Editing Amiga Original Chip Set (section)

==Overview of chips==
The chipset which gave the Amiga its unique graphics features consists of three main "custom" chips: ''Agnus'', ''Denise'', and ''Paula''. Both the original chipset and the enhanced chipset were manufactured using [[NMOS logic]] technology by [[Commodore International|Commodore]]'s [[Semiconductor fabrication|chip manufacturing]] subsidiary, [[MOS Technology]]. According to [[Jay Miner]], the OCS chipset was fabricated in 5&nbsp;μm manufacturing process while [[Amiga Advanced Graphics Architecture|AGA]] Lisa was implemented in 1.5&nbsp;μm process. All three custom chips were originally packaged in 48-pin [[Dual in-line package|DIP]]s; later versions of Agnus, known as Fat Agnus, were packaged in an 84-pin [[plastic leaded chip carrier|PLCC]].

'''Agnus''' is the central chip in the design. It controls all access to [[Amiga Chip RAM|chip RAM]] from both the central [[Motorola 68000|68000]] processor and the other custom chips, using a complicated priority system. Agnus includes sub-components known as the ''blitter'' (fast transfer of data in memory without the intervention of the processor) and the ''Copper'' (video-synchronized co-processor). The original Agnus can address {{nowrap|512 [[Kilobyte|KB]]}} of chip RAM. Later revisions, dubbed 'Fat Agnus', added {{nowrap|512 KB}} pseudo-fast RAM, which for ECS was changed to 1&nbsp;MB (sometimes called 'Fatter Agnus') and subsequently to 2&nbsp;MB chip RAM.

'''Denise''' is the main video processor. Without using [[overscan]], the Amiga's graphics display is 320 or 640 [[pixel]]s wide by 200 ([[NTSC]]) or 256 ([[PAL]]) pixels tall. Denise also supports [[Interlaced video|interlacing]], which doubles the vertical resolution, at the cost of intrusive flickering on typical monitors of that era. (Various [[flicker fixer]] solutions appeared.) [[Planar (computer graphics)|Planar]] [[bitmap graphics]] are used, which splits the individual [[bits per pixel]] into separate areas of memory, called [[bitplanes]]. In normal operation, Denise allows between one and five bitplanes, giving two to 32 unique colors. These colors are selected from a [[Palette (computing)|palette]] of 4096 colors (four bits per [[RGB]] component). A 6th bitplane is available for two special video modes: [[Amiga Halfbrite mode|Halfbrite mode]] and [[Hold-And-Modify]] (HAM) mode. Denise also supports eight [[Sprite (computer graphics)|sprites]], single pixel scrolling, and a "dual-playfield" mode. Denise also handles mouse and digital joystick input.

'''Paula''' is primarily the audio chip, with four independent hardware-mixed [[8-bit computing|8-bit]] [[pulse-code modulation|PCM]] sound channels, each of which supports 65<!--64?--> volume levels (no sound to maximum volume) and waveform output rates from roughly 20 samples per second to almost 29,000 samples per second. Paula also handles [[interrupts]] and various [[input/output|I/O]] functions including the [[floppy disk drive]], the [[serial port]], and [[analog stick|analog joysticks]].

There are many similarities{{snd}} both in overall functionality and in the division of functionality into the three component chips{{snd}} between the OCS chipset and the much earlier and simpler chipset of the [[Atari 8-bit computers]], consisting of the [[ANTIC]], [[Atari 8-bit TIA|GTIA]] and [[POKEY]] chips; both chipsets were conceptually designed by [[Jay Miner]], which explains the similarity.