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]].

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.<ref>[{{Cite web|url=http://www.amigahistory.co.uk/amigaaga.html|title=Amiga TheHistory Guide|website=Amiga AGA Chipset]History}}</ref>

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]].