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Amiga Original Chip Set
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Amiga Original Chip Set
The Original Chip Set (OCS) is a chipset used in the earliest Commodore Amiga computers and defined the Amiga's graphics and sound capabilities. It was succeeded by the slightly improved Enhanced Chip Set (ECS) and the greatly improved Advanced Graphics Architecture (AGA).
The original chipset appeared in Amiga models built between 1985 and 1990: the Amiga 1000, Amiga 2000, Amiga CDTV, and Amiga 500.
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's chip manufacturing subsidiary, MOS Technology. According to Jay Miner, the OCS chipset was fabricated in 5 μm manufacturing process while AGA Lisa was implemented in 1.5 μm process. All three custom chips were originally packaged in 48-pin DIPs; later versions of Agnus, known as Fat Agnus, were packaged in an 84-pin PLCC.
Agnus is the central chip in the design. It controls all access to shared RAM (referred to as "chip RAM" in Amiga contexts) from both the central 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 512 KB of chip RAM. Later revisions, dubbed 'Fat Agnus', added 512 KB pseudo-fast RAM, which for ECS was changed to 1 MB (sometimes called 'Fatter Agnus') and subsequently to 2 MB chip RAM.
Denise is the main video processor. Without using overscan, the Amiga's graphics display is 320 or 640 pixels wide by 200 (NTSC) or 256 (PAL) pixels tall. Denise also supports interlacing, which doubles the vertical resolution, at the cost of intrusive flickering on typical monitors of that era. (Various flicker fixer solutions appeared.) 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 of 4096 colors (four bits per RGB component). A 6th bitplane is available for two special video modes: Halfbrite mode and Hold-And-Modify (HAM) mode. Denise also supports eight 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 PCM sound channels, each of which supports 65 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 I/O functions including the floppy disk drive, the serial port, and analog joysticks.
There are many similarities – both in overall functionality and in the division of functionality into the three component chips – between the OCS chipset and the much earlier and simpler chipset of the Atari 8-bit computers, consisting of the ANTIC, GTIA and POKEY chips; both chipsets were conceptually designed by Jay Miner, which explains the similarity.
The Agnus chip is in overall control of the entire chipset's operation. All operations are synchronized to the position of the video beam. This includes access to the built-in RAM, known as chip RAM because the chipset has access to it. Both the central 68000 processor and other members of the chipset have to arbitrate for access to chip RAM via Agnus. In computing architecture terms, this is Direct Memory Access (DMA), where Agnus is the DMA Controller (DMAC).
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Amiga Original Chip Set
The Original Chip Set (OCS) is a chipset used in the earliest Commodore Amiga computers and defined the Amiga's graphics and sound capabilities. It was succeeded by the slightly improved Enhanced Chip Set (ECS) and the greatly improved Advanced Graphics Architecture (AGA).
The original chipset appeared in Amiga models built between 1985 and 1990: the Amiga 1000, Amiga 2000, Amiga CDTV, and Amiga 500.
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's chip manufacturing subsidiary, MOS Technology. According to Jay Miner, the OCS chipset was fabricated in 5 μm manufacturing process while AGA Lisa was implemented in 1.5 μm process. All three custom chips were originally packaged in 48-pin DIPs; later versions of Agnus, known as Fat Agnus, were packaged in an 84-pin PLCC.
Agnus is the central chip in the design. It controls all access to shared RAM (referred to as "chip RAM" in Amiga contexts) from both the central 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 512 KB of chip RAM. Later revisions, dubbed 'Fat Agnus', added 512 KB pseudo-fast RAM, which for ECS was changed to 1 MB (sometimes called 'Fatter Agnus') and subsequently to 2 MB chip RAM.
Denise is the main video processor. Without using overscan, the Amiga's graphics display is 320 or 640 pixels wide by 200 (NTSC) or 256 (PAL) pixels tall. Denise also supports interlacing, which doubles the vertical resolution, at the cost of intrusive flickering on typical monitors of that era. (Various flicker fixer solutions appeared.) 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 of 4096 colors (four bits per RGB component). A 6th bitplane is available for two special video modes: Halfbrite mode and Hold-And-Modify (HAM) mode. Denise also supports eight 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 PCM sound channels, each of which supports 65 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 I/O functions including the floppy disk drive, the serial port, and analog joysticks.
There are many similarities – both in overall functionality and in the division of functionality into the three component chips – between the OCS chipset and the much earlier and simpler chipset of the Atari 8-bit computers, consisting of the ANTIC, GTIA and POKEY chips; both chipsets were conceptually designed by Jay Miner, which explains the similarity.
The Agnus chip is in overall control of the entire chipset's operation. All operations are synchronized to the position of the video beam. This includes access to the built-in RAM, known as chip RAM because the chipset has access to it. Both the central 68000 processor and other members of the chipset have to arbitrate for access to chip RAM via Agnus. In computing architecture terms, this is Direct Memory Access (DMA), where Agnus is the DMA Controller (DMAC).
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