implemented RUNC routine

STATUS.md

@@ -78,7 +78,7 @@ Not all of them - only these we want to have implemented.
 | `$A004`   | revision str | DONE     |                                                    |
 | `$A533`   | LINKPRG      | DONE     |                                                    |
 | `$A659`   | set txt ptr  | NOT DONE |                                                    |
-| `$A68E`   | RUNC         | NOT DONE |                                                    |
+| `$A68E`   | RUNC         | DONE     |                                                    |
 | `$A7AE`   | NEWSTT       | NOT DONE |                                                    |
 | `$AB1E`   | STROUT       | DONE     |                                                    |
 | `$BDCD`   | LINPRT       | DONE     | temporary implementation                           |

c64/basic/a68e.runc.s

@@ -1,5 +1,27 @@
 
-// XXX provide documentation and implementation
+// Well-known BASIC routine, described in:
+//
+// - [CM64] Compute's Mapping the Commodore 64 - page 97
+// - https://www.c64-wiki.com/wiki/BASIC-ROM
+// - https://csdb.dk/forums/?roomid=11&topicid=137233
+// 
+
+// XXX use this routine to initialize TXTPTR
 
 RUNC:
-    STUB_IMPLEMENTATION()
+	// According to WIKI, we should load $7A/$7B with $2B-1/$2C-1), this does not makes sense.
+	// But CSDB claims, that it should 'set current character pointer to start of basic - 1' - this
+	// looks sane, also checked that the original BASIC routine does this (short test program)
+
+	// Let's initialize TXTPTR using TXTTAB
+
+	lda TXTTAB+0
+	sec
+	sbc #$01
+	sta TXTPTR+0
+	
+	lda TXTTAB+1
+	sbc #$00
+	sta TXTPTR+1
+
+	rts