Llichen-80 (Retrochallenge 2019-03 Update)

I had originally intended for any time I could devote this month to the 2019-03 Retro Challenge was going to be for a new version of my RC2014 Z80 computer emulator, adding support for the TMS9918A video display chip.  But my plans have changed.  With the recent advancements with The 8-bit Guy's "Commander X16" 6502 computer, I decided to start reviving a project I was considering for a while.  The great thing is that there's so many aspects to it that it's like a grab-bag of things to do.

The original idea was to sit down with a C64, floppy drive, monitor, joystick, mouse and reference books, and start with BASIC, writing a text editor, assembler, IDE, etc and work my way up to having a windowed operating system like GEOS with a few utilities and such.

Except now, the spec of the system is designed by me, and I'm building that too.... Which meshes in perfectly with my original intention to build a RC2014+TMS9918 emulator, which I can use for initial development, and kickstarting the boot rom onto it.

Hardware

I wanted some sort of related name for the project, so I was thinking Llama, TMS, which became Llitmus, like the PH sensitive dye.  I was gonna go with Llitmus-80 with the '80' because it uses a z80, but then once I read that the litmus was made from lichen, I just went with the name Llichen-80.

First, let's get into the Llichen-80 system specification.

• RC2014 backplane/base system
• 7.37 MHz clock speed
• 32k RAM from 8000-FFFF
• 8k BASIC ROM from 0000-1FFF (*)
• 32k RAM from 0000-7FFF (*)
• Digital IO board at port 00
• TMS 9918A video board at port 10,11
• ACIA Serial port at 80
• ACIA Serial port 2 at C0 - connected to CH376S USB drive interface

So (*) Indicates something... there's an overlap of these two items. My existing design piggybacks bit 0 of the output from the digital board to select which of the two of these that memory READs come from.  That is to say that writing 0x00 to the digital out board will select ROM, and 0x01 will select RAM.  Writes to these locations always go to RAM.  So one way I can test this is:

• Start up the computer
• select ROM bank
• Write a basic program to peek from the ROM and write to the RAM
• select the RAM bank
• Yank out the ROM board, and everything still works fine

10 OUT 0,0
20 FOR A=0 TO 8192
30 B = PEEK(A)
40 POKE( A, B)
50 NEXT A
60 OUT 0,1
RUN

This all works great!  One addition I made was to intercept the VCC to the RAM chip and have it also connected to some CR2032 batteries for a backup.  It's not perfect, but it works well enough for my system for now.  I also added a switch to force the use of the ROM, since sometimes the bank switching can get into a messed up state and you need to force reboot off of the ROM.

The other thing I created was a second ACIA serial port, essentially duplicating the circuit of the stock ACIA serial port, but changing it to look at ports C0 and C1 instead of 80 and 81.

The TMS could be configured for anywhere, for the most part, but the ACIA chip IO port mapping is messy so they consume essentially from 80 to FF.  I went with the SORD-M5 configuration of putting it at 10 and 11h.  Although there might be issues with this...

Anyway, more about these modifications and such at the end of the month.

Experiments 

I ended up reassembling my RC2014 system, and started working on testing out the configuration.  I Was able to write a few BASIC programs to output to the video chip and to a TV I had hooked up, as you can see in the photo above.  I was noticing some weirdness though. The LEDs on the IO board were flickering when I was doing writes to the TMS, so I may need to move the port around to another location. I think I'm gonna go with the MSX setting of 98 and 99h.

I also became very aware that I do not yet understand how to get the chip to do what I want.

Emulation

I was going to write the emulator using QT Creator for maximum portability, and also using the Z80 emulation core and frameworks I created for my existing RC2014 emulators.  But recently, after thinking about it for a little bit, I realizes that 90% or the emulator I need is already out there and already very well supported.  MSX.

There was really no need to get a whole video display system working with my existing emulator when I could start with an existing, debugged, well supported emulator, openMSX, and just create a new hardware profile for the RC2014 with my system's configuration... and with some changes to support my bank switching hardware and such.

I made a branch of openMSX on github to handle any/all of the changes I've made for it, which at the moment is just getting it to build on OSX/Darwin 10.14 correctly.

I also was messing around with my good old friend, the Texas Instruments TI-99/4A as it uses the TMS9918 chip. I figured that I could experiment around with how color and graphics look.  I'm not a fan of the TI's font, nor the MSX's font for that matter, but I figured it's a good platform to get reacquainted with the weirdness of the TMS and to see how this all might look in the future, perhaps.

