Retrocomputing: hardware, software, games, and stories

I was always interested in the Amiga line, they were very capable computers compared to the competition of the time.

 

Indeed they were. I'd say PCs were not comparable software-wise until Windows 95, although 3.1 was "good enough" for many.

Computers have gone through many cycles of being complicated by additional, custom hardware to do specific tasks, and then the competing impetus of "simple and fast" wins out for a while, and so forth. The Amiga was at the far end of "additional, custom hardware" while the Macintosh and Atari ST were much simpler hardware-wise, to compare the other two popular desktop computers with Motorola 68000 processors and graphical UIs. Later, the Amiga became an object lesson in why documenting your hardware platform in such intimate detail can hurt a company; lots of software was written to take advantage of the custom hardware directly, rather than through OS services, and this meant that upgrading the design and maintaining compatibility was hard. Modern computers have a great deal of specialized hardware, especially graphics and sound processors and disk IO controllers, but they tend to be limited to a very specific supported API and programmers are discouraged from depending too much on specific hardware implementations through there being so many.

 

So, the Amiga's custom hardware. All of it could only operate on memory that was part of "Chip RAM" which was 256k on the early Amiga 1000, 512k through the Amiga's most popular years, then 1M, then 2 or 3M on the last models. This was memory controlled by the chipset's DMA controller, and the custom chipset had priority over the CPU when it came to accessing it. The rest of the memory, if there was any, was designated "Fast RAM" because the CPU had exclusive access to it.

The display frame was made up of up to five bitplanes (and thus up to 32 colors on screen without tricks). What's a bitplane, you ask? Each bitplane was an area of memory describing just one binary digit for each pixel. Color index 0 is 00000 in a 5-bitplane configuration, while color index 32 is 11111. Each bit of that 5 is stored in a different place in memory. This was a common way to do things back then, because you could save memory by cutting down your color palette. Once systems went to 256-color, things changed and a byte array became the preference. Standard resolution was 320x200 in the US, Or 320x256 in Europe. The horizontal resolution could be increased to 640, and the vertical resolution doubled at the cost of interlacing flicker. That wasn't very popular unless working with TV video.

 

gasp more yes

 

The amusing thing is that I'm doing this almost entirely from memory of a system I haven't touched since 1992

 

please morven-sempai more

 

Speaking of TV, the Amiga was designed with a a system clock running at the same speed as the color burst frequency of standard definition television, and the Amiga video connector allowed for it to be synchronized to an external source. This made the hardware to sync video and computer, known as a genlock, really easy and cheap to make for the system. This made the Amiga a natural fit for overlay graphics, captions and titling in the video industry, and this was the market for the high-end models. Many are still in use, 20 years after manufacture ceased, and it's only the switch to HD that's really retiring them. The professional models still sell for well over a thousand dollars on eBay for that reason.

 

All the Amiga custom chips were given girls' names, perhaps because of the computer's name, perhaps because dudes. In the original chipset (OCS), their names were Agnus, Paula and Denise.

Agnus was the core of the system, containing the memory controller for the chip RAM, as well as the blitter, which is a fast memory copy, translation and fill processor that takes a huge amount of graphics load off the CPU. The third component of Agnus was Copper, a video-synchronized coprocessor that could do things based on beam position and trigger functions of all the other chips. It could do things like change the color palette on each scan line, or trigger the moving of graphic elements after they have been shown. Intelligent programming of Copper could produce wonderful results without the CPU having to do anything.

Denise handled the video display, including eight hardware sprites, pixel level scrolling, it also handled mouse and joystick operations.

Paula, meanwhile, handled the Amiga's 4-channel 8-bit PCM sound, revolutionary in the computer market, as well as the floppy disk, serial IO and analog joysticks, as well as being the system interrupt controller.

This left the CPU almost completely free to work on other things. Used right, this could give amazing results; unfortunately it also required software specially written for the Amiga. Many software houses treated the 68k market as one and coded for only the features in both the Atari ST and Amiga, not using most of the Amiga's power.

 

So the Amiga, hardware wise, was pretty similar to the arcade system boards of the time, and for the first time parity between custom arcade hardware and a home computer was achieved. Scrolling, sprite-based games were a perfect match to the hardware and still hold up very well.

Most games took over the hardware, bypassing the OS and sometimes even getting rid of it in memory to free up space. The OS being so unimportant for games, they could have given it a super-simple system and still sold well.

Fortunately, they didn't. They gave the Amiga a powerful modern OS, the most sophisticated ever seen on a personal computer. AmigaOS was built around a pre-emptive multitasking microkernel, Exec, which used message passing for fast inter-process communication and provided essential primitives for maintaining linked lists safely, with which the messages were implemented. It gave a standard structure and calling convention for shared libraries and device drivers, with which the rest of the system was implemented.

This was all utterly fascinating to me. Most of my Amiga programming was to delve deeper into this treasure house of wonders, and I think it was the best introduction to complex systems possible.