implement shift + Vendor key to toggle case

c64/kernal/cint.s

@@ -36,6 +36,8 @@ cint:
 	;; Set maximum keyboard buffer size (Compute's Mapping the 64 p215)
 	lda #10
 	sta key_buffer_size
+	;; Put non-zero value in enable_case_switch
+	sta enable_case_switch
 	
 	;; Fallthrough/jump to screen clear routine (Compute's Mapping the 64 p215)
 	jmp clear_screen

c64/kernal/scan_keyboard.s

@@ -304,6 +304,22 @@ skip0:
 	ora key_bucky_state
 	sta key_bucky_state
 
+	;; Chick for shift + Vendor
+	cmp #$05
+	bne no_case_toggle
+	lda key_last_bucky_state
+	cmp #$05
+	beq no_case_toggle
+
+	lda enable_case_switch
+	beq no_case_toggle
+	
+	lda $D018
+	eor #$02
+	sta $d018
+
+no_case_toggle:	
+	
     ;; //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     ;; // Check for pressed key(s)