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« on: February 22, 2012, 10:55:47 am »
Just realized I could add some info for Amiga500.
Overscan mode allows drawing the image on the screen border. It does not increase the resolution in the sense of making pixels smaller, instead it makes the page bigger. Used whenever you want to avoid the screen border being coloured with whatever is in palette index 0.
There is a third 'special' (6-bitplane) mode on amiga, in addition to extra-halfbright (EHB) and hold-and-modify (HAM). It is called dual-playfield, and allows the bitplanes to be combined in 2 3-bitplaned layers, each with 8 colors (3bp). The advantage is that these can be scrolled by hardware independently with great speed. Some games use this (chuck rock 2, bubba & stix, turrican 2 intro). Hardware sprites can be used to hide the limitations of DPF mode, as well as copper splits.
On the topic of copper splits, it is commonly known that copper can change the entire palette each horizontal line. What is less known is that it can change a single colour index every 8 pixels (or was it 16?). This theoretically enables use of extra colours.
It is also perhaps worth noting that Amiga could easily switch between PAL and NTSC frequencies on the fly (you only needed to poke one register), thus for example stretching the 320x200 ntsc image to fullscreen that would be 320x256 in PAL. The only visible difference would be more prominent scanlines in NTSC, but in turn the clock would increase to 60hz from 50hz.
(*copper = one of amiga's 2 graphic coprocessors)