collapseos/recipes/trs80

TRS-80 Model 4p

The TRS-80 (models 1, 3 and 4) are among the most popular z80 machines. They're very nicely designed and I got my hands on a 4p with two floppy disk drives and a RS-232 port. In this recipe, we're going to get Collapse OS running on it.

Not entirely standalone

Collapse OS uses the TRS-80 drivers rather than its own. On most TRS-80 models, those drivers are on ROM, but in the case of the 4P model, those drivers are on the TRSDOS disk (well, from what I understand, not all of it, but still, a big part of it).

It would be preferable to develop drivers from scratch, but it represents a significant effort for a modest payout (because it's only actually useful when you want to use a 4P model that has no TRSDOS disk).

Maybe those drivers will be developed later, but it's not a priority for now.

Floppy or RS-232?

There are many ways to get Collapse OS to run on it. One would involve writing it to a floppy. I bought myself old floppy drives for that purpose, but I happen to not have any functional computer with a floppy port on it. I still have the motherboard of my old pentium, but I don't seem to have a video card for it any more.

Because my 4p has a RS-232 port and because I have equipment to do serial communication from modern machines (I didn't have a DB-9 to DB-25 adapter though, I had to buy one), I chose that route.

Gathering parts

• A TRS-80 model 4p with a RS-232 port
• A TRSDOS 6.x disk
• A means to do serial communication. In my case, that meant:
  • A USB-to-serial device
  • A null modem cable
  • A DB-9 gender changer
  • A DB-9 to DB-25 adapter

Overview

We need to send sizeable binary programs through the RS-232 port and then run it. The big challenge here is ensuring data integrity. Sure, serial communication has parity check, but it has no helpful way of dealing with parity errors. When parity check is enabled and that a parity error occurs, the byte is simply dropped on the receiving side. Also, a double bit error could be missed by those checks.

What we'll do here is to ping back every received byte back and have the sender do the comparison and report mismatched data.

Another problem is ASCII control characters. When those are sent across serial communication channels, all hell breaks lose. When sending binary data, those characters have to be avoided. We use tools/ttysafe for that.

Does TRSDOS have a way to receive this binary inside these constraints? Not to my knowledge. As far as I know, the COMM program doesn't allow this.

What are we going to do? We're going to punch in a binary program to handle that kind of reception! You're gonna feel real badass about it too...

Keyboard tips

• _ is CLEAR+ENTER.

Building the stage 1

You can start the process by building the stage 1 binary. Running make in this folder will yield a stage1.bin file. You'll need it later.

Testing serial communication

The first step here is ensuring that you have bi-directional serial communication. To do this, first prepare your TRS-80:

set *cl to com
setcomm (word=8,parity=no)

The first line loads the communication driver from the COM/DRV file on the TRSDOS disk and binds it to *cl, the name generally used for serial communication devices. The second line sets communication parameters in line with what is generally the default on modern machine. Note that I left the default of 300 bauds as-is.

Then, you can run COMM *cl to start a serial communication console.

Then, on the modern side, use your favorite serial communication program and set the tty to 300 baud with option "raw". Make sure you have -parenb.

If your line is good, then what you type on either side should echo on the other side. If it does not, something's wrong. Debug.

Punching in the goodie

As stated in the overview, we need a program on the TRS-80 that:

1. Listens to *cl
2. Echoes each character back to *cl
3. Adjusts ttysafe escapes
4. Stores received bytes in memory

That program has already been written, it's in recv.asm in this folder. You can get the binary with zasm < recv.asm | xxd.

It's designed to run from offset 0x5000 and write received data in 0x3000 and onwards.

How will you punch that in? The debug program! This very useful piece of software is supplied in TRSDOS. To invoke it, first run debug (on) and then press the BREAK key. You'll get the debug interface which allows you to punch in any data in any memory address. Let's use 0x5000 which is the offset it's designed for.

For reference: to go back to the TRSDOS prompt, it's o<return>.

First, display the 0x5000-0x503f range with the d5000<space> command (I always press Enter by mistake, but it's space you need to press). Then, you can begin punching in with h5000<space>. This will bring up a visual indicator of the address being edited. Punch in the stuff with a space in between each byte and end the edit session with x.

But wait, it's not that easy! You see those 0xffff addresses? They're placeholders. You need to replace those values with your DCB handle for *cl. See below.

Getting your DCB address

In the previous step, you need to replace the 0xffff placeholders in recv.asm with your "DCB" address for *cl. That address is your driver "handle". To get it, first get the address where the driver is loaded in memory. You can get this by running device (b=y). That address you see next to *cl? that's it. But that's not our DCB.

To get your DBC, go explore that memory area. Right after the part where there's the *cl string, there's the DCB address (little endian). On my setup, the driver was loaded in 0x0ff4 and the DCB address was 8 bytes after that, with a value of 0x0238. Don't forget that z80 is little endian. 38 will come before 02.

