From 9a4c2508f6e7c00a75857eea27d5ebf7a13ec05e Mon Sep 17 00:00:00 2001
From: Konstantin Nazarov <mail@knazarov.com>
Date: Thu, 19 Dec 2024 00:50:10 +0000
Subject: [PATCH] Add a timer device

---
 example/example.c | 24 ++++++++++++++++--------
 src/vm.cpp        | 14 +++++++++++++-
 src/vm.hpp        |  9 +++++++++
 3 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/example/example.c b/example/example.c
index 91aa3f1..b2d7eda 100644
--- a/example/example.c
+++ b/example/example.c
@@ -16,19 +16,27 @@ uint64_t read_mtime_atomic() {
   return ((uint64_t)upper1 << 32) | lower;
 }
 
-int fact(int n) {
-  if (n == 0)
+uint32_t timediff_ms(uint64_t start, uint64_t end) {
+  uint64_t total = end-start;
+  // Clock rate is currently 1MHz
+  return (uint32_t)total / 1000;
+}
+
+int fib(int n) {
+  if (n == 0 || n == 1)
     return 1;
 
-  return n * fact(n-1);
+  return fib(n-1) + fib(n-2);
 }
 
 int main() {
-  int n = 8;
-  int res = fact(8);
-
-  printf("sizeof(long) = %d\n", sizeof(long long));
-  printf("%d! = %d\n", n, res);
+  int n = 24;
+  uint64_t start = read_mtime_atomic();
+  int res = fib(n);
+  uint64_t end = read_mtime_atomic();
+  uint32_t total_msec = timediff_ms(start, end);
 
+  printf("fib(%d) = %d\n", n, res);
+  printf("time: %dms\n", total_msec);
   return 0;
 }
diff --git a/src/vm.cpp b/src/vm.cpp
index 8fc0371..94d210c 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -1,5 +1,6 @@
 #include "vm.hpp"
 
+#include <chrono>
 #include <cstdint>
 #include <cstring>
 #include <fstream>
@@ -14,9 +15,10 @@ inline int32_t sign_extend(int32_t value, int bits) {
 
 VM::VM(const std::vector<uint8_t>& memory, const std::string& file_path)
     : ram(memory), pc(PROGRAM_ADDR), file_path(file_path) {
-  devices = std::vector<Device*>(2, 0);
+  devices = std::vector<Device*>(3, 0);
   devices[0] = &ram;
   devices[1] = &uart;
+  devices[2] = &timer;
 }
 
 void VM::setreg(size_t regnum, uint32_t value) {
@@ -73,6 +75,16 @@ void UART::write_mem(uint8_t* src, size_t addr, size_t size) {
   }
 }
 
+void Timer::read_mem(uint8_t* dst, size_t addr, size_t size) {
+  using Clock = std::chrono::high_resolution_clock;
+  constexpr auto den = Clock::period::den;
+  constexpr auto num = Clock::period::num;
+
+  // Let's assume that clock rate is 1MHz for now
+  set(Clock::now().time_since_epoch().count() / (den / (1000000 * num)));
+  Device::read_mem(dst, addr, size);
+}
+
 void VM::read_mem(uint8_t* dst, size_t addr, size_t size) {
   for (Device* dev : devices) {
     if (addr >= dev->base() && addr + size <= dev->base() + dev->size()) {
diff --git a/src/vm.hpp b/src/vm.hpp
index d8ce4b2..d73c5d3 100644
--- a/src/vm.hpp
+++ b/src/vm.hpp
@@ -11,6 +11,7 @@ class EbreakException : std::exception {};
 const int NUM_REGISTERS = 32;  // Standard RISC-V has 32 registers
 const uint32_t UART_ADDR = 0x10000000;
 const uint32_t PROGRAM_ADDR = 0x80000000;
+const uint32_t MTIME_ADDR = 0x0200BFF8;
 
 class Device {
  public:
@@ -56,6 +57,13 @@ class RAM final : public Device {
   RAM(const std::vector<uint8_t> &memory) : Device(PROGRAM_ADDR, memory) {}
 };
 
+class Timer final : public Device {
+ public:
+  Timer() : Device(MTIME_ADDR, 8) {}
+  virtual void read_mem(uint8_t *dst, size_t addr, size_t size);
+  void set(uint64_t value) { write_mem((uint8_t *)&value, MTIME_ADDR, 8); }
+};
+
 class VM {
  public:
   VM(const std::vector<uint8_t> &memory, const std::string &file_path);
@@ -81,5 +89,6 @@ class VM {
   std::string file_path;
 
   UART uart;
+  Timer timer;
   std::vector<Device *> devices;
 };