Implement prelude-loading mechanism

README.md

@@ -46,3 +46,12 @@ registers: `xBUF`, `xHEAD` and `xTAIL`:
 In order to be distinguishable from the empty state, the ring buffers
 must never be completely full -- there must always be *at least one*
 unoccupied byte between the tail and the head.
+
+# System status
+
+| Name     | Offset | Size / B | Access       |
+|----------|--------|----------|--------------|
+| SYSREADY | 68     | 1        | atomic write |
+
+The `SYSREADY` register is used to indicate when the system has booted
+up and is ready for user input.