add yield-os

kernels/yield-os/Makefile

@@ -0,0 +1,3 @@
+NAME = yield-os
+SRCS = yield-os.c
+include $(AM_HOME)/Makefile

kernels/yield-os/yield-os.c

@@ -0,0 +1,31 @@
+#include <am.h>
+#include <klib-macros.h>
+
+#define STACK_SIZE (4096 * 8)
+typedef union {
+  uint8_t stack[STACK_SIZE];
+  struct { Context *cp; };
+} PCB;
+static PCB pcb[2], pcb_boot, *current = &pcb_boot;
+
+static void f(void *arg) {
+  while (1) {
+    putch("?AB"[(uintptr_t)arg > 2 ? 0 : (uintptr_t)arg]);
+    for (int volatile i = 0; i < 100000; i++) ;
+    yield();
+  }
+}
+
+static Context *schedule(Event ev, Context *prev) {
+  current->cp = prev;
+  current = (current == &pcb[0] ? &pcb[1] : &pcb[0]);
+  return current->cp;
+}
+
+int main() {
+  cte_init(schedule);
+  pcb[0].cp = kcontext((Area) { pcb[0].stack, &pcb[0] + 1 }, f, (void *)1L);
+  pcb[1].cp = kcontext((Area) { pcb[1].stack, &pcb[1] + 1 }, f, (void *)2L);
+  yield();
+  panic("Should not reach here!");
+}