MOS6510 assembly on a C=64

 reset
    LDX #$00
 cycle
    LDA hworld,X
    BEQ reset
    STX cache
    JSR $FFD2
    LDX cache
    INX
    JMP cycle
 hworld
 .text "Hello, World!"
 .byte 13,0
 cache
 .byte 0

And another smaller, better version

	lda #text
	jsr $ab1e ; ROM message out routine
	rts
text	.text "HELLO WORLD!"
	.byte $0d,$00