AVR-C code + makefile. See: http://fab.cba.mit.edu/classes/863.15/section.CBA/people/Schaad/week8-embedded-programming.html

att44_schaad1.c

//
//  att44_schaad1.c
//
//  Created by Raphael Schaad on 2015-11-03.
//  This is free and unencumbered software released into the public domain.
//

// Import headers installed with CrossPack-AVR (/usr/local/CrossPack-AVR/)
#include "stdbool.h" // So we have 'true' and 'false' instead of 1 and 0 (might have memory implications) -- lib/gcc/avr/4.8.1/include/
#include <avr/io.h> // Umbrella header including e.g. iotn44a.h for our chip, defining PORTx/DDRx/Pxn etc. (mapping to addresses from data sheet) -- avr/include/avr/
#include <util/delay.h> // _delay_us/ms convenience functions for actual time values rather than number of cycles -- avr/include/util

// Hardware:
// Based on Neil Gershenfeld's board: http://academy.cba.mit.edu/classes/embedded_programming/index.html#echo
// With my redesign and enhancements: http://fab.cba.mit.edu/classes/863.15/section.CBA/people/Schaad/week6-electronics-design.html
#define LED1 (1 << PB2) // red   (pin 5)
#define LED2 (1 << PA2) // green (pin 11)
#define LED3 (1 << PA3) // green (pin 10)
#define BTN1 (1 << PA7) //       (pin 6)

// Convenience macros
#define output(directions, pin) (directions |= pin)    // set port direction for output
#define input(directions, pin)  (directions &= (~pin)) // set port direction for input
#define set(port, pin)          (port |= pin)          // set port pin
#define clear(port, pin)        (port &= (~pin))       // clear port pin
#define pin_test(pins, pin)     (pins & pin)           // test for port pin
#define bit_test(byte, bit)     (byte & (1 << bit))    // test for bit set

// Copied configuration defines -- but I find this abstraction more confusing
// #define input_port PORTA      // BTN1 is on port A
// #define input_direction DDRA  // DDRA defines input/output for port A
// #define input_pin (1 << PA7)  // BTN1 is on pin 7 of port A
// #define input_pins PINA       // PINA is the register that is read to detect input high or low.
// #define output_port PORTB     // Port B will be used for the LED
// #define output_direction DDRB // DDRB defines input/output for port B
// #define output_pin (1 << PB2) // LED1 is on pin 2 of port B


// Bootloader calls main(void) when the chip gets power
int main() {
   // Set clock divider to 1 (default is 8, from CKDIV8 LFuse).
   // The internal clock of t44/45 is max 8MHz but on this board we configure LFuse to use an external 20MHz resonator.
   // This needs to be reflected in the Makefile with F_CPU = 20000000.
   CLKPR = (1 << CLKPCE);
   CLKPR = (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0);

   // Configure digital I/O pins DDRA & DDRB (bits are 0-based index)
   output(DDRB, LED1); // PB2 as output by setting bit 2 in register DDRB (= 0b00000100)
   output(DDRA, LED2);
   output(DDRA, LED3);

   input(DDRA, BTN1); // PB7 as input by setting bit 7 in register DDRA (= 0b10000000) ...
   set(PORTA, BTN1); // ... and on PORTA (= 0b10000000; turns on pullup resistor)
   
   // Turn LED1 ON by default
   set(PORTB, LED1); // PORTB = 0b00000100;

   // Spin event loop
   while (true) {
      // Blink LED2/3 on button press
      if (pin_test(PINA, BTN1)) { // (PINA & 0b10000000) == 1
         set(PORTB, LED1);
         clear(PORTA, LED2);
         clear(PORTA, LED3);
      } else {
         clear(PORTB, LED1);
         if (pin_test(PINA, LED2)) {
            set(PORTA, LED3);
            clear(PORTA, LED2);
         } else {
            set(PORTA, LED2);
            clear(PORTA, LED3);
         }
         _delay_ms(1000.0);
      }
   }

   // When main(void) exits, the chip goes idle
   return 0;
}

att44_schaad1.make

#
#  att44_schaad1.make
#
#  Created by Raphael Schaad on 2015-11-03.
#  This is free and unencumbered software released into the public domain.
#

PROJECT=att44_schaad1
SOURCES=$(PROJECT).c
MMCU=attiny44
F_CPU = 20000000

CFLAGS=-mmcu=$(MMCU) -Wall -Os -DF_CPU=$(F_CPU)

$(PROJECT).hex: $(PROJECT).out
	avr-objcopy -O ihex $(PROJECT).out $(PROJECT).c.hex;\
	avr-size --mcu=$(MMCU) --format=avr $(PROJECT).out

$(PROJECT).out: $(SOURCES)
	avr-gcc $(CFLAGS) -I./ -o $(PROJECT).out $(SOURCES)

program-usbtiny: $(PROJECT).hex
	avrdude -p t44 -P usb -c usbtiny -U flash:w:$(PROJECT).c.hex

program-usbtiny-fuses: $(PROJECT).hex
	avrdude -p t44 -P usb -c usbtiny -U lfuse:w:0x5E:m