Games - Bally Astrocade

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	Games - Bally Astrocade

The Astrocade is an early video game console and simple computer system designed by a team at Midway, the videogame division of Bally. Originally referred to as the Bally Home Library Computer, it was released in 1977 but available only through mail order. Delays in the production meant none of the units actually shipped until 1978, and by this time the machine had been renamed the Bally Professional Arcade. In this form it sold mostly at computer stores and had little retail exposure (unlike the Atari VCS).

In 1979 Bally grew less interested in the arcade market and decided to sell off their Consumer Products Division, including development and production of the game console.

By this point the unit had gathered a following of dedicated users who had learned of the power of the machine through its BASIC cartridge. A group of them arranged to buy the rights to the system from Bally, and set up in business as Astrovision. In 1981 they re-released the unit with the BASIC cartridge included for free, this time known as the Bally Computer System, and then changed the name again in 1982 to Astrocade. It sold under this name until the video game crash of 1983, and then disappeared around 1985.

Midway had long been planned to release an expansion system for the unit, known as the ZGRASS-100. The system was being developed by a group of computer artists at the University of Illinois known as the Circle Graphics Habitat, along with programmers at Nutting. Midway felt that such a system, in an external box, would make the Astrocade more interesting to the market. However it was still not ready for release when Bally sold off the division in 1980. A small handful may have been produced as the ZGRASS-32 after the machine was re-released by Astrovision.

The system, combined into a single box, would eventually be released as the Datamax UV-1. Aimed at the home computer market while being designed, the machine was now re-targeted as a system for outputting high-quality graphics to video tape. These were offered for sale some time between 1980 and 1982, but it is unknown how many were built.

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In the late 1970s Midway contracted Dave Nutting Associates to design a video display chip that could be used in all of their videogame systems, from standup arcade games, to a home computer system. The system Nutting delivered remains perhaps the most powerful graphics system of the 8-bit generation, and was used in most of Midway's classic arcade games of the era, including Gorf and Wizard of Wor.

The basic systems were powered by a Zilog Z80 driving the display chip with a RAM buffer in between the two. The display chip had two modes, a low-resolution mode at 160x102, and a high-resolution mode at 320x204, both with 2-bits per pixel for four colors. This sort of color/resolution was normally beyond the capabilities of RAM of the era, but a clever trick, technically "holding the RAS high", allowed them to read one "line" at a time at very high speed into a buffer inside the display chip. The line could then be read out to the screen at a more leisurely rate, while also interfering less with the CPU, which was also trying to use the same memory.

Sadly, on the Astrocade the pins needed to use this "trick" were not connected. Thus the Astrocade system was left with just the lower resolution mode that the hardware supported. In this mode the system used up 160x102 x 2bits = 4080 bytes of memory to hold the screen. Since the machine had only 4k of RAM, this left very little room left over for the program's use, which was used for things like holding the score, or game options.

The Astrocade used color registers, or color indirection as it was often referred to then, so the four colors could be picked from a palette of 256 colors. Color animation was possible by changing the values of the registers, and using a horizontal blank interrupt you could change them from line to line. An additional set of four color registers could be "swapped in" at any point along the line, allowing you to easily create two sections of the screen, split vertically. Clever programmers used this feature to emulate 8 color modes.

Unlike the VCS, the Astrocade did not include hardware sprite support. It did, however, include a blitter-like system and software to drive it. Memory above 0x4000 was dedicated to the display, and memory below that to the ROM. If a program wrote to the ROM space (normally impossible, it's "read only" after all) the video chip would take the data, apply a function to it, and then copy the result into the corresponding location in the RAM. Which function to use was stored in a register in the display chip, and included common instructions like XOR and bit-shift. This allowed the Astrocade to support any number of "sprites" independent of hardware, with the downside that it was up to the software to re-draw them when they moved.

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Games - Bally Astrocade

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