small keyboard reading fixes

c64/kernal/keyboard/f142.interop

@@ -0,0 +1,4 @@
+$F142 is a well-known address of keyboard reading routine, described for example here:
+- http://sta.c64.org/cbm64kbdfunc.html
+
+It is used for eexample by a music eeditor here: https://github.com/martinpiper/C64Public/blob/master/MusicEditor/GetKey.a

c64/kernal/keyboard/f142.read_kb_buf.s

@@ -0,0 +1,30 @@
+
+//
+// Well-known Kernal routine, described in:
+//
+// - http://sta.c64.org/cbm64kbdfunc.html
+//
+
+
+read_kb_buf:
+
+	// Check for a key
+	lda NDX
+	bne !+
+
+	// Nothing in keyboard buffer to read
+	sec
+	rts
+	
+!:
+	lda KEYD
+	pha
+	phy_trash_a
+
+	jsr pop_keyboard_buffer
+
+	ply_trash_a
+	pla
+	clc
+
+	rts

c64/kernal/keyboard/keyboard_disconnect.s

@@ -2,6 +2,10 @@
 //
 // Helper routine - disconnects the keyboard from CIA1_PRB,
 // leaves $FF in .X (behavior needed in some places)
+//
+
+
+#if !CONFIG_LEGACY_SCNKEY
 
 
 keyboard_disconnect:
@@ -13,3 +17,6 @@ keyboard_disconnect:
 #endif
 
 	rts
+
+
+#endif

c64/kernal/keyboard/pop_keyboard_buffer.s

@@ -2,7 +2,9 @@
 // Pop key out of keyboard buffer
 // Disable interrupts while reading from keyboard buffer
 // so that no race conditions can occur
-
+//
+// CPU registers that has to be preserved: .X
+//
 
 pop_keyboard_buffer:
 

c64/kernal/keyboard/scnkey.s

@@ -53,7 +53,6 @@ SCNKEY:
 #endif
 
 	// Retrieve SHIFT / VENDOR / CTRL status
-	// Use .X to detect 2 or more keys pressed (should be $FF now)
 
 	ldy #(__kb_matrix_bucky_confmask_end - kb_matrix_bucky_confmask - 1)
 scnkey_bucky_loop:
@@ -154,7 +153,7 @@ scnkey_matrix_loop_inner:
 !:
 	dey
 	bpl scnkey_matrix_loop_inner
-	bmi scnkey_no_keys            // branch always, multiple keys must have been pressed
+	bmi scnkey_no_keys             // branch always, multiple keys must have been pressed
 scnkey_matrix_loop_next:
 	dex
 	bpl scnkey_matrix_loop