quartz/content/notes/6809.md

title	aliases	tags
6809		cosc204

Routines

All variables are global so there is no params or local vars.

There are three way to "Branch" to a subroutine.

• lbsr
• bsr
• jsr

There are subtle differences but we will use bsr mostly

bsr myroutine:

myroutine:
	rts

Iteration

Done using GOTOs

	clra           //A = 0
more:
	cmpa #$06      //compare A to $06 (sets the equal flag in CC if true)
	beq done       //if the equal flag is set, branch to done
	inca           //increment a
	bra more       //branch to more
done:

other instructions set flags too

Input/Output

The screen always displays whatever is in the "screens memory location" which is $0400

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 "message", into the y register
	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 the contents of the Y register into the A register and increment Y by one
	cmpa #$00         ; was it a zero?            ; if the character loaded into A was a zero set the equal flag
	beq done          ; if it was 0 then return   ; if the equal flag is set, branch to the "done" subroutine
	sta 0,x+          ; write                     ; otherwise, store the contents of the A register into memory at the location (hex value) defined in the X register, and increment the hex value stored in the X register
	bra more          ; repeat                    ; branch to the "more" subroutine as there are more character to write
done: 
	puls a,x,y        ; restore a, x, y           ; re load the stored values of the a, x, and y registers that were saved earlier
	rts               ; return from this routine  

Assembler

• compiles assemble to machine code
• can also dissamble machine code

assembler create a listing file that gives the location, machines lcode, and assembly of the program