Introduction

I’ve always regretted losing the original discs (and CPC computer itself) that came with my original CPC 6128 as those discs contained every program I wrote using Z80 machine code. Losing that source code and all the comments I had inserted at the time is a major bummer, I’d love to look through it today.

Other parts of this series, including this one are listed below.

• Disassembling my old code part 1 – Getting started with WinAPE <—- you are here
• Disassembling my old code part 2 – Setting screen mode & Soft 968
• Disassembling my old code part 3 – Text operations
• Disassembling my old code part 4 – Set cursor position & disc cache
• Disassembling my old code part 5 – Get Shift key and set translate
• Disassembling my old code part 6 – Processing disc activities
• Disassembling my old code part 7 – Drawing rectangles
• Disassembling my old code part 8 – Processing HEX in the left pane

I wrote the code using a ROM in a Rombo Rombox called MAXAM, it was an assembler/disassembler/text editor for Amstrad CPC and it was using that hardware that I learned how to program in Z80 machine code.

This blog post is going to explain how I’m trying to get back into my old code and to see why I coded it the way I did, I will slowly work my way through the code and document it here.

This is the first part, how I actually disassembled my old Z80 machine code on a Windows 10 computer using the WINAPE Amstrad CPC Emulator. I wrote this code back in 1988 and I was 21 years old at the time, a long time ago indeed, I’m 53 now at the time of writing this blog post.

I didn’t instantly understand how to program in Z80 machine code, I had to teach myself by trial and error over a period of years, and that involved reading books and acquiring hardware to assist me to program (such as Maxam). Back then the internet was not really a thing so anything you wanted to learn was done via college courses, books, magazines or strangely enough Encyclopedias.

So now you have an idea of what I’m trying to do, let’s get on with it.

Download Winape

Head over to http://www.winape.net/ and download the latest version of the software. Extract the zip file somewhere useful.

Download Disced

Download Disced (which is what I will be using to disassemble) from here.

Now that you’ve got what you need, start Winape by clicking on the winape.exe file.

When the program loads, click on File, select Drive A: then selct Insert Disc Image…

Point it to a disk image file (file.dsk) such as my disc called niall.dsk

Click Open. In Winape type the following

|a

and then press

Enter

Next, type

cat

and then press

Enter

Here’s the output.

So those are 3 programs I wrote back in 1988 and my intention is to disassemble all 3 and re-write (with comments) the source code. But how do we do that ? First of all, you could load one of the applications and then run it, they are three basic listings which poke in the machine code into RAM.

To see what those basic programs look like, let’s load disced.bas and then change the mode of the screen to 2 and then type list.

Type

load "disced.bas

then

mode 2

then

list

This will show all of the text in the basic listing.

If you look at the listing in detail you’ll see it’s pokes the machine code instructions into address memory &9000.

Ok, now that we have the data loaded, let’s run the program.

Type

run

then press

Enter

You should see Disced load.

Disassembling the code

Ok, now that we’ve got a program up on screen, how do we disassemble it. Well I know that my code is loaded into the CPC ram at the following address &9000, so we need to disassemble from that address onwards to see the actual Z80 machine code. To bring up the disassembler built into Winape, press F7.

A little disassembler window should appear like below.

Maximize the window and click on the hand icon to enter an address, enter 9000 and press enter.

The code will be revealed

Select the first line (line &9000) and scroll down to where the code ends (at &962e)

Exporting Z80 code to file

Now that you’ve marked the code, let’s export it to a file. Right click on the marked code and choose Disassemble from the options.

And next select the output location to File, and change the Number format as well if you wish

Once done, open the exported .asm file with Notepad++

Here’s the resulting output and I’ve started adding comments to my code. The first ORG &9000 tells the assembler to load the code at that address

 

and the first call is to a block of code which loads the A register with a value of &0002 and then calls an Amstrad Firmware Jumpblock which does a mode change, let’s look at that little block of code at L933D

how do I know, because I looked up the codes here.

90 #BC0E    SCR SET MODE    Set screen into new mode.

So that’s it ! this is the end of Part 1 of this new mini-series, please continue on to Part 2.

Resources

• Learn Z80 programming – https://www.chibiakumas.com/z80/
• Maxam ROM instructions – http://www.cpcwiki.eu/imgs/e/e2/Maxam_Manual.pdf
• Amstrad Firmware Jumpblocks – http://www.cpcwiki.eu/imgs/7/73/S968se14.pdf
• Notepad ++ – https://notepad-plus-plus.org/downloads/

Cheers

Niall