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An early microcomputer

Bill Marshall

In 1975 while working as a student engineer I built my first microcomputer based on the National Semiconductor 'Simple/Cheap Microprocessor' (SC/MP) chip. This was my first real design effort as distinct from building projects from the pages of Practical Electronics! Its 8Kbyte memory consisted of four 2K boards each containing sixteen 1K x 1bit 2102 static RAM chips. These were practically state-of-the-art but you could buy a bag-full of untested parts from a US dealer for a few pounds. Those were the days. You could also pick up all sorts of scrap computer gear cheaply back then so I soon had a paper tape reader interfaced and my own design of VDU electronics driving a second-hand computer monitor. Initially, it was programmed by entering machine code via switches and buttons on the front panel. By machine code I mean the hexadecimal object code nowadays produced by an Assembler program from a text source code file. In other words, you wrote code in the micro's assembler language (MOV, ADD, JUMP, etc) and 'hand-assembled' it into hex machine code. I couldn't afford manufacturers' software and in any case, you needed another computer to run it on. But then NS released the code for a 4KByte BASIC language interpreter called NIBL - and it was free! Just then, the source listing for a 4KB version of the computer game 'Star Trek' appeared in an electronics magazine. It was in a version of BASIC easily modified to run under NIBL. I used a computer at work to transfer the code to paper-tape. At that point I had gone from tedious hand-assembly of small machine code programs to loading and running BASIC programs from paper tape. Incidentally, Star Trek was one of the earliest computer games - a version of the classic 'Battleships' - designed for playing on a teleprinter or Teletype....

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