Add system timer regs and sync execution at 60fps

2 files changed, 42 insertions, 1 deletions

diff --git a/src/common.h b/src/common.h
index 6f3bd70..b983ca5 100644
--- a/src/common.h
+++ b/src/common.h
@@ -246,7 +246,41 @@ mb_call(unsigned char ch, void *msg) {
 }
 
 //
-// Utilities
+// Timers.
+//
+
+typedef struct TimerRegs {
+    vu32 ctrl_status;
+    vu32 counter_low;
+    vu32 counter_high;
+    vu32 compare[4];
+} TimerRegs;
+
+#define TIMER ((TimerRegs *)(MEM_BASE + 0x00003000))
+
+u64
+timer_get_ticks(void) {
+    u32 high = TIMER->counter_high;
+    u32 low = TIMER->counter_low;
+
+    // Reading is not atomic, since we need to read two 32 bit values. We check
+    // if the high bits changed after reading the low bits for safety.
+    if (high != TIMER->counter_high) {
+        high = TIMER->counter_high;
+        low = TIMER->counter_low;
+    }
+    return ((u64)high << 32) | low;
+}
+
+void
+wait(u64 usec) {
+    u64 current_ticks = timer_get_ticks();
+    // The system timer clock runs at 1 Mhz.
+    while (timer_get_ticks() < (current_ticks + usec));
+}
+
+//
+// Utilities.
 //
 
 void *
diff --git a/src/main.c b/src/main.c
index d72ae20..624d3c8 100644
--- a/src/main.c
+++ b/src/main.c
@@ -155,11 +155,18 @@ void main(void) {
     init_uxn();
 
     uxn_eval(&u, 0x0100);
+    u64 current_ticks = timer_get_ticks();
     while(1) {
         // Echo input to standard output.
         uxn_eval(&u, mempeek16(devscreen->dat, 0));
 
         // Blit ppu.pixels to the framebuffer.
         blit_framebuffer(&ppu);
+
+        // Sync at 60 Hz.
+        if ((timer_get_ticks() - current_ticks) < 16666) {
+            wait(16666 - (timer_get_ticks() - current_ticks));
+        }
+        current_ticks = timer_get_ticks();
     }
 }