Turn on the ACT LED

Author: apinsard

Makefile

@@ -0,0 +1,36 @@
+# Distributed under the terms of the GNU General Public License v2
+
+CC ?= arm-none-eabi
+LD ?= $(CC)-ld
+AS ?= $(CC)-as
+
+BUILD  = build
+SOURCE = src
+TARGET = kernel.img
+LIST   = kernel.list
+MAP    = kernel.map
+LINKER = kernel.ld
+
+OBJECTS := $(patsubst $(SOURCE)/%.s,$(BUILD)/%.o,$(wildcard $(SOURCE)/*.s))
+
+all: $(TARGET) $(LIST)
+
+$(LIST): $(BUILD)/output.elf
+	$(CC)-objdump -d $< > $@
+
+$(TARGET): $(BUILD)/output.elf
+	$(CC)-objcopy $< -O binary $@
+
+$(BUILD)/output.elf: $(OBJECTS) $(LINKER)
+	$(LD) --no-undefined $< -Map $(MAP) -o $@ -T $(LINKER)
+
+$(BUILD)/%.o: $(SOURCE)/%.s
+	$(AS) -I $(SOURCE) $< -o $@
+
+clean:
+	rm -f $(BUILD)/*.o
+
+mrproper: clean
+	rm -f $(TARGET)
+	rm -f $(LIST)
+	rm -f $(MAP)

kernel.ld

@@ -0,0 +1,13 @@
+/* Distributed under the terms of the GNU General Public License v2 */
+
+SECTIONS {
+  /* Init section at 0x8000 according to the bootloader expectations. */
+  .init 0x8000: {
+    *(.init)
+  }
+
+  /DISCARD/: {
+    *(*)
+  }
+}
+

src/main.s

@@ -0,0 +1,27 @@
+# Distributed under the terms of the GNU General Public License v2
+.section .init
+.glob _start
+
+_start:
+
+# Put the GPIO address in a registry
+ldr r0,=0x20200000
+
+# Enable output to the 16th GPIO pin
+# => 2^18 stored at 0x20200004
+# The 16th GPIO pin is represented by the 6th set of 3 bits (#18=3×6) of the
+# second set of 4 bytes (#4=4×1).
+# Finally, we want to put #1 in the selected set of 3 bits to enable output
+mov r1,#1
+lsl r1,#18
+str r1,[r0,#4]
+
+# Turn off (#40) the 16th pin (#16) of the GPIO to turn on the ACT LED.
+mov r1,#1
+lsl r1,#16
+str r1,[r0,#40]
+
+# End
+loop$:
+b   loop$
+