Amiga Advanced Graphics Architecture

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Amiga Advanced Graphics Architecture (AGA) is the third generation Amiga graphic chip set, first used in the Amiga 4000 in 1992. before release AGA was codenamed Pandora by Commodore International.

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.[1]

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 262 144 colors (18-bit) in HAM-8 (Hold-And-Modify) modes. The palette for the AGA chipset has 256 entries from 16 777 216 colors (24-bit), whereas previous chip sets (OCS and ECS) only allowed 32 colors out of 4096 (64 colors in EHB mode). 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 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 is a speed impediment to graphics operations not tailored for planar modes. In practice, the AGA HAM (Hold-And-Modify) mode is mainly useful in painting programs, picture viewers and for video playback. Workbench in 256 colors is much slower than ECS operation modes for normal application use; a workaround is to use multiple screens with different color depths. AGA also lacks flicker free higher resolution modes, being only able to display 640 × 480 at 72 Hz flicker-free operation. 800 × 600 mode is rarely used as it could only operate at a flickering 60 Hz interlaced mode. In contrast, higher-end PC systems of this era could operate 1024 × 768 at 72 Hz with a full 256-color display. AGA's highest resolution is 1440 × 580 (262 144 colors) in interlaced 50 Hz PAL mode, when overscan was used.

These missed opportunities in the AGA upgrade 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.

Technical details[edit]

In order to increase memory bandwidth, the Chip RAM data bus was extended to 32-bit width (as in the A3000[2]) and the Alice chip (replacing OCS/ECS Agnus) was improved to be able to support full width access for bitplane DMA. Additionally, the memory clock was doubled.

Lisa (replacing former Denise) added support for 8-bit bitplane data fetches, 256 instances of 24-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, still working on 16-bit data.

See also[edit]

References[edit]

  1. ^ The Amiga AGA Chipset
  2. ^ Unlike AGA, the A3000's Chip RAM is 32-bit for CPU access only.