add os[1-8]-ref for os refereces, add guide, add README

os4-ref/src/timer.rs

@@ -0,0 +1,23 @@
+//! RISC-V timer-related functionality
+
+use crate::config::CLOCK_FREQ;
+use crate::sbi::set_timer;
+use riscv::register::time;
+
+const TICKS_PER_SEC: usize = 100;
+const MICRO_PER_SEC: usize = 1_000_000;
+
+/// read the `mtime` register
+pub fn get_time() -> usize {
+    time::read()
+}
+
+/// get current time in microseconds
+pub fn get_time_us() -> usize {
+    time::read() / (CLOCK_FREQ / MICRO_PER_SEC)
+}
+
+/// set the next timer interrupt
+pub fn set_next_trigger() {
+    set_timer(get_time() + CLOCK_FREQ / TICKS_PER_SEC);
+}