Custom Chips

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WinUAE Documentation


Custom Chips

In the first Amiga, the A1000, the chips "Agnus", "Paula", and "Denise" did the hard work. They together build a specialized multiprocessor system that takes much of the time-critical load off of the main processor. This includes the plain display of image informations, 4-channel sound output, floppy control, general DMA control, and much more. In the newest 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).


Chipset Generations



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. With its data compression, the HAM mode is also responsible for the Amiga being so famous for its 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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