Cleaned up boot.S so it's clearer where the changes are from part9

part10-multicore/boot.S

@@ -3,9 +3,10 @@
 #define OSC_FREQ        54000000
 #define MAIN_STACK      0x400000
 
-.section ".text.boot"
+.section ".text.boot"  // Make sure the linker puts this at the start of the kernel image
+
+.global _start  // Execution starts here
 
-.globl _start
 _start:
     ldr     x0, =LOCAL_CONTROL   // Sort out the timer
     str     wzr, [x0]
@@ -16,37 +17,43 @@ _start:
     msr     cntfrq_el0, x0
     msr     cntvoff_el2, xzr
 
-	mrs	x6, mpidr_el1		
-	and	x6, x6,#0xFF		// Check processor id
-	cbz	x6, primary_cpu         // Hang for all non-primary CPU
+    // Check processor ID is zero (executing on main core), else hang
+    mrs     x1, mpidr_el1
+    and     x1, x1, #3
+    cbz     x1, 2f
 
+    // We're not on the main core, so hang in an infinite wait loop
     adr     x5, spin_cpu0
-proc_hang:
-	wfe
-	ldr     x4, [x5, x6, lsl #3]
-	cbz     x4, proc_hang
+1:  wfe
+    ldr     x4, [x5, x1, lsl #3]
+    cbz     x4, 1b
 
     ldr     x1, =__test_stack    // Get ourselves a fresh stack
     mov     sp, x1
-secondary_cpu:
+
     mov     x0, #0
     mov     x1, #0
     mov     x2, #0
     mov     x3, #0
     br      x4
-        b       proc_hang
-primary_cpu:
-	ldr	x1, =__bss_start
-	ldr	w2, =__bss_size
-memzero:
-        cbz     w2, startup
+    b       1b
+2:  // We're on the main core!
+
+    // Set stack to start somewhere safe
+    mov     sp, #MAIN_STACK
+
+    // Clean the BSS section
+    ldr     x1, =__bss_start     // Start address
+    ldr     w2, =__bss_size      // Size of the section
+3:  cbz     w2, 4f               // Quit loop if zero
     str     xzr, [x1], #8
     sub     w2, w2, #1
-	cbnz    w2, memzero
-startup:
-	mov	sp, #MAIN_STACK
-	bl	main
-	b 	proc_hang		// should never come here
+    cbnz    w2, 3b               // Loop if non-zero
+
+    // Jump to our main() routine in C (make sure it doesn't return)
+4:  bl      main
+    // In case it does return, halt the master core too
+    b       1b
 
 .ltorg