Games - Original Amiga chipset

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The Original Chip Set (OCS) is a chipset used in the earliest Commodore Amiga computers. It was succeeded by the modestly improved Enhanced Chip Set (ECS) and greatly improved Advanced Graphics Architecture (AGA).

The original chipset appeared in the Amiga 1000, Amiga 2000 and Amiga 500.

Overview of chips

The chipset which gave the Amiga its unique graphics features consisted of three main custom chips, Agnus, Denise, and Paula. The OCS (and ECS) chipset was manufactured using NMOS technology by Commodore's chip manufacturing subsidiary, MOS Technology. All three were originally packaged in 48-pin DIPs; later versions of Agnus, known as Fat Agnus, were packaged in an 84-pin PLCC.

Agnus – the central chip in the design. It controlled all access to chip RAM from both CPU and other chips, using a complicated priority system. It also included the Blitter and the Copper. The original Agnus and Fat Agnus could address 512 KiB of chip RAM. Later revisions of Fat Agnus could address 1 MiB of chip RAM.
Denise – the main video processor. Without using overscan, the display was 320 (lowres) or 640 (hires) pixels wide by 200 (NTSC) or 256 (PAL) tall. It also supported interlacing which doubled the vertical resolution. Anything between 2 and 32 unique colors (1 to 5 bitplanes) from a 12 bit (4096 color) palette, was supported. A 6th bitplane was available for either the Halfbrite mode
that added a copy of the first 32 colors but with half the intensity or Hold And Modify mode which allowed access to all 4096 colors at once. Denise supported eight sprites, smooth scrolling, and "dual playfield". Denise also handles mouse and digital joystick inputs.
Paula – primarily a sound chip, with 4 independent hardware mixed 8-bit PCM sound channels with 65 volume levels and any sample rate from roughly 20 Hz to 29 kHz. Paula also handled interrupts and various I/O functions including floppy disk drive, serial port, and analog joystick.

Agnus

The central aspect of the chipset's power is that all operations are synchronised with the output of the video beam. This includes access to the built-in RAM, which is known as chip RAM because the chipset has access to it. The CPU and other members of the chipset have to arbitrate for access with Agnus. From the perspective of system architecture, this is known as Direct Memory Access (DMA), and Agnus is the DMA Controller (DMAC) in that respect.

Agnus has a complex priority-based memory access policy. Bitplane data fetches are more important than blitter transfers, for example. As the original 68000 in Amigas can only access memory on every second clock cycle, Agnus operates a system where the time-critical custom chips access get the "odd" cycle and the CPU gets the "even" cycle, thus the CPU does not get locked out of memory access and does not appear to slow down. However, non-time-critical custom chip access, such as blitter transfers, can use up any unused odd or even cycles and, if the "BLITHOG" (blitter hog) flag is set, Agnus will lock out the even cycles from the CPU in deference to the blitter.

Agnus's timings were measured in "colour clocks" of 280 ns. This is equivalent to two lowres (140 ns) pixels or four hi-res (70 ns) pixels. Like Denise, these timings were designed for display on household TVs, and can be synchronised to an external clock source.

Blitter

The blitter—"blit" is shorthand for "block image transfer" (or bit blit). The Blitter is a highly parallel memory transfer and logic operation unit. It has three modes of operation: copying blocks of memory, filling blocks (e.g. polygon filling) and line drawing.

Block copying mode takes zero to three data sources in memory (A, B and C), and writes to a destination area (D). Any of these areas can overlap. All blocks are on 16-bit (two byte) boundaries. The blitter either runs from the start of the block to the end, known as "ascending" mode, or in reverse, "descending" mode. Blocks are "rectangular"; they have a "width" in 16-bit words (not pixels), a height measured in "lines", and a "stride" distance to move from the end of one line to the next. This allows the blitter to operate on any conceivable video resolution. The copy automatically performs a per-pixel logical operation. These operations are described generically using minterms. This is most commonly used to do direct copies (D = A), or apply a pixel mask around blitted objects (D = A AND C). The copy can also barrel shift each line by 0 to 15 pixels. This allows the blitter to draw at pixel offsets that are not exactly multiples of 16.

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