This is of course just a screenshot from the impressive web-javascript TI emulator at js99er.net.

More as it develops...

The RC2014 Computer: 1. Emulation

As you may know, I'm bigtime into Z80 computerey stuff.  For the past 20 years or so, I've been hacking Pac-Man ROMs, been maintaining the Ms Pac Disassembly, and have made my own Z80-Pac based programes over the years.

Fairly recently, I got a Kaypro II from a friend at interlock, and it worked perfectly, and looked brand new.  I felt like I couldn't hold on to it... "it belongs in a museum!" ...so I donated it to ICHEG/Strong Museum of Play.  But it helped whet my appetite for a CP/M computer.

Another project I've been wanting to do was to start with a Commodore 64 (I know it's not Z80, it's 6502ish), a floppy drive, some blank disks and a hardware manual and code up, from scratch, an OS.  Start out by making a text editor, assembler, GEOS-like GUI, etc.

These projects recently had an opportunity to overlap, and they all seem to converge on the RC2014 modular Z80 computer.

The RC2014 computer is a backplane-based modular computer created by Spencer Owen, based on the Z80SBC by Grant Searle.  There are modules for the CPU, RAM, ROM, Serial Terminal interface, and so on.  It is available as a kit from tindie.com.  Once assembled, you hook up a serial terminal to it, power it on, and you get a 1980s-esque BASIC prompt onto which you can write your 32kbytes of program.  This is based on Grant's simplified Z80 computer, so there is no off-line storage.

My general plan for the RC2014 is:

1. Emulation:
1. Create an emulator for the system to aid with rapid development
1. also bring my "bleu-romtools" from Google Code to github
2. Add a serial-based storage solution to the RC2014 emulation to confirm proof-of-concept
3. Add ROM swap out to the emulation
4. Add 32k of ram to give a full flat 64k of ram to the emulation
2. Hardware:
1. Get a RC2014 kit
2. Build the RC2014 kit
3. Make a test ROM to run on real hardware to verify my toolchain is working
4. Create a new serial card that sits at port 0xC0 (second serial)
5. Create the SD Drive firmware for the serial arduino
6. Hack the ROM and Digital IO boards to allow for disabling the ROM
7. Add 32k hacked RAM to the system
3. Name: RC2014/LL
1. At this point, the architecture is different enough and well defined enough that I think a new name for this configuration is in order. I call this configuration "RC2014/LL".
4. Port CP/M
1. Create the BIOS
2. Create the sector-based emulation layer in the SD drive
3. Boot CP/M
4. Play Zork

I started out by making an emulator using the Z80 Pack emulation system.  Once I got this running, I realized the limitations of this emulator and looked around and found another emulator that suited my needs better. (I wanted a way to "swap" memory around, which Z80 Pack would not do in a way that wasn't a major hack.)

I created a layer that adds disableable memory regions, and added emulation of the 6850 ACIA serial chip, and threw the 32k RAM BASIC ROM at it, and it started right up, running BASIC!

I added a second 32k of RAM (easy to do when you're emulating it!), and started creating the SD interface, also using the 6850 ACIA for communications.  I then added a port, emulating the IO card, on which bit 0 (0x01), when set, will disable the ROM... So any reads to the low area of memory will read from the ROM.  Whether this is set or not, all writes to that area of memory will actually happen to the RAM... they will just be hidden from reads until that bit is set.

I now have the basics of the RC2014/LL system emulated in software!  I created a boot/diagnostic ROM which can be used for all RC2014 systems which can probe memory to determine type (ROM, RAM, unpopulated), peek and poke memory, In and out IO, and other utility functions for the SD card interface.

Currently, I'm writing the SD card API which I will port to the Arduino Leonardo and SD breakout board which I have ordered, once those come this weekend, I'll shove them into my own serial board and burn a test ROM and see how it goes....

Introduction

Hi all!  I'm Scott, and I like to make things.  I hope to show you here a few things that I've made, and I'll try to explain how I made them so that you can get something out of it too... Make things for yourself, or at least spark a bit of your imagination to make things for yourself.

I'm a big fan of retro computing, re purposing gadgets, making things do what they weren't intended to do, and generally have fun learning some new things.  I hope to take you along with me on making some of this stuff.