basically you are talking about overlaying text onto a composite output and there are quite complex devices made by people such as Dallas semiconductors which will do that for you, in principle you can do it quite simply by wire oring the video with the text that you want to display but timing is the critical issue.Generating video timing loops in software is tricky and they have to be very stable because any change in the timing due to interrupts etcetera will be very obvious,and you cannot do much else with the processor when it is doing the video timing so you have to do your real application during the video blanking phases.