feat: Add initial Supervisor Mode Trap Handler

Implement the initial mechanism for handling traps and exceptions in Supervisor Mode.

Set up the stvec Control and Status Register (CSR) to direct all S-Mode Traps (Interrupts and Exceptions) to a unified assembly entry point.

Implemented assembly code for saving and restoring the Trap Context (register state).

Provides the foundational framework necessary for future Timer Interrupts and System Calls.

3 files changed, 240 insertions, 16 deletions

diff --git a/src/kernel/main.cc b/src/kernel/main.cc
index beda9dc..2fa10a2 100644
--- a/src/kernel/main.cc
+++ b/src/kernel/main.cc
@@ -1,20 +1,21 @@
 #include "drivers/uart.h"
 #include "kernel/pmm.h"
 #include "kernel/mm.h"
+#include "kernel/trap.h"
 
-void print_hex(uint64_t val)
-{
-    char buf[17];
-    buf[16] = 0;
-    const char *digits = "0123456789ABCDEF";
-    for (int i = 15; i >= 0; i--)
-    {
-        buf[i] = digits[val & 0xf];
-        val >>= 4;
-    }
-    Drivers::uart_puts("0x");
-    Drivers::uart_puts(buf);
-}
+// void print_hex(uint64_t val)
+// {
+//     char buf[17];
+//     buf[16] = 0;
+//     const char *digits = "0123456789ABCDEF";
+//     for (int i = 15; i >= 0; i--)
+//     {
+//         buf[i] = digits[val & 0xf];
+//         val >>= 4;
+//     }
+//     Drivers::uart_puts("0x");
+//     Drivers::uart_puts(buf);
+// }
 
 extern "C" void kernel_main()
 {
@@ -33,11 +34,11 @@ extern "C" void kernel_main()
     void *p2 = PMM::alloc_page();
 
     Drivers::uart_puts("Allocated Page 1: ");
-    print_hex((uint64_t)p1);
+    Drivers::print_hex((uint64_t)p1);
     Drivers::uart_puts("\n");
 
     Drivers::uart_puts("Allocated Page 2: ");
-    print_hex((uint64_t)p2);
+    Drivers::print_hex((uint64_t)p2);
     Drivers::uart_puts("\n");
 
     Drivers::uart_puts("[Kernel] Freeing Page 1...\n");
@@ -45,7 +46,7 @@ extern "C" void kernel_main()
 
     void *p3 = PMM::alloc_page();
     Drivers::uart_puts("Allocated Page 3: ");
-    print_hex((uint64_t)p3);
+    Drivers::print_hex((uint64_t)p3);
     Drivers::uart_puts("\n");
 
     if (p3 == p1)
@@ -63,6 +64,19 @@ extern "C" void kernel_main()
     VM::kvminithart();
     Drivers::uart_puts("[Boot] MMU ENABLED! We are now using virtual addresses.\n");
 
+    Trap::init();
+    Drivers::uart_puts("[Boot] Trap Handler Installed.\n");
+
+    Drivers::uart_puts("\n[Test] Triggering Page Fault by reading 0x0...\n");
+    volatile int *p = (int *)0x0;
+    int crash = *p;
+    (void)crash;
+
+    Drivers::uart_puts("[Test Failed] Shuold not reach here.\n");
+
+    Drivers::uart_puts("\n[Test] Triggering Store Fault on Text Segment...\n");
+    *(int*)0x80000000 = 1234;
+
     while (1)
     {
 
