Amiga Original Chip Set


The Original Chip Set 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 and the greatly improved Advanced Graphics Architecture.
The original chipset appeared in Amiga models built between 1985 and 1990: the Amiga 1000, Amiga 2000, Amiga CDTV, and Amiga 500.

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'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 chip RAM from both the central 68000 processor and the other custom chips, using a complicated priority system. Agnus includes sub-components known as the blitter and the Copper. The original Agnus can address of chip RAM. Later revisions, dubbed 'Fat Agnus', added pseudo-fast RAM, which for ECS was changed to 1 MB 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 or 256 pixels tall. Denise also supports interlacing, which doubles the vertical resolution, at the cost of intrusive flickering on typical monitors of that era. 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. A 6th bitplane is available for two special video modes: Halfbrite mode and Hold-And-Modify 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 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.

Agnus

The Agnus chip is in overall control of the entire chipset's operation. All operations are synchronised 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, where Agnus is the DMA Controller.
Agnus has a complex and priority-based memory access policy that attempts to best coordinate requests for memory access among competing resources. For example, bitplane data fetches are prioritized over blitter transfers as the immediate display of frame buffer data is considered more important than the processing of memory by the blitter. Agnus also attempts to order accesses in such a way so as to overlap CPU bus cycles with DMA cycles. As the original 68000 processor in Amigas tended only to access memory on every second available memory cycle, Agnus operates a system where "odd" memory access cycles are allocated first and as needed to time-critical custom chip DMA while any remaining cycles are available to the CPU, thus the CPU does not generally 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 spare odd or even cycles and, if the "BLITHOG" flag is set, Agnus can lock out the even cycles from the CPU in deference to the blitter.
Agnus's timings are measured in "color clocks" of 280 ns. This is equivalent to two low resolution pixels or four high resolution pixels. Like Denise, these timings were designed for display on household TVs, and can be synchronized to an external clock source.

Blitter

The blitter is a sub-component of Agnus. "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 and line drawing.
The blitter allows rapid copying of video memory, meaning that the CPU can be freed for other tasks. The blitter was primarily used for drawing and redrawing graphics images on the screen, called "bobs", short for "blitter objects".
The blitter's block copying mode takes zero to three data sources in memory, called A, B and C, performs a programmable Boolean function on the data sources and writes the result to a destination area, D. Any of these four areas can overlap. The blitter runs either from the start of the block to the end, known as "ascending" mode, or in reverse, "descending" mode. This type of operation is known today as a Bitwise ternary logic instruction.
Blocks are "rectangular"; they have a "width" in multiples of 16 bits, 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 video resolution up to 1,024×1,024 pixels. 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, or apply a pixel mask around blitted objects. 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.
These functions allow the Amiga to move GUI windows around the screen rapidly as each is represented in graphical memory space as a rectangular block of memory which may be shifted to any required screen memory location at will.
The blitter's line mode draws single-pixel thick lines using Bresenham's line algorithm. It can also apply a 16-bit repeating pattern to the line. The line mode can also be used to draw rotated bobs: each line of bob data is used as line pattern while the line mode draws the tilted bob line by line.
The blitter's filling mode is used to fill per-line horizontal spans. On each span, it reads each pixel in turn from right to left. Whenever it reads a set pixel, it toggles filling mode on or off. When filling mode is on, it sets every pixel until filling mode is turned off or the line ends. Together, these modes allow the blitter to draw individual flat-shaded polygons. Later Amigas tended to use a combination of a faster CPU and blitter for many operations.

Copper

The Copper is another sub-component of Agnus; The name is short for "co-processor". The Copper is a programmable finite-state machine that executes a programmed instruction stream, synchronized with the video hardware.
When it is turned on, the Copper has three states; either reading an instruction, executing it, or waiting for a specific video beam position. The Copper runs a program called the Copper list in parallel with the main CPU. The Copper runs in sync with the video beam, and it can be used to perform various operations which require video synchronization. Most commonly it is used to control video output, but it can write to most of the chipset registers and thus can be used to initiate blits, set audio registers, or interrupt the CPU.
The Copper list has three kinds of instructions, each one being a pair of two bytes, four bytes in total:
  • The MOVE instruction writes a 16-bit value into one of the chipset's hardware registers and is also used to strobe a new address into the Copper's instruction pointer.
  • The WAIT instruction halts Copper execution until a given beam position is reached, thus making possible to synchronize other instructions with respect to screen drawing. It can also wait for a blitter operation to finish. During a wait state the copper is off the bus and doesn't use DMA cycles.
  • The SKIP instruction will skip the following Copper instruction if a given beam position has already been reached. This can be used to create Copper list loops.
The length of the Copper list program is limited by execution time. The Copper restarts executing the Copper list at the start of each new video frame. There is no explicit "end" instruction; instead, the WAIT instruction is used to wait for a location which is never reached.

Uses of the Copper

  • The Copper is most commonly used to set and reset the video hardware registers at the beginning of each frame.
  • It can be used to change video settings mid-frame. This allows the Amiga to change video configuration, including resolution, between scanlines. This allows the Amiga to display different horizontal resolutions, different color depths, and entirely different frame buffers on the same screen. The AmigaOS graphical user interface allows two or more programs to operate at different resolutions in different buffers, while all are visible on the screen simultaneously. A paint program might use this feature to allow users to draw directly on a low resolution Hold-And-Modify screen, while offering a high resolution toolbar at the top or bottom of the screen.
  • The Copper can also change color registers mid-frame, creating the "raster bars" effect seen commonly in Amiga games. The Copper can go further than this and change the background color often enough to make a blocky graphics display without using any bitmap graphics at all.
  • The Copper allows "re-use" of sprites; after a sprite has been drawn at its programmed location, the Copper can then immediately move it to a new location and it will be drawn again, even on the same scanline.
  • The Copper can trigger an interrupt when the video beam reaches a precise location on the display. This is useful for synchronizing the CPU to the video beam.
  • The Copper can also be used to program and operate the blitter. This allows blitter operation and control to proceed independently of, and concurrently with, the CPU. With copper and blitter programming each other a text mode can be emulated.
  • The Copper can be used to produce "sliced HAM", or S-HAM, this consists of building a Copper list that switches the palette on every scanline, improving the choice of base colors in Hold-And-Modify mode graphics.