6502/hello-world.s

PORTB = $6000
PORTA = $6001
DDRB = $6002
DDRA = $6003

E  = %10000000
RW = %01000000
RS = %00100000

  .org $8000


	
reset:
  lda #%11111111 ; Set all pins on port B to output
  sta DDRB

  lda #%11100000 ; Set top 3 pins on port A to output
  sta DDRA

  lda #%00111000 ; Set 8-bit mode; 2-line display; 5x8 font
  jsr send_inst
	
  lda #%00001110 ; Display on; cursor on; blink off
  jsr send_inst

  lda #%00000001  ; Reset display
  jsr send_inst

  lda #%00000110 ; Increment and shift cursor; don't shift display
  jsr send_inst
	
  ldx #0
print:
  lda message,x
  beq loop
  jsr send_data	
  inx
  jmp print
	
loop:
  jmp loop

message: .asciiz "Hello again,                            Kiddies!"

lcd_wait:
  pha
  lda #%00000000 ; PORT B to INPUT
  sta DDRB
lcd_busy:
  lda #RW
  sta PORTA
  lda #(RW | E)
  sta PORTA
  lda PORTB
  and #%10000000
  bne lcd_busy
  
  lda #RW
  sta PORTA
  lda #%11111111 ; PORT B to output
  sta DDRB
  pla
  rts

send_inst:	
  jsr lcd_wait
  sta PORTB
  lda #0         ; Clear RS/RW/E bits	
  sta PORTA
  lda #E         ; Set E bit to send instruction
  sta PORTA
  lda #0         ; Clear RS/RW/E bits
  sta PORTA
  rts

send_data:
  jsr lcd_wait
  sta PORTB
  lda #RS         ; Set RS; Clear RW/E bits
  sta PORTA
  lda #(RS | E)   ; Set E bit to send instruction
  sta PORTA
  lda #RS         ; Clear E bits
  sta PORTA
  rts


  .org $fffc
  .word reset
  .word $0000