The so-called "Custom Chips" are arguably the most famous and differentiating feature of Amigas, compared to other computing architectures. They allowed the - for it's time - very advanced capabilities.
In the first one, the A1000, the chips "Agnus", "Paula", and "Denise" did the hard work. Together, they formed a specialized multiprocessor system that took much of the time-critical load off of the main processor. This included the display of image information, 4-channel sound output, floppy control, general DMA control, and much more.
In the latest Amigas you find the "AGA chipset", which consists of the original "Paula" and the advanced chips named "Alice" (the replacement for Agnus) and "Lisa" (the replacement for Denise).
- OCS (Original Chip Set): This was the first chipset, used in the Amiga 1000. It consists of the chips Paula, Agnus, and Denise. With these, you can display graphics in resolutions up to 640x512 and in up to 4096 colors.
- ECS (Enhanced Chip Set): This was the next generation of Amiga chipsets. The chip names remained unchanged, except some minor additions like "Fat Agnus". Only Agnus and Denise were changed. Compared to OCS, they provided higher resolutions, but no expansion of the color palette so far.
- AGA (Advanced Graphics Architecture): This is the latest and most advanced chipset. The current generation was created in the labs as AA chipset (for Advanced Amiga), but in some countries it was later marketed as AGA (Advanced Graphics Architecture).
Agnus was replaced by Alice and Denise by Lisa. The resolutions now reach up to 1280x512 in VGA scan frequencies. The color palette is now 24 bits or 16.8 million colors. You can display up to 256 colors simultaneously or in HAM mode more than 640,000. Backwards compatibility with Video and OCS modes is assured.
- Discrete graphics and RTG. After AGA, the Amiga used discrete graphics cards to improve display resolutions and color depths but lost some of the features like HAM, Copper effects and Sprites on the earlier boards.
The Amiga's third custom chip, Paula took care of audio generation and I/O. There were four voices of sound, output as two (stereo) channels. Nine octaves were available with both amplitude (e.g. speech playback) and frequency modulation.
Amplitude modulation was the normal way of generating sound, with the DMA chip reading amplitude data from memory and the I/O chip translating the digital data to an analog voltage level. Frequency modulation always took place "on top of" amplitude modulation, modulating the frequency with which the DMA chip would read new amplitude data. One voice's amplitude data was usurped to modulate the frequency of another voice. Frequency modulation was not needed to play normal tones, and usually had little to do with the perceived pitch of the actual sound played, at least as experienced by a human listener.
The HAM (Hold And Modify) mode is one of several graphics modes that's provided by the Amiga custom chips from their start. With HAM, you can display many colors simultaneously while using up very little memory. Simply put, it's sort of a hardware data compression method by a factor of ca. three. Still you can display practically all colors of the palette on screen. This modification of one color component serves to render soft color transitions very accurately, as they are typical for natural pictures like portraits. In the HAM8 mode you can theoretically display more than 640,000 colors simultaneously on screen. The dependence on previous pixel contents sets certain limits, so that the HAM mode is less suited for realtime action. But in still pictures or precalculated animations, it lives with its many colors. Due to data compression, the HAM mode is also responsible for the Amiga being so famous for animations that can run at full speed, even on smaller Amiga models. Basically you can display all 16.8 million colors of the 24-bit palette in HAM8 mode.
Copper is the short name for a "Coprocessor" which is integrated into the Amiga custom chips. It makes fast action games possible, as well as unbelievable color effects, and the capability to drag down a screen partially on the Amiga's display and thus show several screens at once. Technically, the Copper is a highly specialized microprocessor which knows only three different commands, but can process these extremely fast and efficiently. It provides the ability to change e.g. graphics mode, color values, or sprite definitions somewhere at a well defined location within the screen. This way the Amiga can display many different actions on one screen simultaneously without activity by the main CPU.
Blitter is the short name for another graphics coprocessor which is part of the Amiga custom chips. With it you can copy rectangular image parts extremely fast to other areas. During this, you can take up to three source data areas and perform logical operations on them. This allows e.g. to define certain color values of the copied object as transparent, thus not modifying the destination area. As an add-on, the Blitter can also draw lines and fill areas fast. All in all it serves to provide extremely fast, animated graphics that can be designed to be especially realistic.
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