Why don't emulators use the full screen?

[mirrorsawlljk]

I don't mean stretching the image. I'm talking about the fact that several emulators have the image centered in the screen. Why doesn't it reach the top and bottom? It seems like you could use the full height of the screen and still preserve a 4:3 aspect ratio.

[pirloui]

that's because of the amount of pixels that the emulated console generates..
The SNES for example, has 256x240pixels, so if you stretch those to the PSP's 272 pixels in height; it would either have to be antialiased which would blur the image, or double lines here and there; which would be ugly..
(IMHO)

[RobbieTheRabbit]

Several emulators do this. With time every emulators will have this feature : )