vram/VRAMCore: cycle using prime numbers in tests

VRAMs are powers of two, so if memory wiring is wrong and we end up
with ram blocks mirrored at several points in the address space, we
want a write pattern that doesn't repeat cleanly on power of two
blocks. That way, a mirrored memory block cannot contain values that
are valid for all its locations.

vram/VRAMCore_Test.bsv

@@ -38,8 +38,16 @@ module mkIncrementingValue(ValFn);
 
    function ActionValue#(Bit#(8)) next();
       return (actionvalue
-                 val <= val+1;
+                 // Cycle through 101 values. 101 is prime, so the
-                 return val;
+                 // pattern it generates doesn't align to a power of
+                 // two and should detect any memory mapping errors.
+                 if (val == 100)
+                    val <= 0;
+                 else
+                    val <= val+1;
+                 // Add another number to get all nonzero values, to
+                 // detect writes that don't stick.
+                 return 23+val;
               endactionvalue);
    endfunction