SYSCALL cleanup and ivt rewrite

[cortex-from-scratch] / syscall.c

diff --git a/syscall.c b/syscall.c
index f6e1cba..9f33ad5 100644 (file)
--- a/syscall.c
+++ b/syscall.c
@@ -37,52 +37,46 @@
 #include <lib/tinyprintf.h>
 
 
-/* the function gets called for 
- * */
+/*
+ * This is a so-called first interrupt handler
+ * The naked attribute makes sure the compiler doesn't
+ * places registers on the stack.  */
+
 __attribute__ ((naked))
-void * __svc_handler__(int x) {
+void * _svc_handler(void) {
 
-       uint8_t svc_nr;
+       uint32_t * current_sp;
 
+       /* Test whether system call was invoked from supervisor (use MSP) or
+        * user (use PSP) mode */
        asm volatile (
        "tst lr, #4" "\n\t"
        "ite eq" "\n\t"
-       "mrseq r0, msp" "\n\t"
-       "mrsne r0, psp" "\n\t"
-       "ldr r0, [r0, #24]" "\n\t"
-       "ldrb %0, [r0, #-2]" : "=r" (svc_nr) );
+       "mrseq %0, msp" "\n\t"
+       "mrsne %0, psp" : "=r" (current_sp));
+
+       /* An exception (or interrupt) before entering this handler
+        * places the following on the stack 
+        * 
+        * R0
+        * R1
+        * R2
+        * R3
+        * R12
+        * LR
+        * PC <- placed at current_sp[6]
+        * PSR 
+        *
+        * PC contains the return address that will continue after this SVC handler
+        * is finised. The previous address (the svc # call) is at PC - 2, the
+        * first byte contains the svc number.
+        * */
+
+       uint8_t svc_nr = ((char *) current_sp[6])[-2];
 
        printf("SYSTEM CALL NR: %d", svc_nr);
 
-       for(;;);
-
-       volatile uint32_t * sp;
-       
-       asm volatile (
-       "tst lr, #4" "\n\t" 
-       "ite eq" "\n\t"
-       "mrseq %0, msp" "\n\t" 
-       "mrsne r0, psp" : "=r" (sp) );
-
-
-       for (int i = 0; (sp + i) < 0x20010000; i++) {
-               printf("ADDRESS: %p, VALUE: %x\n", (sp + i), *(sp + i));
-       }
-
-       for (;;);       
-       //asm ("mov %0, pc" : "=r" (link_register));
-       //printf("%x\n", link_register);
-       
-       volatile uint32_t * svc_number = (uint32_t *) 0x20022222;
-       *svc_number = sp[-6];
-       //              = sp[6];
-       //uint8_t tmp  = link_register[-2];
-       printf("SVC nr: %x", *svc_number);
-       //printf("%d, %d, %d, %d, %d, %d\n", msp[6], msp[0], msp[1], msp[2], msp[3], msp[4]);
-       
        for (;;);
-       //return NULL;  
-       return 0;
 
 }
 
@@ -104,13 +98,8 @@ void syscall(unsigned int * args) {
 
 void syscall_init() {
 
-
        /* SVC is located at position 11 in the interrupt vector table  */
-//     extern void * _syscall;
-       extern void * hardfault;
-
-       ivt_set_gate(11, __svc_handler__, 0);
-
+       ivt_set_gate(11, _svc_handler, 0);
 
 }