WOZMON

 Apple 1

Steve Wozniak was the brains behind the first Apple computers.  In conjunction with Steve Jobs (the Salesman) they formed Apple computers and in 1976 released the Apple 1.  As you can see from the advertisement below it came with a TV and keyboard interface so that you could communicate with it.

The computer is based on a 6502 microprocessor which was first sold in 1975 and was significantly cheaper than any similar product.  It opened the gate to hobbyist computing.  The 6502 was also the heart of the Apple ][ computer which became both a hobbyist and a business machine the following year.

My BEN2 computer also contains a 6502 processor which means that it runs the same machine code instructions as Apple1.  

WOZMON

A small program, called the monitor, was provided in Apple1 ROM.  It allowed the user to look at memory, type programs into memory and run programs.  It is famous because it was written by Steve Wozniak and packs lots of functionality into only 248 bytes of ROM.  An assembly listing of the monitor was provided in the Apple1 user manual.

For some time I have been thinking about trying to make wozmon work on BEN2. Ben Eater had  the same thoughts and has kindly created a couple of brilliant tutorials on using wozmon and amending it to run on his 6502 system.  

Conversion

The Ben Eater 6502 (and mine) use an ACIA serial interface chip to communicate with a PC so that it can use the PC screen and keyboard and doesn't require us to manually attach them.   In contrast Apple1 uses a physical screen and keyboard.  In order to use wozmon we need to change the keyboard character input and screen character output subroutines.

Using Bens amended wozmon version I was easily able to build and run wozmon on BEN2.
Here is a sample session showing the first few bytes of wozmon code at FF00-FF1F.
I then change the contents of address 1000 from CC to AA and check the results.
Finally I return to run my original monitor.

The Apple1 manual even gives you a sample program you can type in and run.  The results are shown below.

 

Conclusion

This is an amazing connection back over 50 years to the genius who was the brains behind Apple.
As always Kudos is due to Ben Eater who makes it easy for us to follow these steps from the past.