Back in 1978 I built my first computer from a magazine article in Popular
Electronics.  The computer was called the Cosmac Elf and was based on RCA's
CDP1802 microprocessor.  There is a users group where the old timers and
some new kids are still active.

        http://groups.yahoo.com/group/cosmacelf

Recently, I wanted to build another Elf so that I could keep my existing
machine in it's original shape but thought about using a PIC instead of the
antique processor.  The goal was to create a bus accurate full speed version
of the 1802.  Also, RCA created the CDP1861 video display processor that
teamed up with the DMA features of the 1802 to produce low resolution video. 
I wanted to emulate this chip as well with another PIC.  

The results can be found here:

    http://www.tedrossin.atbhost.net/Electronics/RCA/RCA.html#ElfClone

It seems that other antique processors could be emulated (6502, 6800, Z80
...)  as well as creating new "processors" using the same techique. 
Emulating the processors allows new instructions to be added as well as
adding debug features that would not be possible with the original
processors.





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