quartz/content/notes/06-6809-programming.md

title	aliases	tags
06-6809-programming		lecture cosc204

difficult

• no variables
• GOTO not loops
• only compare ints and bytes
• not really routines
• all vars global
• case sensitive

Hello World

• need a routine that given a string will print each chracter one at a time
• need to know how to print

Routines

routine:
	rts

• no params
• no local vars

calling routine

• lbsr, bsr, jsr
• very subtle differences
• we use bsr (branch to subroutine)

bsr routine

Interation

use GOTOs

	clra           //A = 0
more:
	cmpa #$06      //compare A to $06
	beq done       //if equal, branch to done
	inca           //increment a
	bra more       //branch to more
done:

• comparison only compares and sets flags in CC
• you then branch on the result
  • beq bge etc
• other intructions set flags to
  • lda etc
• so

lda $45
beq somewhere

6809.uk text screen

write h to top left

lda #'H'
sta $0400

add an 'e' beside that

lda #'E'
sta $0401

print routine

• 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() 1

;
; 
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

using a as a temp variable

read from y to x

constants

ldx #$0400 ; start of screen

• this is a hard coded constant
• the assembler replaces instances of screen with $0400 when it assembles the code

screen EQU $0400 ; start of screen
ldx #screen

post increment

loaded A with 0 bytes past where Y point then increment Y

lda 0,y ; current char -> A 
leay 1,y ; increment y

can do this in one go

lda 0,y+

can do the same with x

sta 0,x+

PUTS() 2

;
; 
Routine: PUTS 
; Pass the address of the string in Y 
; 

screen EQU #$0400 ;start of screen

PUTS: 
	ldx #$screen ; start of screen 
more: 
	lda 0,y+ ; current char -> A
	cmpa #$00 ; was it a zero? 
	beq done ; if it was 0 then return 
	sta 0,x+ ; write
	bra more ; repeat 
done: 
	rts ; return from this routine

CC set on load

CC is set on an lda so if we loaded a zero then the Z flag will be set so we dont need cmpa

lda 0,y+ ; current char -> A 
beq done ; if it was 0 then return

storing x, y, a

need to add an insruction to save them at the start and resotre them at the end

pshs a,x,y
puls a,x,y

there are other ways to do this

; 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 
	ldx #screen ; start of screen more: lda 0,y+ ; current char -> A 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