi  r31, %1       \n" //LED bit mask
    "eor  r30, r31      \n" //toggle LED bit
    "out  %0, r30       \n" //write port

    //restore old SREG
    "pop  r24           \n"
    "out  __SREG__, r24 \n"
    //restore r30, r31
    "pop r24            \n"
    "pop  r30           \n"
    "pop  r31           \n"
    "reti               \n"
    : : "I" (kPORTB), "I" (_BV(PORTB5)), 
    "M" (kTCNT1), "M" (TCNT_BASE_L), "M" (TCNT_BASE_H)

The initiation code required for the Timer #1 interrupt (setting the prescaler, loading the counter and enabling the overflow interrupt) is completely contained inside the Setup function. Obviously, it is not necessary to write this in inline assembly, it’s just good practice:

#include "k328p.h"

#define TCNT_BASE   0x0bdc
#define TCNT_BASE_H (((TCNT_BASE)>>8)&0xff)
#define TCNT_BASE_L ((TCNT_BASE)&0xff)

void setup() {
  uint16_t TNCTBase = TCNT_BASE;

  asm (
    "cli                  \n" //disable gloal interrupts 
    "sbi %0, %1           \n" //pinMode(13, OUTPUT);

    //set 256 prescale (CS12)
    "st  Z+, __zero_reg__ \n" //TCCR1A
    "ldi r24, %3          \n"
    "st  Z+, r24          \n" //zero TCCR1B
    "st  Z, __zero_reg__  \n" //zero TCCR1C
    //load counter for 1sec interrupt
    "ldi r30, %4          \n"
    "st  Z+, %A5          \n" //TCNT1L
    "st  Z, %B5           \n" //TCNT1H
    //enable overflow interrupt
    "ldi r30, %6          \n"
    "ldi r24, %7          \n"
    "st  Z, r24           \n" //TIMSK1

    "sei                  \n" //enable global interrupts 
    : : "I" (_SFR_IO_ADDR(DDRB)), "I" (PORTB5),
    "z" (_SFR_MEM_ADDR(TCCR1A)), "I" (_BV(CS12)),
    "M" (kTCNT1), "r" (TNCTBase),
    "M" (kTIMSK1), "I" (_BV(TOIE1)) : "r24", "memory"

void loop() { }

Finally, we are introducing a new header file “k328p.h” (contents listed below) which contains all of the IO register defines in such a way that we can use them inside our inline assembly routines. The definitions in this file use the same standard ATMEL mnemonics for the IO registers with the letter ‘k’ pre-pended. They are the LSB of the IO register address, and allow greater flexibility in inline assembler code when referring to the IO registers (when using pointer registers with the LD/ST instructions). A close examination of the above code will reveal the method of use.

Arduino IO Register Defines

//k328p.h - definitions for ATmega328P
#ifndef _k328P_H_
#define _k328P_H_ 

//standard registers 
//0-0x1f: bit addressable
//0-0x3f: IN/OUT compatible 
//0-0x3f: add 0x20 when using LD/ST
#define kPINB   0x03
#define kDDRB   0x04
#define kPORTB  0x05
#define kPINC   0x06
#define kDDRC   0x07
#define kPORTC  0x08
#define kPIND   0x09
#define kDDRD   0x0A
#define kPORTD  0x0B

#define kTIFR0  0x15
#define kTIFR1  0x16
#define kTIFR2  0x17

#define kPCIFR  0x1B
#define kEIFR   0x1C
#define kEIMSK  0x1D
#define kGPIOR0 0x1E
#define kEECR   0x1F
//end bit addressable

#define kEEDR   0x20
#define kEEAR   0x21
#define kEEARL  0x21
#define kEEARH  0x22
#define kGTCCR  0x23
#define kTCCR0A 0x24
#define kTCCR0B 0x25
#define kTCNT0  0x26
#define kOCR0A  0x27
#define kOCR0B  0x28

#define kGPIOR1 0x2A
#define kGPIOR2 0x2B
#define kSPCR   0x2C
#define kSPSR   0x2D
#define kSPDR   0x2E

