Steve Wozniak
Article Index
Steve Wozniak
Apple I and II
Disk Drive designer

Banner

Then came the personal computer. The Altair made the cover of Popular Electronics and Woz was one of the first 32 members of the Homebrew computer club. Components were now almost cheap enough for him to realise his dream of designing, building and owning a computer. He had already built a TV terminal based on a design by Don Lancaster, a similar spirit to Woz who never had the good fortune to bump into a Steve Jobs! From a TV terminal it would be a shortish step to a complete home computer - if only he could find a processor that was cheap enough. The 6800 he preferred was too expensive so a $25 6502 was the only possibility. Woz bought a 6502 and started his design.


Rather than follow the low cost per chip/high chip count approach, Woz decided that the fewer chips the better but the main problem was memory. At the time video displays used shift registers to hold the video data which was shifted out at video speeds but each screen location could only be updated once per frame, i.e. one character every 1/50th of a second or so. Woz wanted something faster because he had in mind writing programs to implement the sort of video games he had designed for Atari. A faster video design implied using RAM rather than shift registers but at the time the standard RAM chip only provided 1024 Bytes of storage. Steve Jobs suggested to Woz that he looked at the new 16-pin dynamic RAMs that were just beginning to become available.

Dynamic RAM is more difficult to design with and at the time making it work at all was considered to be the height of sophisticated digital design. Woz liked the idea because, although it was more complicated, it reduced the chip count to only 8 chips per 16KBytes. Other people were designing with dynamic RAMs but they were concentrating on providing memory boards for modular S100 type machines. Woz was designing a complete computer on a single circuit board and this was a very different way of thinking.
In the end he managed to get the whole lot onto a single board, six inches by eight using only 30 or 40 chips. The Apple I, as it came to be known was poor by today's standards but it was a complete computer and for the time it was a breakthrough. It used a 6502 processor, had 8KBytes of RAM and could run a simple BASIC in 4KBytes. It needed a keyboard, a monitor and two transformers for 5V and 12V. It had no speaker, no graphics and no colour. Its one expansion connector was used for a cassette interface. BASIC had to be loaded from tape and before that by hand because only a 256 Byte PROM was included with only memory load and examine functions. Steve Jobs and Woz sold around 200 Apple Is and out of this grew the entire Apple empire - but the next step was the Apple II.

Woz realised that the compact single board design had a lot of advantages but now he needed to add colour. The S100 modular machines of the time were just beginning to use colour displays but they were expensive. Woz had designed a seven chip colour circuit that would work with a colour TV. He also knew that domestic TVs were very sloppy in terms of their timing so he managed to produce a very low chip count 40 character display that didn't quite meet the TV standards specification but would work in most cases. He also cheated on the addressing for the memory mapped graphics to save two chips at the cost of a strangely mangled memory layout. For years to come users of the Apple II would wonder why memory adjacent memory locations didn't always control adjacent screen locations. A similar trick but this time adding two chips allowed him to shift out the memory locations directly into the video generator and so produce high resolution colour graphics. If you're impressed by the attention to detail the amazing fact is that Woz also wrote the system software including Apple Integer Basic - yes that's right they didn't need Bill Gates and Microsoft.


Banner


Even the prototype Apple II was impressive but Woz still wasn't happy. He wanted to play breakout, the game he had implemented for Atari - so he added a pair of game paddles. No he didn't bother with an AtoD converter, that would have been expensive. Instead he used a chip containing four (555) timers and used the paddles to control the width of the pulse that they produced - a habit that the IBM PC was to catch some years later. With this addition breakout was not only possible but relatively easy using BASIC. When enthusiasts were shown the machine they were more than impressed - they wanted one.

An odd feature of the design was the provision of expansion slots. These looked superficially like a bus but instead each slot had a pre-decoded memory location and the ability to include a 256Byte PROM and a 2KByte memory space. This allowed Apple II expansion cards to be simple because they didn't need to include their own memory decoding. This wasn't a completely new feature, for example the South West Technical Products (SWTP)  6800 machine used the same idea, but it helped make the Apple II great.

apple1

Apple I

<ASIN:0596007191>

<ASIN:0137059671>



Last Updated ( Wednesday, 11 August 2010 )
 
 
   
RSS feed of all content
I Programmer - full contents
Copyright © 2014 i-programmer.info. All Rights Reserved.
Joomla! is Free Software released under the GNU/GPL License.