Mask control keys from first keyboard scan row.

libsrc/osic1p/kbhit.s

@@ -1,19 +1,35 @@
 ;
 ; unsigned char kbhit (void);
+;
+; The method to detect a pressed key is based on the documentation in
+; "Section 3 Programmed Key Functions" in "The Challenger Character Graphics
+; Reference Manual"
+; We only want to return true for characters that can be returned by cgetc(),
+; but not for keys like <Shift> or <Ctrl>. Therefore a special handling is
+; needed for the first row. This is implemented by a bit mask that is stored
+; in tmp1 and that is set to zero after the first round.
 ;
 
         .export _kbhit
         .include "osic1p.inc"
+        .include "extzp.inc"
+        .include "zeropage.inc"
 
 _kbhit:
-        lda     #%11111110      ; Select first keyboard row
+        lda     #%11011111      ; Mask for only checking the column for the
+        sta     tmp1            ; ESC key in the first keyboard row.
+
+        lda     #%11111110      ; Mask for first keyboard row
 scan:
         sta     KBD             ; Select keyboard row
         tax                     ; Save A
         lda     KBD             ; Read keyboard columns
-        ora     #$01            ; Mask out lsb (Shift Lock), since we ignore it
+        ora     tmp1            ; Mask out uninteresting keys (only relevant in
+                                ; first row)
         cmp     #$FF            ; No keys pressed?
         bne     keypressed
+        lda     #$00            ; For remaining rows no keys masked
+        sta     tmp1
         txa                     ; Restore A
         sec                     ; Want to shift in ones
         rol     a               ; Rotate row select to next bit position