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Amiga Advanced Graphics Architecture (AGA) is the third-generationAmiga graphic chipset, first used in theAmiga 4000 in 1992. Before release AGA was codenamedPandora byCommodore International.
AGA was originally calledAA forAdvanced 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.[1]
AGA is able to display graphics modes with a depth of up to8 bits per pixel. This allows for256 colors in indexed display modes and262,144 colors (18-bit) inHold-And-Modify (HAM-8) modes. The palette for the AGA chipset has 256 entries from16,777,216 colors (24-bit), whereas previous chipsets, theOriginal Chip Set (OCS) andEnhanced Chip Set (ECS), only allow32 colors out of 4096 or 64 colors in AmigaExtra Half-Brite (EHB mode). Other features added to AGA over ECS are super-hi-res smooth scrolling and 32-bit fast page memory fetches to supply the graphics data bandwidth for 8 bitplane graphics modes and widersprites.
AGA is an incremental upgrade, rather than the dramatic upgrade of the other chipset that Commodore had begun in 1988, theAmiga Advanced Architecture chipset (AAA), lacking many features that would have made it competitive with other graphic chipsets of its time. Apart from the graphics data fetches, AGA still operates on 16-bit data only, meaning that significant bandwidth is wasted during register accesses andcopper andblitter operations. Also the lack of achunky graphics mode is a speed impediment to graphics operations not tailored forplanar modes, resulting in ghost artifacts during the common productivity task ofscrolling. In practice, the AGA HAM mode is mainly useful in paint programs, picture viewers, and for video playback. Workbench in 256 colors is much slower thanECS operation modes for normal application use; a workaround is to use multiple screens with different color depths. AGA lacks flicker free higher resolution modes, being only able to display640 × 480 at72 Hz flicker-free operation.800 × 600 mode is rarely used as it can only operate at a flickering60 Hz interlaced mode. In contrast, higher-end PC systems of this era can operate1024 × 768 at72 Hz with a full 256-color display. AGA's highest resolution is1440 × 580(262 144 colors) in interlaced50 HzPAL mode, when overscan is used.
These missed opportunities in the AGA upgrade contributed to theAmiga ultimately losing technical leadership in the area ofmultimedia. After the long-delayed AAA was finally suspended, AGA was to be succeeded by theHombre chipset, but this was ultimately cancelled due to Commodore'sbankruptcy.
AGA is present in theCD32,Amiga 1200, andAmiga 4000.
In order to increase memory bandwidth, theChip RAM data bus was extended to 32-bit width as in theA3000 (unlike AGA, the A3000's Chip RAM is 32-bit for CPU access only) and the Alice chip (replacingOCS/ECSAgnus) was improved to be able to support full-width access for bitplane DMA. Bandwidth was doubled again (to 4x) by usingFast Page Mode RAM.Lisa (replacing formerDenise) adds support for 8-bit bitplane data fetches, 256 instances of 24-bit palette registers, and for 32-bit data transfer for bitplane graphic andsprites.
The rest of the chipset remains unchanged, as do the Blitter and Copper coprocessors in Alice, still working on 16-bit data.
