Add Verilog MMIO GCD peripheral example

tests/gcd.c

@@ -0,0 +1,42 @@
+#include "mmio.h"
+
+#define GCD_STATUS 0x2000
+#define GCD_X 0x2004
+#define GCD_Y 0x2008
+#define GCD_GCD 0x200C
+
+unsigned int gcd_ref(unsigned int x, unsigned int y) {
+  while (y != 0) {
+    if (x > y)
+      x = x - y;
+    else
+      y = y - x;
+  }
+  return x;
+}
+
+// DOC include start: GCD test
+int main(void)
+{
+  uint32_t result, ref, x = 20, y = 15;
+
+  // wait for peripheral to be ready
+  while ((reg_read8(GCD_STATUS) & 0x2) == 0) ;
+
+  reg_write32(GCD_X, x);
+  reg_write32(GCD_Y, y);
+
+
+  // wait for peripheral to complete
+  while ((reg_read8(GCD_STATUS) & 0x1) == 0) ;
+
+  result = reg_read32(GCD_GCD);
+  ref = gcd_ref(x, y);
+
+  if (result != ref) {
+    printf("Hardware result %d does not match reference value %d\n", result, ref);
+    return 1;
+  }
+  return 0;
+}
+// DOC include end: GCD test