added status LED signal after disabling three cores

Makefile

@@ -12,11 +12,11 @@ all: $(KERNEL)
 %.o: %.c
 	${CROSS}gcc ${CFLAGS} -c -o $@ $<
 
-$(NAME): main.o startup.o
-	${CROSS}ld $(LDFLAGS) -o $@ -T linkerscript.ld $<
+$(NAME): main.o startup.o led.o
+	${CROSS}ld $(LDFLAGS) -o $@ -T linkerscript.ld $^
 
 $(KERNEL): $(NAME)
 	${CROSS}objcopy --gap-fill=0xff -j .text -j .rodata -j .data -O binary $< $@
 
 clean:
-	rm -f $(KERNEL) $(NAME) main.o startup.o
+	rm -f $(KERNEL) $(NAME) main.o startup.o led.o

led.asm

@@ -0,0 +1,62 @@
+/*
+
+ -> taken fram the led01 example of https://github.com/adamransom/bare_metal
+ -> ... and adapted
+
+@;
+@; This turns on the ACT LED for the Raspberry Pi 3 Model B v1.2
+@;
+@; The ACT LED is no longer wired directly to a GPIO pin and now belongs on
+@; the GPIO expander, which is controlled by the GPU. In order to communicate
+@; with the GPIO expander, we need to use the GPU's mailbox interface (in
+@; particular, we need to send a message to the property tag channel).
+@;
+@; Mailbox base address: 0x3f00b880
+@; Mailbox 1 write address: [0x3f00b880, #0x20]
+@; Property tag channel: 8
+@; Property tag ID: 0x00038041 (SET_GPIO_STATE)
+@; Property tag message: 130 1 (ACT_LED pin number followed by state)
+@;
+*/
+
+.globl _set_led// @; Make _set_led available to the outside world
+
+.section .data
+.align 4// @; This ensures lowest 4 bits are 0 for the following label
+PropertyInfo:
+//  @; = Message Header =
+  .int PropertyInfoEnd - PropertyInfo// @; Calculate buffer size
+  .int 0// @; Request code: Process Request
+//  @; = Tag Header =
+  .int 0x00038041// @; Tag ID (SET_GPIO_STATE)
+  .int 8// @; Value buffer size
+  .int 0// @; Request/response size
+//  @; = Tag Value Buffer =
+  .int 130// @; ACT_LED pin number
+  .int 1// @; Turn it on
+  .int 0// @; End tag
+PropertyInfoEnd:
+
+.section .text
+_set_led:
+  mailbox .req x0// @; Alias mailbox to r0
+  ldr mailbox, =0x3f00b880// @; Load the mailbox's base address into r0
+
+  wait1$:
+    status .req x1// @; Alias status to r1
+    ldr status, [mailbox, #0x18]// @; Load the Mailbox 0 status address
+    tst status, #0x80000000// @; Check the status against the FULL bit
+    .unreq status// @; Unset the alias
+    bne wait1$// @; Keep checking the mailbox until it isn't full
+
+  message .req x1// @; Alias message to r1
+  ldr message, =PropertyInfo// @; Load r1 with address of our message buffer
+  add message, message, #8// @; Put the channel in the last 4 bits
+  str message, [mailbox, #0x20]// @; Put the message in the mailbox
+  .unreq message// @; Unset the alias
+  .unreq mailbox
+
+  ret // return from the led setting function
+
+//  hang:
+//    b hang// @; Give the CPU something to do ad infinitum

startup.asm

@@ -35,7 +35,10 @@ _start:
 	and x2, x2, #0xFF
 	cbnz x2, hang
 
-	// then, core0 can move on to execute the main c function
+	// then, core0 can move on to call the function setting the status LED, and ...
+	bl _set_led
+
+	// ... to execute the main c function
 	ldr x5, =0x00100000
 	mov sp, x5
 	bl main