Friday, September 23, 2016

Bally Astrocade

The Astrocade is a second generation home video game console and simple computer system designed by a team at Midway, at that time the videogame division of Bally. It was marketed only for a limited time before Bally decided to exit the market. The rights were later picked up by a third-party company, who re-released it and sold it until around 1984. The Astrocade is particularly notable for its very powerful graphics capabilities for the time of release, and for the difficulty in accessing those capabilities. There are 28 officially released video games for the system.

Nutting and Midway

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 was used in most of Midway's classic arcade games of the era, including Gorf and Wizard of Wor.

Console use

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.

At about the same time a 3rd party group had been unsuccessfully attempting to bring their own console design to market as the Astrovision. A corporate buyer from Montgomery Ward who was in charge of the Bally system put the two groups in contact, and a deal was eventually arranged. 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 planning 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 at Chicago 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. 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.



The basic system was 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 160 × 102, and a high-resolution mode at 320 × 204, both with 2-bits per pixel for four colors. This sort of color/resolution was normally beyond the capabilities of RAM of the era, which could not read out the data fast enough to keep up with the TV display. The chip used a common solution to work around this problem, technically known "holding the RAS high", allowing 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.

On the Astrocade the pins needed to use this "trick" were not connected. Thus the Astrocade system was left with just the lower resolution 160 × 102 mode. In this mode the system used up 160 × 102 × 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 rest of the program would have to be placed in ROM.

The Astrocade used color registers, or color indirection, 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 create two "halves" of the screen, split vertically. Originally intended to allow you to easily create a score area on the side of the screen, programmers also 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 is "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 sprite-like objects independent of hardware, with the downside that it was up to the software to re-draw them when they moved.

The Astrocade was one of the early cartridge-based systems, using cartridges known as Videocades that were designed to be as close in size and shape as possible to a cassette tape. The unit also included two games built into the ROM, Gunfight and Checkmate, along with the simple but useful Calculator and a "doodle" program called Scribbling.

The Astrocade featured a relatively complex input device incorporating several types of control mechanisms: the controller was shaped as a pistol-style grip with trigger switch on the front; a small 4-switch/8-way joystick was placed on top of the grip, and the shaft of the joystick connected to a potentiometer, meaning that the stick could be rotated to double as a paddle controller. By most reports the controllers were excellent[citation needed], but had the downside of breaking frequently.

On the front of the unit was a 24-key "hex-pad" keyboard used for selecting games and options. Most cartridges included two games, and when they were inserted the machine would reset and display a menu starting with the programs on the cartridge and then listing the four built-in programs. On the back were a number of ports, including connectors for power, the controllers, and an expansion port. One oddity was that the top rear of the unit was empty, and could be opened to store up to 15 cartridges. The system's ability to be upgraded from a Videogame console to Personal computer along with its library of nearly 30 games in 1982 are some reasons that made it more versatile than its main competitors, and was listed by Jeff Rovin as one of the seven major video game suppliers.


The Astrocade also included a BASIC programming language cartridge which was based on Li-Chen Wang's Tiny BASIC. Supporting BASIC on the system was difficult, because the display alone used up almost all the available RAM. The solution to this problem was to store the BASIC program code in the video RAM.

This was accomplished by interleaving every bit of the program along with the display itself; BASIC used all the even-numbered bits, and the display got the odd-numbered bits. The interpreter would read out two bytes, drop all the odd-numbered bits, and assemble the results into a single byte of code. This was rendered invisible by setting two of the colors to be the same as the other two, such that colors 01 and 11 would be the same (white), so the presence, or lack, of a bit for BASIC had no effect on the screen. Additional memory was scavenged by using fewer lines vertically, only 88 instead of the full 102. The end result of all this was to manage to squeeze out 1760 bytes of RAM for BASIC programs. The downside was that most of the graphics system's power was unavailable.

Programs were entered via the keyboard, with each of the keys assigned to a single command, number, and several alpha characters. These were selected through a set of 4 colored shift keys. This way you simply typed "WORD"(gold) shift then the "+" key and got.


The ZGRASS unit sat under the Astrocade and turned it into a "real" computer, including a full keyboard, a math co-processor (FPU), 32k of RAM, and a new 32k ROM containing the GRASS programming language (sometimes referred to as GRAFIX on this machine). The unit also added I/O ports for a cassette and floppy disk, allowing it to be used with CP/M.

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