Entries tagged retro

In the past, I've talked about building a Z80-based computer. I made some progress towards that goal, in the sense that I took the initial (trivial steps) towards making something:

• I built a clock-circuit.
• I wired up a Z80 processor to the clock.
• I got the thing running an endless stream of NOP instructions.
  • No RAM/ROM connected, tying all the bus-lines low, meaning every attempted memory-read returned 0x00 which is the Z80 NOP instruction.

But then I stalled, repeatedly, at designing an interface to RAM and ROM, so that it could actually do something useful. Over the lockdown I've been in two minds about getting sucked back down the rabbit-hole, so I compromised. I did a bit of searching on tindie, and similar places, and figured I'd buy a Z80-based single board computer. My requirements were minimal:

• It must run CP/M.
• The source-code to "everything" must be available.
• I want it to run standalone, and connect to a host via a serial-port.

With those goals there were a bunch of boards to choose from, rc2014 is the standard choice - a well engineered system which uses a common backplane and lets you build mini-boards to add functionality. So first you build the CPU-card, then the RAM card, then the flash-disk card, etc. Over-engineered in one sense, extensible in another. (There are some single-board variants to cut down on soldering overhead, at a cost of less flexibility.)

After a while I came across https://8bitstack.co.uk/, which describes a simple board called the the Z80 playground.

The advantage of this design is that it loads code from a USB stick, making it easy to transfer files to/from it, without the need for a compact flash card, or similar. The downside is that the system has only 64K RAM, meaning it cannot run CP/M 3, only 2.2. (CP/M 3.x requires more RAM, and a banking/paging system setup to swap between pages.)

When the system boots it loads code from an EEPROM, which then fetches the CP/M files from the USB-stick, copies them into RAM and executes them. The memory map can be split so you either have ROM & RAM, or you have just RAM (after the boot the ROM will be switched off). To change the initial stuff you need to reprogram the EEPROM, after that it's just a matter of adding binaries to the stick or transferring them over the serial port.

In only a couple of hours I got the basic stuff working as well as I needed:

• A z80-assembler on my Linux desktop to build simple binaries.
• An installation of Turbo Pascal 3.00A on the system itself.
  • I used this to build some simple utilities.
  • There's a good overview of using turbopascal here which covers how to use/quit the editor, compile binaries, etc.
• An installation of FORTH on the system itself.
  • Which is nice.
• A couple of simple games compiled from Pascal
  • Snake, Tetris, etc.
• The Zork trilogy installed, along with Hitchhikers guide.

I had some fun with a CP/M emulator to get my hand back in things before the board arrived, and using that I tested my first "real" assembly language program (cls to clear the screen), as well as got the hang of using the wordstar keyboard shortcuts as used within the turbo pascal environment.

I have some plans for development:

• Add command-line history (page-up/page-down) for the CP/M command-processor.
• Add paging to TYPE, and allow terminating with Q.

Nothing major, but fun changes that won't be too difficult to implement.

Since CP/M 2.x has no concept of sub-directories you end up using drives for everything, I implemented a "search-path" so that when you type "FOO" it will attempt to run "A:FOO.COM" if there is no file matching on the current-drive. That's a nicer user-experience at all.

I also wrote some Z80-assembly code to search all drives for an executable, if not found in current drive and not already qualified. Remember CP/M doesn't have a concept of sub-directories) that's actually pretty useful:

  B>LOCATE H*.COM
  P:HELLO   COM
  P:HELLO2  COM
  G:HITCH   COM
  E:HYPHEN  COM

I've also written some other trivial assembly language tools, which was surprisingly relaxing. Especially once I got back into the zen mode of optimizing for size.

I forked the upstream repository, mostly to tidy up the contents, rather than because I want to go into my own direction. I'll keep the contents in sync, because there's no point splitting a community even further - I guess there are fewer than 100 of these boards in the wild, probably far far fewer!

• My fork
• The original

Tags: cpm, retro, single-board computer, z80 | 6 comments

In my previous post I wrote about how I'd been running CP/M on a Z80-based single-board computer.

I've been slowly working my way through a bunch of text-based adventure games:

• The Hitchhiker's Guide To The Galaxy
• Zork 1
• Zork 2
• Zork 3

Along the way I remembered how much fun I used to have doing this in my early teens, and decided to write my own text-based adventure.

Since I'm not a masochist I figured I'd write something with only three or four locations, and solicited facebook for ideas. Shortly afterwards a "plot" was created and I started work.

I figured that the very last thing I wanted to be doing was to be parsing text-input with Z80 assembly language, so I hacked up a simple adventure game in C. I figured if I could get the design right that would ease the eventual port to assembly.

I had the realization pretty early that using a table-driven approach would be the best way - using structures to contain the name, description, and function-pointers appropriate to each object for example. In my C implementation I have things that look like this:

{name: "generator",
 desc: "A small generator.",
 use: use_generator,
 use_carried: use_generator_carried,
 get_fn: get_generator,
 drop_fn: drop_generator},

A bit noisy, but simple enough. If an object cannot be picked up, or dropped, the corresponding entries are blank:

{name: "desk",
 desc: "",
 edesc: "The desk looks solid, but old."},

Here we see something that is special, there's no description so the item isn't displayed when you enter a room, or LOOK. Instead the edesc (extended description) is available when you type EXAMINE DESK.

