Implement loops

prelude.f

@@ -20,6 +20,20 @@
   SWAP !
 ; IMMEDIATE
 
+\ Loops
+
+: BEGIN HERE @ ; IMMEDIATE
+
+: UNTIL
+  LIT 0BRANCH ,
+  HERE @ - ,
+; IMMEDIATE
+
+: AGAIN
+  LIT BRANCH ,
+  HERE @ - ,
+; IMMEDIATE
+
 \ Peripheral register addresses
 
 : TXBUF 0 ;