begin implementing printf-like debug function

c64/basic/printf.s

@@ -0,0 +1,61 @@
+	;; printf()-like routine for simplifying debugging
+
+	;; Prints the literal zero-terminated string that follows,
+	;; which may also include special tokens that allow
+	;; printing of hex byte and word values stored at the indicated
+	;; addresses.
+
+printf:
+	;; Temporary storage is between end of screen at $0400
+	;; and the sprite pointers at $7f8
+	sta $7f0
+	stx $7f1
+	sty $7f2
+	php
+	pla
+	sta $7f3
+
+	;; Set up read routine
+	lda #$ad 		; LDA $nnnn
+	sta $7f4
+	lda #$60
+	sta $7f7		; RTS
+	
+	;; Get PC of caller from the stack, so that
+	;; we can parse through it
+	pla
+	clc
+	adc #$01
+	sta $7f5
+	pla
+	adc #$00
+	sta $7f6
+
+printf_loop:
+	jsr $7f4
+	cmp #$00
+	bne printf_continues
+	;; Put return address on stack and restore
+	;; registers
+	lda $7f6
+	pha
+	lda $7f5
+	pha
+	lda $7f3
+	pha
+	lda $7f0
+	ldx $7f1
+	ldy $7f2
+	rts
+
+printf_continues:
+	;; Print character
+	jsr $ffd2
+
+printf_nextchar:
+	;; Advance pointer to next character
+	inc $7f5
+	bne +
+	inc $7f6
+*
+	jmp print_loop