Add ACT/OK led program

kernel.ld

@@ -1,42 +1,9 @@
-/******************************************************************************
-*	kernel.ld
-*	 by Alex Chadwick
-*
-*	A linker script for generation of raspberry pi kernel images, with C
-*	code.
-******************************************************************************/
-
 SECTIONS {
-	/*
-	* First and formost we need the .init section, containing the code to 
-        * be run first. We allow room for the ATAGs and stack and conform to 
-        * the bootloader's expectation by putting this code at 0x8000.
-	*/
-	.init 0x8000 : {
-		*(.init)
-	}
-
-	/* 
-	* Next we put the data.
-	*/
-	.data : {
-		*(.data)
-		*.c.o(*)
-	}
-
-	/* 
-	* Next we put the rest of the code.
-	*/
-	.text : {
-		*.c.o(.text)
-		*(.text)
-	}
+  .text 0x8000 : {
+    *(.text)
+  }
 
-	/*
-	* Finally comes everything else. A fun trick here is to put all other 
-	* sections into this section, which will be discarded by default.
-	*/
-	/DISCARD/ : {
-		*(*)
-	}
-}
+  .data : {
+    *(.data)
+  }
+}

src/main.S

@@ -0,0 +1,37 @@
+/*
+ *  A simple program to turn on the OK/ACT LED on Raspberry Pi 3
+ */
+
+.globl _start       @ define _start label globally available for the linker
+
+.section .data
+.align 4    @ last 4 bits of the next label set to 0 (16-byte alligned)
+message:
+    .int    size        @ message header contains the size of the message
+    .int    0           @ request code 0
+
+    .int    0x00038041  @ header tag ID
+    .int    8           @ size of tag data
+    .int    0           @ request/response size 
+
+    .int    130         @ pin number
+    .int    1           @ pin state
+    .int    0           @ signal the GPU that the message is over
+size:
+    .int    . - message @ size of the message
+
+.section .text
+_start:
+        ldr         r0, =0x3f00b880     @ load the hex number =0x3f00b880 into register r0
+                                        @ this is the base address of the mailboxes
+        wait$:
+            ldr     r1, [r0, #0x18]     @ load r1 with the address of the offset 0x18 for mailbox 0 (read mailbox)
+            tst     r1, #0x80000000     @ check if the full flag is set
+            bne     wait$               @ branch to wait$ label if the full flag is not set
+
+        ldr         r1, =message        @ load the message into r1
+        add         r1, #8              @ add the channel 8 as the last 4 bits of the message
+        str         r1, [r0, #0x20]     @ put the message into mailbox 1 write register, which is at offset 0x20 from the base address
+
+        loop$:                          @ keep the cpu busy forever in the loop
+        b loop$