Advanced Graphics Architecture
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Advanced Graphics Architecture (AGA) was the name used for the improved graphics chipset of the third generation Amiga computers at the beginning of the 1990s (Second generation had ECS). In the United States AGA was originally called AA for Advanced Architecture, whereas in Europe the name was changed to AGA to reflect that it largely improved the graphical subsystem (opposed to the architecture in its entirety), and to avoid trademark issues. [1]
AGA is able to do 8-bit pixels, which gives 256 colors in normal display mode and 262144 colors in HAM-8 (Hold-And-Modify) mode (18-bit color, 6 bits per RGB channel). Palette for AGA chipset is 256 entries from 16777216 colors (24-bit). The original Amiga chipset (OCS) had 4096 colors (12-bit, 4 bits per RGB channel), of which 32 could be displayed unless in half-bright (which provided an additional 32 colors fixed at half the brightness of the first 32) or HAM mode. Other features added to AGA over ECS were superhires smooth scrolling and 32-bit fast page memory fetches to supply the graphics data bandwidth for 8 bitplane graphics modes and wider sprites.
The AGA chipset was a basic evolutionary upgrade, lacking many features that would have made it competitive with PC graphic chipsets of its time. Apart from the graphics data fetches the chipset still operates on 16-bit data only, meaning that a lot of bandwidth is wasted during register accesses and copper and blitter operations. Also the lack of a chunky graphics mode was a speed impediment to graphics operations not tailored for planar modes. Missed opportunities in the AGA upgrade may have contributed to the Amiga ultimately losing technical leadership in the multimedia area.
AGA was included in the CD32, Amiga 1200 and Amiga 4000.
