vault backup: 2022-08-01 10:20:54

content/notes/6809.md

@@ -40,7 +40,42 @@ other instructions set flags too
 # Input/Output
 ![screen](https://i.imgur.com/IEiGKtj.png)
 
+The screen always displays whatever is in the "screens memory location" which is $0400
 
+``` 6809 assembly
+lda #'H'          ; load the character (indicated by '') into the a register
+sta $0400         ; store the contents of the a register in memory at location $0400
+```
+
+## printing a whole string
+
+```
+message:
+	.byte "HELLO WORLD", 0
+
+START:
+	ldy #message                                 ; load 
+	bsr puts                                     ; branch to the "puts" subroutine
+	rts                                          ; return
+
+; Routine: PUTS 
+; Pass the address of the string in Y 
+; 
+screen EQU $0400      ; start of screen 
+
+PUTS: 
+	pshs a,x,y        ; save a, x, and y          ; store the contents of the a, x, and y registers so they are not overwritten
+	ldx #screen       ; start of screen           ; load the hex value of the memory location of the screen ($0400) into the x register
+more: 
+	lda 0,y+          ; current char -> A         ; load 
+	cmpa #$00         ; was it a zero? 
+	beq done          ; if it was 0 then return 
+	sta 0,x+          ; write 
+	bra more          ; repeat 
+done: 
+	puls a,x,y        ; restore a, x, y 
+	rts               ; return from this routine
+```
 
 # Hello World