Saving that program for later

If you want to save yourself typing for later sessions, why not save the program you've painfully typed to disk? TRSDOS enables that easily. Let's say that you typed your program at 0x5000 and that you want to save it to RECV/CMD on your second floppy drive, you'd do:

dump recv/cmd:1 (start=x'5000',end=x'5030',tra='5000')

A memory range dumped this way will be re-loaded at the same offset through load recv/cmd:1. Even better, TRA indicates when to jump after load when using the RUN command. Therefore, you can avoid all this work above in later sessions by simply typing recv in the DOS prompt.

Note that you might want to turn debug off for these commands to run. I'm not sure why, but when the debugger is on, launching the command triggers the debugger.

Sending binary through the RS-232 port

Once you're finished punching your program in memory, you can run it with g5000<enter> (not space). If you've saved it to disk, run recv instead. Because it's an infinite loop, your screen will freeze. You can start sending your data.

To that end, there's the tools/pingpong program. It takes a device and a filename to send. Before you send the binary, make it go through tools/ttysafe first (which just takes input from stdin and spits tty-safe content to stdout):

./ttysafe < stage1.bin > stage1.ttysafe

On OpenBSD, the invocation can look like:

doas ./pingpong /dev/ttyU0 os.ttysafe

You will be prompted for a key before the contents is sent. This is because on OpenBSD, TTY configuration is lost as soon as the TTY is closed, which means that you can't just run stty before running pingpong. So, what you'll do is, before you press your key, run doas stty -f /dev/ttyU0 300 raw and then press any key on the pingpong invocation.

If everything goes well, the program will send your contents, verifying every byte echoed back, and then send a null char to indicate to the receiving end that it's finished sending. This will end the infinite loop on the TRS-80 side and return. That should bring you back to a refreshed debug display and you should see your sent content in memory, at the specified address (0x3000 if you didn't change it).

If there was no error during pingpong, the content should be exact. Nevertheless, I recommend that you manually validate a few bytes using TRSDOS debugger before carrying on.

debugging tip: Sometimes, the communication channel can be a bit stubborn and always fail, as if some leftover data was consistently blocking the channel. It would cause a data mismatch at the very beginning of the process, all the time. What I do in these cases is start a COMM *cl session on one side and a screen session on the other, type a few characters, and try pingpong again.

Saving to disk

If everything went well, you could run Collapse OS with g3000<return>. You would get a usable Collapse OS prompt. But don't do that just yet. That executable bootstraps itself from code and it takes a while to do that every time you launch it. You don't want that right? Let's save a compiled version of it to disk.

Turn off the debugger (which can mess up some things) and save your sent content as-is by following the instructions you had for the RECV program. This way, if you mess up a step below, you can quickly start over. Now you can launch Collapse OS. Then, we need to:

• Reclaim wasted memory
• Create hook entry
• Update LATEST
• Write down initialization code
• Write memory to floppy

Reclaim wasted memory

During initialization, RDLN$ allocated memory in HERE for its input buffer. If you don't reclaim that space, that will be dead space in your binary.

You can reclaim that space with FORGET _ which will rewind to before we defined our initialization routine (see xcomp.fs).

However, that is not enough. If you only do that, there will be a conflict between the rdln input buffer and your HERE space! So we need to go put that input buffer somewhere else first. Therefore, your commands will be:

500 ALLOT RDLN$ FORGET _

Create hook entry

That one is easy:

(entry) _

Update LATEST

At this point, both HERE and CURRENT point to your future LATEST.

H@ 0x08 BIN+ !

Done.

Write down initialization code

You'll do something similar to what we do in xcomp, except you'll have to add "HERE rewinding" code because by default, HERE starts at RAMSTART+0x80. So:

," CURRENT @ HERE ! (ok) RDLN$ "

As soon as RDLN$ is called, the C< pointer changes and gives control to keyboard, giving us our full forth interpreter.

Write memory to floppy

What you currently have in memory is gold. You want that on floppy. First, run H@ .X to know your upper bound (and 0 BIN+ .X to know your lower one, but you're already supposed to know that one). Then, run BYE to return to TRSDOS (the TRSDOS driver overrides BYE so that it calls the proper SVC instead of just halting). Then, you can dump memory to floppy as you already did for RECV.

Configuration

In addition to the generic basic shell, this build of Collapse OS has support for floppy drive :1 as a block device (mapped to device 0). Block device commands work as expected.

In addition to this, there is a flush command to ensure that dirty buffers are synced to disk. Make sure you run this after a write operation or before swapping disks.

On top of that, there's CFS support builtin. To enable a FS, type fson while the active block device is properly placed (you can initialize a new FS by writing CFS\0\0\0\0 to the disk). If it doesn't error out, commands like fls and fnew will work. Don't forget to flush when you're finished :)

There is also a custom recv command that does the same "ping pong" as in recv.asm, but once. It puts the result in A. This can be useful to send down a raw CFS: you just need a while loop that repeatedly call recv:putb a.

Assembling programs

Running make will yield a floppy.cfs file that you can dump on a disk. This CFS contains a properly configured zasm as well as a test hello.asm file.

By mounting this CFS (running fson with the active device properly placed), you can assemble and run a binary from hello.asm in the same way that you would in any CFS-enabled shell. You'll then see those sweet "Assembled from a TRS-80" words!