mbv: now with functioning timer interrupts!

And a few other nice things.
The bootloader now has an embedded wozmon!
If you know its offset, you can jump to it from the app.

mbv/hal/interrupts.cc

@@ -0,0 +1,52 @@
+#include <cstdint>
+
+#include "interrupts.h"
+
+namespace {
+
+constexpr uint32_t kMstatusMieMask = 1 << 3;
+constexpr uint32_t kMieExternalInterruptMask = 1 << 11;
+constexpr uint32_t kExternalInterruptCause = 0x0b;
+constexpr uint32_t kInterruptCauseMask = 0xff;
+
+Isr external_handler = nullptr;
+
+__attribute__((interrupt))
+void TrapHandler() {
+    uint32_t mcause;
+    uint32_t mip;
+    asm volatile("csrr %0, mcause" : "=r"(mcause));
+    asm volatile("csrr %0, mip" : "=r"(mip));
+
+    // check for external interrupt
+    if ((mcause & kInterruptCauseMask) == kExternalInterruptCause) {
+        if (external_handler != nullptr) {
+            external_handler();
+        }
+
+        mip &= ~(kMieExternalInterruptMask);
+        asm volatile("csrw mip, %0" :: "r"(mip));
+    }
+}
+
+}
+
+void SetExternalInterruptHandler(Isr handler) {
+    external_handler = handler;
+}
+
+void EnableExternalInterrupts() {
+    uint32_t mstatus;
+    uint32_t mie;
+    Isr trap = TrapHandler;
+
+    asm volatile("csrr %0, mstatus" : "=r"(mstatus));
+    asm volatile("csrr %0, mie" : "=r"(mie));
+
+    asm volatile("csrw mtvec, %0" :: "r"(trap));
+
+    mie |= kMieExternalInterruptMask;
+    asm volatile("csrw mie, %0" :: "r"(mie));
+    mstatus |= kMstatusMieMask;
+    asm volatile("csrw mstatus, %0" :: "r"(mstatus));
+}