Original Amiga chipset
In computing, OCS is Original (or Old) Amiga Chipset. It was followed by the ECS (Enhanced chipset).It came bundled with A1000, as well with older A500 and A2000. Newer A500 and A2000 came with ECS, or a mix of OCS and ECS.
The chipset which gave the Amiga its unique graphics features consisted of three so-called custom chips, Agnus, Denise, and Paula. The two CIA chips are not considered part of either the old or enhanced chipsets, having first been used with the later model Commodore 64 and all Commodore 128 systems, and thus, predating the Amiga.
Denise was a non-programmable chip responsible for transferring the raw bit data through a hybrid IC to the RGB port.
Agnus was in fact the master of the multimedia capabilities:
- Providing DMA for sound and graphics
- Handling memory (512KBytes of Chip RAM (Memory that can be acceded by the processor and the chipsets)
- Video timing (PAL or NTSC versions)
- Contained the logic for the copper and blitter coprocessors.
A newer version of Agnus was used in most of the A500, and A2000 versions, called Fat Agnus, that could handle 1 MBytes of CHIP RAM on the mainboard. Super Agnus, an even more recent version was able to support 2 MBytes of CHIP RAM, and was able to change between PAL and NTSC video signal timing.
The basic resolution of an Amiga display (sometimes called LoRes for low resolution) is 320x200 (by 60 Hz, NTSC timing), or 320x256 (by 50 Hz, PAL timing). In these resolutions, it was able to display 2, 4, 8, 16 or 32 colors in a palette based manner simultaneously from 4096 colors (4 bit for each of the RGB components). To achieve this flexibility, the display memory buffers were operated in planar mode.
The number of colors on screen could also be increased by two methods:
- EHB, or Extra HalfBright - an additional bit plane was used to display 64 colors, but the second 32 colors were half the brightness of the first 32 ones, useful only with careful palette setup.
- HAM, or Hold-And-Modify, where again six bit planes were used, but only 16 colors were defined by the palette, the remaining 48 "colors" described how to modify the previous pixel color. It was possible to display the entire 4096 color palette, even with the palette registers loaded with all block. But, a badly mismatched color palette configuration can produce a lot of fringing. With a more careful color selection, it was able to show photo-like images and possibly using all of the 4096 colors with a minimum of fringing; many viewers don't even notice them.
(describe other special video modes here)
The resolution can also be increased horizontally (HiRes or high resolution modes), from 320 to 640, but this limited the maximum number of usable colors to 16.
The vertical resolution was 200/256 and could be doubled to 400/512 with the use of video interlace technology.
An Amiga is also able to extend the display by a few more pixels in every direction, effectively using the area outside of the 320x200 frames, thus making all of the display accessible to programs. This feature is called overscan.
Under normal circumstances, The Amiga generates its own video timings, but the chipset also supports external sync from the RGB monitor connector so as to achieve genlocking with external video. There is also an 1 bit output on this connector that indicated whether the Amiga was outputting an image (as opposed to background colour), permitting easy overlaying of Amiga video onto external video. This made the Amiga particularly attractive as a character generator for titling videos and broadcast work, as it avoided the use and expense of AB roll and chromakey units that would be required without the genlock support.
The support of overscan, interlacing and genlocking capabilities, and the fact that the display timing was very close to broadcast standards (NTSC or PAL), made the Amiga the first ideal computer for video purposes, and indeed, it was used in many studios for digitizing video data (sometimes called frame-grabbing), subtitling and interactive video news.
Other features of the chipsets:
- framebuffers operated in planar bitfield modes.
- framebuffers were often linear, but not limited to that.
- hardware graphics potential is very hard to explain briefly since it's pretty complicated due to copper co-processor.
- It was possible to use different 32 hardware colours many times during the screen draw.
- One could have had screen splitted from center to two or more different screen modes with different palettes/depth).
- 8 separate hardware sprites (used for mouse pointer for example) with 16 pixel width and arbitrary height with 4 colours.
- It is possible to simulate having arbitrary-width sprites by running sprite hardware with copper co-processor, up to a maximum of 128 pixels wide for 3-color sprites, and 64 pixels for 15-color sprites, by laying each sprite adjacent to each other. The wider the "meta-sprite," however, the fewer other sprites could be displayed on a single scanline.
- It is possible to simulate having arbitrary-width sprites by running sprite hardware with copper co-processor, up to a maximum of 128 pixels wide for 3-color sprites, and 64 pixels for 15-color sprites, by laying each sprite adjacent to each other. The wider the "meta-sprite," however, the fewer other sprites could be displayed on a single scanline.
- 4 channel hardware DMA driven stereo output with 8-bit PCM samples and up to 28867Hz sample rate.
- sample rates and pointers separate for each of 4 DMA channels.
- Possible to emulate 14-bit sounds with hardware DMA by modulating 2 channels with inline 6-bit volume data.
- sound dynamics of 8-bits == 48.165 dB.
- Each channel has a 6-bit volume register.
- Blitter (block image transferer had 3 operations: copy, fill and draw lines).
- Copper (3 instruction co-processor running in parallel with main CPU made possible to create special graphics effects with minimal CPU load and intervention).