Anyway over a couple of days I hacked up the C-game, then I started work porting it to Z80 assembly. The implementation changed, the easter-eggs were different, but on the whole the two things are the same.

Certainly 99% of the text was recycled across the two implementations.

Anyway in the unlikely event you've got a craving for a text-based adventure game I present to you:

• The Lighthouse Of Doom
  • https://github.com/skx/lighthouse-of-doom/

Tags: cpm, retro, single-board computer, z80 | 6 comments

Back in April 2021 I introduced a simple text-based adventure game, The Lighthouse of Doom, which I'd written in Z80 assembly language for CP/M systems.

• My game is on github here:
• https://github.com/skx/lighthouse-of-doom
• It was discussed on hacker-news at the time:
  • https://news.ycombinator.com/item?id=26946130

As it was recently the 40th Anniversary of the ZX Spectrum 48k, the first computer I had, and the reason I got into programming in the first place, it crossed my mind that it might be possible to port my game from CP/M to the ZX Spectrum.

To recap my game is a simple text-based adventure game, which you can complete in fifteen minutes, or less, with a bunch of Paw Patrol easter-eggs.

• You enter simple commands such as "up", "down", "take rug", etc etc.
• You receive text-based replies "You can't see a telephone to use here!".

My code is largely table-based, having structures that cover objects, locations, and similar state-things. Most of the code involves working with those objects, with only a few small platform-specific routines being necessary:

• Clearing the screen.
• Pausing for "a short while".
• Reading a line of input from the user.
• Sending a $-terminated string to the console.
• etc.

My feeling was that I could replace the use of those CP/M functions with something custom, and I'd have done the 99% of the work. Of course the devil is always in the details.

Let's start. To begin with I'm lucky in that I'm using the pasmo assembler which is capable of outputting .TAP files, which can be loaded into ZX Spectrum emulators.

I'm not going to walk through all the code here, because that is available within the project repository, but here's a very brief getting-started guide which demonstrates writing some code on a Linux host, and generating a TAP file which can be loaded into your favourite emulator. As I needed similar routines I started working out how to read keyboard input, clear the screen, and output messages which is what the following sample will demonstrate..

First of all you'll need to install the dependencies, specifically the assembler and an emulator to run the thing:

# apt install pasmo spectemu-x11

Now we'll create a simple assembly-language file, to test things out - save the following as hello.z80:

    ; Code starts here
    org 32768

    ; clear the screen
    call cls

    ; output some text
    ld   de, instructions                  ; DE points to the text string
    ld   bc, instructions_end-instructions ; BC contains the length
    call 8252

    ; wait for a key
    ld hl,0x5c08        ; LASTK
    ld a,255
    ld (hl),a
wkey:
    cp (hl)             ; wait for the value to change
    jr z, wkey

    ; get the key and save it
    ld a,(HL)
    push af

    ; clear the screen
    call cls

    ; show a second message
    ld de, you_pressed
    ld bc, you_pressed_end-you_pressed
    call 8252

    ;; Output the ASCII character in A
    ld a,2
    call 0x1601
    pop af
    call 0x0010

    ; loop forever.  simple demo is simple
endless:
    jr endless

cls:
    ld a,2
    call 0x1601  ; ROM_OPEN_CHANNEL
    call 0x0DAF  ; ROM_CLS
    ret

instructions:
    defb 'Please press a key to continue!'
instructions_end:

you_pressed:
    defb 'You pressed:'
you_pressed_end:

end 32768

Now you can assemble that into a TAP file like so:

$ pasmo --tapbas hello.z80 hello.tap

The final step is to load it in the emulator:

$ xspect -quick-load -load-immed -tap hello.tap

The reason I specifically chose that emulator was because it allows easily loading of a TAP file, without waiting for the tape to play, and without the use of any menus. (If you can tell me how to make FUSE auto-start like that, I'd love to hear!)

I wrote a small number of "CP/M emulation functions" allowing me to clear the screen, pause, prompt for input, and output text, which will work via the primitives available within the standard ZX Spectrum ROM. Then I reworked the game a little to cope with the different screen resolution (though only minimally, some of the text still breaks lines in unfortunate spots):

• bios.z80

The end result is reasonably playable, even if it isn't quite as nice as the CP/M version (largely because of the unfortunate word-wrapping, and smaller console-area). So now my repository contains a .TAP file which can be loaded into your emulator of choice, available from the releases list.

Here's a brief teaser of what you can expect:

Outstanding bugs? Well the line-input is a bit horrid, and unfortunately this was written for CP/M accessed over a terminal - so I'd assumed a "standard" 80x25 resolution, which means that line/word-wrapping is broken in places.

That said it didn't take me too long to make the port, and it was kinda fun.

Tags: cpm, retro, spectrum, z80 | 4 comments