My previous post on the subject of building a Z80-based computer briefly explained my motivation, and the approach I was going to take.

This post describes my progress so far:

• On the hardware side, zero progress.
• On the software-side, lots of fun.

To recap I expect to wire a Z80 microprocessor to an Arduino (mega). The arduino will generate a clock-signal which will make the processor "tick". It will also react to read/write attempts that the processor makes to access RAM, and I/O devices.

The Z80 has a neat system for requesting I/O, via the use of the IN and OUT instructions which allow the processor to read/write a single byte to one of 255 connected devices.

To experiment, and for a memory recap I found a Z80 assembler, and a Z80 disassembler, both packaged for Debian. I also found a Z80 emulator, which I forked and lightly-modified.

With the appropriate tools available I could write some simple code. I implemented two I/O routines in the emulator, one to read a character from STDIN, and one to write to STDOUT:

IN A, (1)   ; Read a character from STDIN, store in A-register.
OUT (1), A  ; Write the character in A-register to STDOUT

With those primitives implemented I wrote a simple script:

;
;  Simple program to upper-case a string
;
org 0
   ; show a prompt.
   ld a, '>'
   out (1), a
start:
   ; read a character
   in a,(1)
   ; eof?
   cp -1
   jp z, quit
   ; is it lower-case?  If not just output it
   cp 'a'
   jp c,output
   cp 'z'
   jp nc, output
   ; convert from lower-case to upper-case.  yeah.  math.
   sub a, 32
output:
   ; output the character
   out (1), a
   ; repeat forever.
   jr start
quit:
   ; terminate
   halt

With that written it could be compiled:

 $ z80asm ./sample.z80 -o ./sample.bin

Then I could execute it:

 $ echo "Hello, world" | ./z80emulator ./sample.bin
 Testing "./sample.bin"...
 >HELLO, WORLD

 1150 cycle(s) emulated.

And that's where I'll leave it for now. When I have the real hardware I'll hookup some fake-RAM containing this program, and code a similar I/O handler to allow reading/writing to the arduino's serial-console. That will allow the same code to run, unchanged. That'd be nice.

I've got a simple Z80-manager written, but since I don't have the chips yet I can only compile-test it. We'll see how well I did soon enough.

Tags: computer-building, z80 | 3 comments