Как-то так, но это не точно:
.ORG 0
LDI ZL, LOW(DATAROM)
LDI ZH, HIGH(DATAROM)
LDI XL, LOW(DATARAM)
LDI XH, HIGH(DATARAM)
AGAIN:
LDI R20, Z+ (или LD R20, Z+)
CPI R20, 0
BREQ END
STS X+, R20
RJMP AGAIN
END: RJMP END
.ORG 0x07FF
DATAROM: .DB "This is a beautiful world!",0