diff --git a/src/kernel/trap/handler.cc b/src/kernel/trap/handler.cc
new file mode 100644
index 0000000..9cc19d1
--- /dev/null
+++ b/src/kernel/trap/handler.cc
@@ -0,0 +1,123 @@
+#include "kernel/trap.h"
+#include "common/types.h"
+#include "drivers/uart.h"
+
+void panic(const char *s)
+{
+    Drivers::uart_puts("\n\n!!! KERNEL PANIC !!!\n");
+    Drivers::uart_puts(s);
+    Drivers::uart_puts("\nSystem Halted.\n");
+    while (1)
+        ;
+}
+
+namespace Trap
+{
+    void init()
+    {
+        uint64 stvec_val = (uint64)kernelvec;
+        asm volatile("csrw stvec, %0" : : "r"(stvec_val));
+
+        uint64 sstatus;
+        asm volatile("csrr %0, sstatus" : "=r"(sstatus));
+        sstatus &= ~(1L << 1);
+        asm volatile("csrw sstatus, %0" : : "r"(sstatus));
+
+        Drivers::uart_puts("[Trap] Initialized stvec to: ");
+        Drivers::print_hex(stvec_val);
+        Drivers::uart_puts("\n");
+    }
+
+    extern "C" void kerneltrap(struct Trapframe *tf)
+    {
+        uint64 scause, stval, sepc;
+
+        asm volatile("csrr %0, scause" : "=r"(scause));
+        asm volatile("csrr %0, stval" : "=r"(stval));
+        asm volatile("csrr %0, sepc" : "=r"(sepc));
+
+        if (scause & (1L << 63))
+        {
+            uint64 cause_code = scause & 0x7FFFFFFFFFFFFFFFL;
+            switch (cause_code)
+            {
+            case 1:
+                Drivers::uart_puts("[IRQ] Software Interrupt\n");
+                asm volatile("csrc sip, %0" : : "r"(1 << 1));
+                break;
+
+            case 5:
+                Drivers::uart_puts("[IRQ] Timer Interrupt\n");
+
+                break;
+            
+            case 9:
+                Drivers::uart_puts("[IRQ] External Interrupt\n");
+                break;
+
+            default:
+                Drivers::uart_puts("[IRQ] Unknown Interrupt: ");
+                Drivers::print_hex(cause_code);
+                Drivers::uart_puts("\n");
+                break;
+            }
+        }
+        else
+        {
+            switch (scause)
+            {
+            case 8:
+                Drivers::uart_puts("[Syscall] ecall for User Mode\n");
+                tf->epc += 4;
+                break;
+            
+            case 12:
+            case 13:
+            case 15:
+                Drivers::uart_puts("\n--- PAGE FAULT ---\n");
+                Drivers::uart_puts("Cause Code: ");
+                Drivers::print_hex(scause);
+                Drivers::uart_puts("\n");
+                Drivers::uart_puts("Fault Addr (stval): ");
+                Drivers::print_hex(stval);
+                Drivers::uart_puts("\n");
+                Drivers::uart_puts("Instruction (sepc): ");
+                Drivers::print_hex(sepc);
+                Drivers::uart_puts("\n");
+
+                if (stval == 0)
+                {
+                    panic("Null Pointer Dereference!");
+                }
+                else if (stval < 0x80000000)
+                {
+                    panic("Invalid Low Memory Access!");
+                }
+                else
+                {
+                    panic("Segmentation Fault (Unmapped Area)");
+                }
+                break;
+
+            case 2:
+                Drivers::uart_puts("\n--- ILLEGAL INSTRUCTION ---\n");
+                Drivers::uart_puts("Instruction (sepc): ");
+                Drivers::print_hex(sepc);
+                Drivers::uart_puts("\n");
+                panic("CPU encountered an unknown opcode");
+                break;
+
+            default:
+                Drivers::uart_puts("\n--- UNHANDLED EXCEPTION ---\n");
+                Drivers::uart_puts("Cause: ");
+                Drivers::print_hex(scause);
+                Drivers::uart_puts("\n");
+                Drivers::uart_puts("sepc: ");
+                Drivers::print_hex(sepc);
+                Drivers::uart_puts("\n");
+                panic("System Halt");
+                break;
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/kernel/trap/trap.S b/src/kernel/trap/trap.S
new file mode 100644
index 0000000..ffce52a
--- /dev/null
+++ b/src/kernel/trap/trap.S
@@ -0,0 +1,87 @@
+.section .text
+.global kernelvec
+.align 4
+
+kernelvec:
+    
+    addi sp, sp, -256
+    
+    sd ra, 40(sp)
+    sd sp, 48(sp)
+    sd gp, 56(sp)
+    sd tp, 64(sp)
+    sd t0, 72(sp)
+    sd t1, 80(sp)
+    sd t2, 88(sp)
+    sd s0, 96(sp)
+    sd s1, 104(sp)
+    sd a0, 112(sp)
+    sd a1, 120(sp)
+    sd a2, 128(sp)
+    sd a3, 136(sp)
+    sd a4, 144(sp)
+    sd a5, 152(sp)
+    sd a6, 160(sp)
+    sd a7, 168(sp)
+    sd s2, 176(sp)
+    sd s3, 184(sp)
+    sd s4, 192(sp)
+    sd s5, 200(sp)
+    sd s6, 208(sp)
+    sd s7, 216(sp)
+    sd s8, 224(sp)
+    sd s9, 232(sp)
+    sd s10, 240(sp)
+    sd s11, 248(sp)
+    sd t3, 256(sp)
+    sd t4, 264(sp)
+    sd t5, 272(sp)
+    sd t6, 280(sp)
+    
+    csrr t0, sepc
+    csrr t1, sstatus
+    sd t0, 24(sp)   # Trapframe.epc
+    sd t1, 32(sp)   # Trapframe.kernel_hartid
+    
+    mv a0, sp
+    call kerneltrap
+    
+    ld t0, 24(sp)
+    ld t1, 32(sp)
+    csrw sepc, t0
+    csrw sstatus, t1
+
+    ld ra, 40(sp)
+    ld gp, 56(sp)
+    ld tp, 64(sp)
+    ld t0, 72(sp)
+    ld t1, 80(sp)
+    ld t2, 88(sp)
+    ld s0, 96(sp)
+    ld s1, 104(sp)
+    ld a0, 112(sp)
+    ld a1, 120(sp)
+    ld a2, 128(sp)
+    ld a3, 136(sp)
+    ld a4, 144(sp)
+    ld a5, 152(sp)
+    ld a6, 160(sp)
+    ld a7, 168(sp)
+    ld s2, 176(sp)
+    ld s3, 184(sp)
+    ld s4, 192(sp)
+    ld s5, 200(sp)
+    ld s6, 208(sp)
+    ld s7, 216(sp)
+    ld s8, 224(sp)
+    ld s9, 232(sp)
+    ld s10, 240(sp)
+    ld s11, 248(sp)
+    ld t3, 256(sp)
+    ld t4, 264(sp)
+    ld t5, 272(sp)
+    ld t6, 280(sp)
+    
+    addi sp, sp, 256
+    
+    sret
\ No newline at end of file