Geometry Raster Texture RAM Engines (GEs) Engines (REs) per RE (in MB) SolidIMPACT: 1 1 0 HighIMPACT: 1 1 1 or 4 HighIMPACT-AA: 2 1 1 or 4 (very rare) MaxIMPACT: 2 2 1 or 4 Thus: HighIMPACT: Physical TRAM Logical TRAM (never discussed) (user visible) Without TRAM Option: 1 1 With TRAM Option: 4 4 MaxIMPACT: Physical TRAM Logical TRAM (never discussed) (user visible) Without TRAM Option: 2 1 With TRAM Option: 8 4
Without the texture option, HighIMPACT has 1MB TRAM, while MaxIMPACT has 2MB TRAM, but the latter is just a way of parallelising the texture mapping process, ie. the user only 'sees' a visible/usable 1MB TRAM in a MaxIMPACT system without the texture option (2MB physical, 1MB logical). What actually happens is that texture data is sent to *both* TRAM units in parallel (because there are 2 Raster Engines, each having 1MB physical TRAM) so that each RE can access its own local copy of the texture data. With the texture option installed (an extra 3MB TRAM per RE), HighIMPACT has 4MB physical and 4MB logical TRAM, while MaxIMPACT has 8MB physical and 4MB logical. Thus, in terms of how the user thinks of the actual available TRAM for programming purposes, both systems have either 1 or 4 MB, but the actual physical amount present is not the same.
Anyway, this is important for sales information because many buyers don't understand why the '4MB TRAM option' for MaxIMPACT costs roughly twice as much as the '4MB TRAM option' for HighIMPACT - it's simply because the actual physical upgrades consist of +6MB (Max) and +3MB (High), even though both give the user a logical visible 4MB.
With 4MB logical TRAM, High and Max IMPACT can do 24bit RGB or 32bit RGBA (8 bits per channel) instead of just 16bit colour, can handle more textures, and can also do 36bit RGB for advanced 2D imaging (12 bits per channel) as well as more advanced video operations, etc. Also, a 4MB TRAM system has the TRAM organised as four 1MB banks, allowing an RGBA texture to be stored as red in bank 0, green in bank 1, etc. which speeds up computation.
Lastly, 4MB TRAM makes Quake2, etc. look much nicer! And that's what really matters... ;)