Advanced Graphics Architecture (AGA) is the third generation Amiga graphic chip set, first used in the Amiga 4000 in 1992. AGA was codenamed the Pandora chipset by Commodore International internally.
AGA was originally called AA for Advanced Architecture in the United States. The name was later changed to AGA for the European market to reflect that it largely improved the graphical subsystem, and to avoid trademark issues. 
AGA is able to display graphics modes with a depth of up to 8 bit per pixel. This allows for 256 colors in indexed display modes and 262144 colors (18-bit) in HAM-8 (Hold-And-Modify) modes. The palette for the AGA chipset has 256 entries from 16777216 colors (24-bit), whereas previous chip sets (OCS and ECS) only allowed 32 colors out of 4096. Other features added to AGA over ECS were super hires smooth scrolling and 32-bit fast page memory fetches to supply the graphics data bandwidth for 8 bitplane graphics modes and wider sprites.
AGA was an incremental upgrade, rather than the dramatic upgrade of the other chipset that Commodore had begun in 1988, AAA, lacking many features that would have made it competitive with other graphic chip sets of its time. Apart from the graphics data fetches, AGA still operated on 16-bit data only, meaning that a lot of bandwidth was 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. In practice, the AGA HAM (Hold-And-Modify) mode was too slow to be useful except for displaying static images. Also, Workbench in 256 color AGA was of concern, being much slower than ECS operation modes for normal application use. AGA also lacked flicker free higher resolution modes; being only able to display 640x480 at 72Hz flicker-free operation. 800x600 mode was left useless as it could only operate at a flickering 60Hz. In contrast, PC systems of this era would easily operate 1024x768 at 72Hz with a full 256 color display (AGA highest resolution 1280 x 512). This left the AGA systems appear amateurish at a time when PCs were being priced more affordably than a comparable Amiga.
These missed opportunities in the AGA upgrade may have contributed to the Amiga ultimately losing technical leadership in the multimedia area. AGA was to be succeeded by the Hombre chipset, after the long delayed AAA was finally shelved, which was ultimately cancelled due to Commodore's bankruptcy.
AGA was used in the CD32, Amiga 1200 and Amiga 4000.
In order to increase memory bandwidth, the Chip RAM data bus was extended to 32 bit width (as in the A3000) and the Alice chip (replacing OCS/ECS Agnus) was improved to be able to handle full width access. Additionally, the memory clock was doubled.
Lisa (replacing former Denise) added support for 8 bit bitplane data fetches, 256 25-bit palette registers, and for 32 bit data transfer for bitplane graphic and sprites.
The rest of the chipset remained unchanged, as did the Blitter and Copper coprocessors in Alice.
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