#define kACSR   0x30

#define kMCUSR  0x34
#define kMCUCR  0x35

#define kSPMCSR 0x37

#define kSPL    0x3D
#define kSPH    0x3E
#define kSREG   0x3F
//end IN/OUT compatible

//extended registers begin
#define kWDTCSR 0x60
#define kCLKPR  0x61

#define kPRR    0x64

#define kOSCCAL 0x66

#define kPCICR  0x68
#define kEICRA  0x69

#define kPCMSK0 0x6B
#define kPCMSK1 0x6C
#define kPCMSK2 0x6D
#define kTIMSK0 0x6E
#define kTIMSK1 0x6F
#define kTIMSK2 0x70

#define kADC    0x78
#define kADCW   0x78
#define kADCL   0x78
#define kADCH   0x79
#define kADCSRA 0x7A
#define kADCSRB 0x7B
#define kADMUX  0x7C

#define kDIDR0  0x7E
#define kDIDR1  0x7F

#define kTCCR1A 0x80
#define kTCCR1B 0x81
#define kTCCR1C 0x82

#define kTCNT1  0x84
#define kTCNT1L 0x84
#define kTCNT1H 0x85
#define kICR1   0x86
#define kICR1L  0x86
#define kICR1H  0x87
#define kOCR1A  0x88
#define kOCR1AL 0x88
#define kOCR1AH 0x89
#define kOCR1B  0x8A
#define kOCR1BL 0x8A
#define kOCR1BH 0x8B

#define kTCCR2A 0xB0
#define kTCCR2B 0xB1
#define kTCNT2  0xB2
#define kOCR2A  0xB3
#define kOCR2B  0xB4
#define kASSR   0xB6

#define kTWBR   0xB8
#define kTWSR   0xB9
#define kTWAR   0xBA
#define kTWDR   0xBB
#define kTWCR   0xBC
#define kTWAMR  0xBD

#define kUCSR0A 0xC0
#define kUCSR0B 0xC1
#define kUCSR0C 0xC2

#define kUBRR0  0xC4
#define kUBRR0L 0xC4
#define kUBRR0H 0xC5
#define kUDR0   0xC6
//end extended registers

//0-0x3f for LD/ST instructions
#define k2PINB   0x23
#define k2DDRB   0x24
#define k2PORTB  0x25
#define k2PINC   0x26
#define k2DDRC   0x27
#define k2PORTC  0x28
#define k2PIND   0x29
#define k2DDRD   0x2A
#define k2PORTD  0x2B
#define k2TIFR0  0x35
#define k2TIFR1  0x36
#define k2TIFR2  0x37
#define k2PCIFR  0x3B
#define k2EIFR   0x3C
#define k2EIMSK  0x3D
#define k2GPIOR0 0x3E
#define k2EECR   0x3F
#define k2EEDR   0x40
#define k2EEAR   0x41
#define k2EEARL  0x41
#define k2EEARH  0x42
#define k2GTCCR  0x43
#define k2TCCR0A 0x44
#define k2TCCR0B 0x45
#define k2TCNT0  0x46
#define k2OCR0A  0x47
#define k2OCR0B  0x48
#define k2GPIOR1 0x4A
#define k2GPIOR2 0x4B
#define k2SPCR   0x4C
#define k2SPSR   0x4D
#define k2SPDR   0x4E
#define k2ACSR   0x50
#define k2MCUSR  0x54
#define k2MCUCR  0x55
#define k2SPMCSR 0x57
#define k2SPL     0x5D
#define k2SPH     0x5E
#define k2SREG    0x5F

#endif //_k328P_H_


Arduino Interrupts
Newbie’s Guide to AVR Interrupts
PJRC Guide to Interrupts
AVR Libc Information on Interrupts
University of Maryland, BC, C Programming and Embedded Systems Course, Interrupt Information
AVR 8-bit Instruction Set
AVR-GCC Inline Assembler Cookbook
Extended Asm – Assembler Instructions with C Expression Operands
Mixing C and Assembly Language
ATMEL ATmega328P Datasheet

Also available as a book, with greatly expanded coverage!

[click on the image]

About Jim Eli

µC experimenter
This entry was posted in arduino, assembly language, avr, avr inline assenbly and tagged , , , , , . Bookmark the permalink.

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