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Hold and modify

Jay had always had a passion for flight simulators, and it was something that would stay with him for the rest of his life. A friend of his took him on a field trip to Link, a company that made multimillion-dollar flight simulators for the military. Jay was enthralled by the realistic sights and sounds and vowed that he would make the Amiga computer capable of playing the best flight simulators possible.

Two major design decisions came out of this trip: the blitter and HAM mode. Jay had already read about blitters in electronic design magazines and had taken a course at Stanford on their use, so they were not a new idea for him. However, the flight simulator experience had made him determined to create the best possible blitter for the Amiga.

A blitter is a dedicated chip that can move large chunks of graphics around on the screen around very quickly without having to involve the CPU. All modern video cards have what is essentially an advanced descendant of a blitter inside them. Again, Jay was ahead of his time.

HAM mode, which stood for Hold And Modify, was a way of getting more colors to display on the screen than could normally fit into the display memory. At the time, memory chips were very expensive, and the cost for displaying millions of colors at once was too high even for military applications like the Link simulator. So instead of storing all the color information for each dot (or pixel) on the display, the hardware could be programmed to start with one color and then change only one component of it (Hue, Saturation or Luminosity) for each subsequent pixel along each line. Jay decided to put this into the Amiga.

Later on in the design process, Jay would become concerned that HAM mode was too slow and even asked his chip layout artist if he could take it out. The chip designer replied that it would take many months and leave an aesthetically unappealing "hole" in the middle of the chip. Jay decided to keep the feature in, and later admitted that this was a good decision. The Amiga shipped with the ability to display 4096 colors in this mode, far more than any of its competitors, with clever programmers squeezing even more colors out of future Amiga chipset revisions. Despite HAM being suitable only for displaying pre-calculated images, a software company would even develop a graphics editor that operated in HAM mode. Like the chess game on the Atari 2600 before it, programmers would make the impossible possible on the Amiga.

Screens like no other

Another new invention for the Amiga computer was the "copper" chip. This was essentially a special-purpose CPU designed specifically for direct manipulation of the display. It had only three instructions, but it could directly access any part of the other display chips at any time. What's more, it could turn amazing tricks in the fraction of a second that it took for the monitor to refresh the display. This allowed a trick that no other computer has ever reproduced: the ability to view multiple different screens, opened at different resolutions, at the same time. These "pull-down" screens would amaze anyone who saw them. Modern computers can open different screens at different resolutions (say, for example, to open a full-screen game at a lower resolution than the desktop is displaying, in order to play the game faster or at a higher frame rate) but they can only switch between these modes, not display multiple modes at once.

The design eventually coalesced down to three chips named Agnes, Denise, and Paula. Agnes handled direct access to memory and contained both the blitter and copper chips. Denise ran the display and supported "sprites," or graphical objects that could be displayed and moved over a complex background without having to redraw it. Finally, Paula handled sound generation using digitally-sampled waveforms and was capable of playing back four channels at once: two on the left stereo channel and two on the right. It would be years before competing computer sound capabilities came anywhere close to this ability. Paula also controlled the Amiga's floppy disk drive.

These chips formed the core of what would be referred to as the Amiga's "custom chipset." However, they did not yet exist, except on paper. While the software development team was able to get started planning and writing programs that would support the chipset's features, the hardware team needed some way to test that their chips would actually work before committing to the expense of manufacturing them. In addition, the operating software could not be fully tested without having real Amiga hardware to run it on.

Come back next week for Part 3: The first prototype.

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