Editing Amiga Original Chip Set (section)

==Denise==
[[File:Commodore Amiga 1000 - main board - MOS 8362R5-7823.jpg|thumb|MOS 8362R5 - Denise]]
Denise is programmed to fetch [[Planar (computer graphics)|planar]] video data from one to five bitplanes and translate that into a [[color look-up table|color lookup]]. The number of bitplanes is arbitrary, thus if 32 colors are not needed, 2, 4, 8 or 16 can be used instead. The number of bitplanes (and resolution) can be changed on the fly, usually by the Copper. This allows for very economical use of RAM, and balancing of CPU processing speed vs graphical sophistication when executing from Chip RAM (as modes beyond 4bpp in low resolution, or 2bpp in high resolution, use extra DMA channels that can slow or temporarily halt the CPU in addition to the usual non-conflicting channels). There can also be a sixth bitplane, which can be used in three special graphics modes:

* In [[Amiga Halfbrite mode|Extra-HalfBrite]] (EHB), if a pixel is set on the sixth bitplane, the brightness of the regular 32 color pixel is halved. Early versions of the [[Amiga&nbsp;1000]] sold in the [[United States]] did not have the Extra-HalfBrite mode.<ref>{{Cite web |url=http://www.old-computers.com/museum/computer.asp?st=1&c=28 |title=OLD-COMPUTERS.COM : The Museum<!-- Bot generated title --> |access-date=2005-05-27 |archive-url=https://web.archive.org/web/20100528010507/http://www.old-computers.com/museum/computer.asp?st=1&c=28 |archive-date=2010-05-28 |url-status=dead }}</ref>

* In [[Hold-And-Modify]] (HAM) mode, each 6-bit pixel is interpreted as two control bits and four data bits. The four possible permutations of control bits are "set", "modify red", "modify green" and "modify blue". With "set", the four data bits act like a regular 16-color display look up. With one of the "modify"s, the red, green or blue component of the previous pixel is modified to the data value, and the other two components are held from the previous pixel. This allows all 4096 colors on screen at once and is an example of [[lossy compression|lossy image compression]] in hardware.

* In dual-playfield mode, instead of acting as a single screen, two "playfields" of eight colors each (three bitplanes each) are drawn on top of each other. They are independently scrollable and the background color of the top playfield "shines through" to the underlying playfield.

There are two horizontal graphics resolutions, "lowres" with 140&nbsp;ns pixels and "hires" with 70&nbsp;ns pixels, with a default of 320 or 640 horizontal pixels wide without using overscan. As the pixel output is regulated by the main system clock, which is based directly on the NTSC colorburst clock, these sizes very nearly fill the width of a standard television with only a thin "underscan" border between the graphics and the screen border when compared to many other contemporary home computers, for an appearance closer to a games console but with finer detail. On top of this, Denise supports reasonably extensive overscan; technically modes with enough data for up to 400 or 800 pixels (+25%) may be specified, although this is only actually useful for scrolling and special effects that involve partial display of large graphics, as a separate hardware limit is met at 368 (or 736) pixels, which is the maximum that will fit between the end of one blanking period and the start of the next - although it is unlikely that even this many pixels will be visible on any display other than a dedicated monitor that allows adjustment of horizontal scan width, as much of the image will, by design, disappear seamlessly behind the screen bezel (or, on LCDs, be cropped off at the edge of the panel).<ref>{{Cite web|url=http://www.retroplatform.com/kb/19-115|title = Display Sizes, Offsets and Clipping}}</ref><ref>{{Cite web|url=http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0085.html|title=3 / Bitplanes and Windows of All Sizes / Maximum Display Window Size}}</ref> Because of the highly regular structure of the Amiga's timing in relation to scanlines and allocation of DMA resources to various uses besides normal "playfield" graphics, increased horizontal resolution is also a tradeoff between number of pixels and how many hardware sprites are available, as increasing the DMA slots dedicated to playfield video ends up stealing some (from 1 to 7 of the total 8) the sprite engine.[http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node02D4.html].

Vertical resolution, without overscan, is 200 pixels for a 60&nbsp;Hz NTSC Amiga or 256 for a 50&nbsp;Hz PAL Amiga. This can be doubled using an [[interlaced video|interlace]]d display, and, as with horizontal resolution, increased using overscan, to a maximum of 241 (or 483) for NTSC, and 283 (or 567) for PAL (interlaced modes gaining one extra line as the maximum is determined by how many lines are taken from the available total by blanking and sync, and the total scanlines in non-interlaced modes are half the original, broadcast-spec odd-numbered interlaced counts, rounded down). 

Starting with ECS, Denise was upgraded to support "Productivity" mode which allowed for 640x400 non-interlaced albeit with only 4 colors.

Denise can composite up to eight 16-pixel-wide [[sprite (computer graphics)|sprites]] per scan line (in automatic mode) on top, underneath, or between playfields, and detect collisions between sprites and the playfields or between sprites. These sprites have three visible colors and one transparent color. Optionally, adjacent pairs of sprites can be "attached" to make a single 15-color sprite. The Sprite DMA reads data to form a sprite channel as controlled by its registers, enabling the vertical reuse of sprites. There has to be one empty scanline in between two successive list entries to allow for updating sprite data. Using Copper or CPU register manipulations, each sprite ''channel'' can be reused multiple times in a single frame to increase the total sprites per frame. Sprite ''position'' registers may also be changed during a scanline, increasing the total number of sprites on a single scanline. However, the sprite ''data'', or shape, is only fetched a single time per scanline and can't change. The first Amiga game to utilize the sprite re-position registers during a scanline was ''[[Hybris (video game)|Hybris]]'' released in 1988.

The Denise chip does not support a dedicated text mode.<ref>{{Cite web|title=The Amiga Museum » Denise|url=https://theamigamuseum.com/the-hardware/the-ocs-chipset/denise/|access-date=2021-02-28}}</ref>

Finally, Denise next to the CIAs is responsible for handling mouse/joystick X/Y inputs.

The notion that Denise fetches bit plane and sprite data is a simplification. It is Agnus who is maintaining horizontal and vertical screen position counters and initiating the DRAM read operations. Denise has a number of bit plane registers which hold 16 bits of data each, enough to draw 16 pixels. When Agnus issues a write to register 1, all registers are then transferred into separate shift registers from which pixels are generated (at the same time new values are loaded from DRAM). Denise is not aware of any memory addresses either.