vault backup: 2022-07-28 11:24:22

content/notes/06-6809-programming.md

@@ -64,3 +64,54 @@ beq somewhere
 
 ## 6809.uk text screen
 ![](https://i.imgur.com/dC1idMA.png)
+
+write h to top left
+```
+lda #'H'
+sta $0400
+```
+
+add an 'e' beside that
+```
+lda #'E'
+sta $0401
+```
+
+## print routine
+![](https://i.imgur.com/IEiGKtj.png)
+
+- pass the address of the string in Y
+- "address" - the index into the memory array the hold the first character of the string
+- use $00 to end the string
+
+## algorithm
+- print a string
+	- Load a character from memory\[Y] 
+	- Move on to the next character (Y++) 
+	- Compare the character to $00 If equal exit this routine 
+	- Write the character to the screen 
+	- Move on to the next screen location 
+	- GOTO start
+
+PUTS()
+
+```
+;
+; 
+Routine: PUTS 
+; Pass the address of the string in Y 
+; 
+PUTS: 
+	ldx #$0400 ; start of screen 
+more: 
+	lda 0,y ; current char -> A  (load y into a)
+	leay 1,y ; increment y 
+	cmpa #$00 ; was it a zero? 
+	beq done ; if it was 0 then return 
+	sta 0,x ; write  (store a in x)
+	leax 1,x ; increment x 
+	bra more ; repeat 
+done: 
+	rts ; return from this routine
+```
+