The WozMon

In 1976, Steve Wozniac wrote what’s commonly known simply as Wozmon.

Wozmon is a machine-code monitor program and for those who may not be aware, this is essentially code that starts to run in a microprocessor at initial power on. Lets be more specific, an older microprocessor system, not usually your modern Intel, AMD or ARM based system. We’re taking the old 8-bit micros here, and I’m specifically referring to the original 6502.

The monitor would let you examine and change memory. Some even had built-in disassemblers, debuggers and a whole host of tools to help you get going in a new machine. They would include subroutines that could be used by other programs, these basic input output subroutines, or BIOS as it came to be known were essential for bigger programs as it took that burden away from the programmer and helped to make programs run on different systems, or at least easier to port from one system to another.

Before the monitor there were LEDs (or even older; filament bulbs) and switches. Before that? you might have ended up soldering diodes to circuit boards. I kid you not.

Wozmon may not have been the first, but it was compact. Wozniac had 250 bytes to squeeze it all into. (6 bytes were reserved for the reset and interrupt vectors) ROMs and the means to program them were expensive in those days. He ended up with 2 bytes spare.

By todays standards Wozmon was crude. But it was also compact – if it didn’t understand what you typed then it would just print a backslash and let you start again, but however crude it may have been, it worked and let many early programmers enter code and use the computer that Wozmon ran on – the Apple 1.

After the Apple 1, there as the Apple][ (sometimes written as II, latterly as //, but rarely 2). That had more ROM space, so Wozmon was expanded and as computer got bigger and more complex eventually (we’re into the mid 1980’s now), the monitor for the various microprocessors was replaced by nothing more than the basic input/output subroutines and the BIOS as we know it today was born, although nowadays (c2018), the BIOS is bigger and more complex than an entire Apple ][ computer.

You may be wondering why I’m writing this – well, for no reason other than this year marks my 40th year of programming the 6502, I’ve built a little 6502 computer and written a monitor for it. The monitor resembles Wozmon – mostly because I used the Apple ][ variant of it back in 1978.

More to come – details of Ruby – my 40th anniversary 6502 computer.

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