The Editors and
Readers of COMPUTE!
Amiga Graphics Update
We enjoyed your article on the graphics for Commodore's new Amiga computer ["Amigo's Amazing Graphics," COMPUTE!, November 1985]. Most of the information was very accurate, and the picture describing bit plane graphics was particularly effective. There was some inaccuracy, however. The actual color map at 640 X 200 resolution is the same as it is at 320 X 200, and there are no restrictions on adjacent pixels. Somehow, you must have gotten some old documentation. Also, although Intuition supports dual playfield mode, it does not do multiple screens using dual playfield mode-it uses the video coprocessor to rewrite the display parameters. This is much more flexible, as any display parameter can be changed instead of just the color map. You can change resolution, color map, or number of bit planes between screens running simultaneously.
We appreciate your comments. At the time we wrote the article, we were working with the prerelease Hardware Manual, which described an earlier version of the graphics chips.
As you say, the final graphics hardware does not impose any restrictions on color resolutions. Early graphics chips could not fully change the color signal between adjacent pixels in 640 X 200 mode, but the current hardware permits any of 16 colors (from the palette of 4,096 colors) at any pixel position, or 32 simultaneous colors for the 320 X 200 mode. The 400-line interlaced modes have the same color capability as 320 X 200 and 640 X 200.
Since the Workbench uses only four colors (two bit planes), we assumed that dual playfields (where two independent screens can be overlapped and merged) were used to support pull-down custom screens. Since every multitasking application can call for its own custom bitmap, Intuition allows these custom screens to be overlapped and repositioned vertically with the mouse. This effect can be seen when you click and drag the menu bar with the left mouse button. When the left button is held down, pulling the mouse downward reveals the background AmigaDOS screen. It's also possible to switch between. a custom screen and the Workbench screen by pressing either Left Amiga-M or Left Amiga-N.
The special copper (as in coprocessor) circuitry tracks the video beam on the fly. The copper's instruction list, similar in concept to Atari 800 display lists, can perform any video change at any line, as with display list interrupts (Atari) or raster interrupts (Commodore). The operating system permits applications to modify the copper list, giving full video control to the application, while using the copper list itself for the graphic effect of overlapping screens. A copper wait instruction tells the copper to wait until the video beam reaches a certain line, and then the video registers are reset to display another screen. The normal display is automatically reset at the top, so you get two overlapping screens, even with different colors and resolutions.