commit 135f785aab0ef96c5b85e5b6a3f1bca6a2645c27
Author: lucic71 <lucian.popescu187@gmail.com>
Date:   Tue Jun 2 19:32:16 2020 +0300

    added hello world

diff --git a/README.md b/README.md
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index 0000000..419f64d
--- /dev/null
+++ b/README.md
@@ -0,0 +1 @@
+# Here are some of my attempts to learn how AVR architecture works
diff --git a/hello_world/Makefile b/hello_world/Makefile
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+++ b/hello_world/Makefile
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+all:
+	avr-as -g -mmcu=atmega328p -o hello_world.o hello_world.s
+	avr-ld -o hello_world.elf hello_world.o
+	avr-objcopy -O ihex -R .eeprom hello_world.elf hello_world.hex
+
+upload:
+	avrdude -C /etc/avrdude.conf -p atmega328p -c arduino -P /dev/ttyACM0 -b 115200 -D -U flash:w:hello_world.hex:i
+
+run: all upload
+
+clean:
+	ls | grep hello_world | grep -v *.s | xargs rm -f --
diff --git a/hello_world/README.md b/hello_world/README.md
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index 0000000..9e9c597
--- /dev/null
+++ b/hello_world/README.md
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+This program blinks the LED on pin 5 at an interval of approximately .7 seconds.
+It comes with a Makefile to build and upload the code into the Arduino board
+using binutils-avr for building and avrdude for uploading.
diff --git a/hello_world/hello_world.s b/hello_world/hello_world.s
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+++ b/hello_world/hello_world.s
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+; RAMEND is pretty self explanatory.
+
+.equ RAMEND, 0x08FF
+
+; Check [1] and [2] for more info about the following three
+; symbols
+
+.equ SREG, 0x3F
+.equ SPL,  0x3D
+.equ SPH,  0x3E
+
+; Check [3] for more info about the following three symbols.
+
+.equ PORTB, 0x05
+.equ DDRB,  0x04
+.equ PINB,  0X03
+
+.org 0x0000
+    rjmp MAIN
+
+MAIN:
+
+    ; Reset system status. Check this link for more details:
+    ; http://www.rjhcoding.com/avr-asm-sreg.php [1]
+
+    clr r16
+    out SREG, r16
+
+    ; Initialize stack pointer. Put the low byte of RAMEND in SPL (Stack Pointer Low)
+    ; and the high byte in SPH (Stack Pointer High). Check this link for more details:
+    ; http://www.rjhcoding.com/avr-asm-functions.php [2]
+
+    ldi r16, lo8(RAMEND)
+    out SPL, r16
+
+    ldi r16, hi8(RAMEND)
+    out SPH, r16
+
+    ; Set port bits to output mode. We will use LED 5, so we need to set the fifth bit
+    ; in DDRB (Data Direction Register B). Check this link for more details:
+    ; https://web.ics.purdue.edu/~jricha14/Port_Stuff/PortB_general.htm [3]
+
+    ldi r16, 0x20
+    out DDRB, r16
+
+    ; In the main loop we will toggle the fifht bit in PORTB to turn the LED on and off
+    ; using exclusive or. After that we will wait for some time because we won't be able
+    ; to see the LED blink otherwise.
+
+    ldi r17, 0x20
+    clr r16
+
+MAINLOOP:
+
+    eor r16, r17
+    out PORTB, r16
+    call WAIT
+    rjmp MAINLOOP
+
+WAIT:
+
+    push r16
+    push r17
+    push r18
+
+    ; Loop 0x400000 times which takes approximately 12 milion cycles which is
+    ; approximately 0.7s. For cycles/instruction info check this link, this is
+    ; actually the AVR Instruction Set Manual:
+    ; http://ww1.microchip.com/downloads/en/devicedoc/atmel-0856-avr-instruction-set-manual.pdf [4]
+
+    ldi r16, 0x20
+    ldi r17, 0x00
+    ldi r18, 0x00
+
+_WAIT:
+
+    dec r18
+    brne _WAIT
+
+    dec r17
+    brne _WAIT
+
+    dec r16
+    brne _WAIT
+
+    pop r18
+    pop r17
+    pop r16
+
+    ret