brainwagon

A couple of days ago I got intrigued by this cool project which ran CP/M on an AVR. The basic idea was to equip an AVR with an external RAM, and then write a compact 8080 emulator to run on the AVR. Instead of floppy drives, the AVR accesses data stored on a small SD card. I thought it was really cool.

So I decided to see how far I could get.

A couple of hours work spent watching TV had the basic framework in place, and it’s beginning to execute code. For instance, here is the simple “Hello World” program, written in 8080 assembler (it presumes that you are running on a CP/M system to supply a printing system call):

bdos    equ     0005H
        org 0x100
start:  mvi c,9
        lxi d,msg
        call bdos
        hlt
msg:    db 'Hello World!', 0xA, '$'
end     start

Unless you like assembling code by hand, you need an assembler. I thought about using a simulated CP/M system running on simh, but that quickly got annoying, so instead I searched around and ultimately stumbled on asmx, a multi-CPU assembler which seemed to work just fine. It produced a nice little Intel hex format file, which my emulator can then load. I trap calls to the BDOS entry vector, and implement them with standard C calls (at the moment), and the net result is the rather unimpressive:

::: brainwagon 8080 emulator version 0.0
::: 23 bytes of code loaded.
Hello World!

Most of the control functions work, I need to get the conditional returns to work properly. I don’t have interrupt support yet, nor does it compute cycle counts, but the overall framework is quite good. I suspect that another couple of hours of work will get 95% of the instructions working, and then I could consider doing something like implementing Space Invaders. Ultimately, I kind of want to run a simple simulated CP/M system. We’ll see how it goes: stay tuned.