some ugly instructions (I wouldn't call this code), that switch to PL0 and check that MMU really works here - it's temporary, don't merge! (merge commit b4 PL0_test.c was added, if you really want)

2 files changed, 23 insertions, 2 deletions

diff --git a/PL0_test.c b/PL0_test.c
index 1a3d49d..7609294 100644
--- a/PL0_test.c
+++ b/PL0_test.c
@@ -1,9 +1,25 @@
 #include "uart.h"
+#include "psr.h"
 
 void PL0_main(void)
 {
-  uart_puts("hello PL0!\n\r");
+  uart_puts("hello PL0! Switching to user mode!\n\r");
 
+  asm("cps #0b10000\n\r"
+      "isb" ::: "memory");
+
+  // if all went correct, Success! gets printed
+  uart_puts("Success!\n\r");
+
+  // if we're indeed i PL0, we should crash now, when trying to access
+  // memory we're not allowed to
+  char first_kernel_byte[2];
+
+  first_kernel_byte[0] = *(char*) ((uint32_t) 0x8000);
+  first_kernel_byte[1] = '\0';
+  
+  uart_puts(first_kernel_byte);
+    
   while (1)
     {
       char c;
diff --git a/kernel.c b/kernel.c
index bd4426d..5d4e6d7 100644
--- a/kernel.c
+++ b/kernel.c
@@ -229,7 +229,12 @@ void kernel_main(uint32_t r0, uint32_t r1, uint32_t atags)
       (&_binary_PL_0_test_img_start)[i];
 
   // jump to that copied code (no switch to PL0 yet, just testing)
-  ((void(*)(void)) (((uint32_t) 0b101010101010) << 20))();
+  asm volatile("mov  r5, #0\n\r"
+  	       "movt r5, #0b1010101010101111\n\r"
+  	       "mov  sp, r5\n\r"
+  	       "mov  r5, #0\n\r"
+  	       "movt r5, #0b1010101010100000\n\r"
+  	       "blx  r5\n\r");
   
   while(1);