Multiple cleanups and improvements

- Cleanup exception trigger code
- Cleanup division to divide by 0
- Cleanup SRET code
- Cleanup CSR code
- Added interrupts
- Added TIMER interrupt

src/cpu/exception.c

@@ -2,24 +2,48 @@
 
 #include <stdio.h>
 
-void exception_trigger(rv32_cpu_t* cpu, uint32_t scause)
+void exception_trigger(rv32_cpu_t* cpu, uint32_t scause, uint32_t tval)
 {
-    // An exception can only be triggered by the CPU itself,
-    // so we know we already own the mutex
-    // We are in the CPU thread itself, but we need
-    // the return of this function to be the beginning of
-    // the cpu loop
-    // To achieve that, we can just call cpu_loop (noreturn)
-    // at the end of this function
+	// An exception can only be triggered by the CPU itself,
+	// so we know we already own the mutex
+	// We are in the CPU thread itself, but we need
+	// the return of this function to be the beginning of
+	// the cpu loop
+	// To achieve that, we can just call cpu_loop (noreturn)
+	// at the end of this function
 
-    // Save execution context, so that 'mret/sret/..' can restore it
+	// Exceptions cannot be disabled
+
+	// Set xCAUSE : exception cause, with interrupt bit set to null
+	cpu->csr[CSR_SCAUSE] = scause;
+
+	// Save previous interrupt enable in xSTATUS.xPIE
+	if(cpu->csr[CSR_SSTATUS] & 0b10U)
+		cpu->csr[CSR_SSTATUS] |= 0x80;
+
+	// Set previous privilege mode in xSTATUS.xPP
     // TODO
 
-    // Set PC to STVEC, and set SCAUSE
-    // TODO: If PC cannot be mmu_resolved, throw a 'double fault' ?
-    cpu->pc = cpu->csr[CSR_STVEC];
-    cpu->csr[CSR_SCAUSE] = scause;
+	// Unset SIE (interrupt enable) bit
+	cpu->csr[CSR_SSTATUS] &= ~0b10U;
 
-    pthread_mutex_unlock(&cpu0_mutex);
-    cpu_loop(cpu);
+	// Set privilege mode for exception handling, checking for delegation
+    // TODO
+
+	// Set xTVAL, exception-specific information related to xCAUSE
+	cpu->csr[CSR_STVAL] = tval;
+
+	// Set SEPC to instruction that caused exception
+	cpu->csr[CSR_SEPC] = cpu->pc;
+
+	// Set PC to xTVEC : exception handling code
+	// xTVEC: [Base(30bits) Mode(2 bits)], address 4-byte aligned in base
+	// Exceptions are not vectored (we can safely ignore mode)
+	cpu->pc = cpu->csr[CSR_STVEC] & 0xFFFFFFFC;
+
+    // Unlock cpu mutex, cpu_loop will lock it just after
+	pthread_mutex_unlock(&cpu0_mutex);
+
+    // cpu loop (attribute noreturn should erase previous stack)
+	cpu_loop(cpu);
 }