Amiga Advanced Graphics Architecture: Difference between revisions

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{{Short description|Amiga graphic chipset}}

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{{about-distinguish-text|the Amiga graphics chipset released in 1992|the planned but never-released [[Advanced Amiga Architecture chipset|Advanced Amiga Architecture (AAA) chipset]]}}

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{{About|the Commodore Amiga graphics hardware|the oven|Aga cooker}}

'''Amiga Advanced Graphics Architecture''' ('''AGA''') is the third-generation [[Amiga]] graphic chipset, first used in the [[Amiga 4000]] in 1992. Before release AGA was codenamed '''Pandora''' by [[Commodore International]].

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AGA is able to display graphics modes with a depth of up to {{nowrap|8 bit}}s per pixel. This allows for {{nowrap|256 colors}} in indexed display modes and {{nowrap|262,144 colors}} (18-bit) in [[Hold-And-Modify]] (HAM-8) modes. The palette for the AGA chipset has 256 entries from {{nowrap|16,777,216 colors}} (24-bit), whereas previous chipsets, the [[Original Chip Set]] (OCS) and [[Amiga Enhanced Chip Set|Enhanced Chip Set]] (ECS), only allow {{nowrap|32 colors}} out of 4096 or 64 colors in Amiga [[Extra 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 wider [[Sprites (computer graphics)|sprites]].

AGA is an incremental upgrade, rather than the dramatic upgrade of the other chipset that Commodore had begun in 1988, the [[Amiga 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 a lot ofsignificant bandwidth is wasted during register accesses and [[Original Chip Set#Copper|copper]] and [[blitter]] operations. Also the lack of a [[packed pixel|chunky]] graphics mode is a speed impediment to graphics operations not tailored for [[Planar (computer graphics)|planar]] modes, resulting in ghost artifacts during the common productivity task of [[scrolling]]. In practice, the AGA HAM mode is mainly useful in paintingpaint programs, picture viewers, and for video playback. Workbench in 256 colors is much slower than [[Amiga Enhanced Chip Set|ECS]] 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 display {{nowrap|640 × 480}} at {{nowrap|72 Hz}} flicker-free operation. {{nowrap|800 × 600}} mode is rarely used as it can only operate at a flickering {{nowrap|60 Hz}} interlaced mode. In contrast, higher-end PC systems of this era can operate {{nowrap|1024 × 768}} at {{nowrap|72 Hz}} with a full 256-color display. AGA's highest resolution is {{nowrap|1440 × 580}} {{nowrap|(262 144 colors)}} in interlaced {{nowrap|50 Hz}} {{nowrap|PAL mode,}} when overscan is used.

These missed opportunities in the AGA upgrade contributed to the [[Amiga]] ultimately losing technical leadership in the area of [[multimedia]]. After the long-delayed AAA was finally suspended, AGA was to be succeeded by the [[Hombre chipset]], but this was ultimately cancelled due to Commodore's [[bankruptcy]].

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== Technical details ==

In order to increase memory bandwidth, the [[Chip RAM]] data bus was extended to 32-bit width as in the [[Amiga 3000|A3000]] (unlike AGA, the A3000's Chip RAM is 32-bit for CPU access only) and the Alice chip (replacing [[Original Chip Set|OCS]]/[[Amiga Enhanced Chip Set|ECS]] [[MOS Technology Agnus|Agnus]]) was improved to be able to support full-width access for bitplane DMA. Additionally,Bandwidth thewas memorydoubled clockagain was(to doubled4x) by using [[Dynamic_random-access_memory#Page_mode_DRAM|Fast Page Mode RAM]].

Lisa (replacing former [[Original Chip Set#Denise|Denise]]) adds support for 8-bit bitplane data fetches, 256 instances of 24-bit palette registers, and for 32-bit data transfer for bitplane graphic and [[Sprite (computer graphics)|sprites]].