Sol-20 cassette games

A Processor Technology Sol-20 with a 1977 game in the cassette deck. Four of them, each in its own machine file — pick one and run it:

altairsim trek80.toml        # TREK80  -- Star Trek
altairsim atc.toml           # ATC     -- Air Traffic Controller
altairsim pacman.toml        # PAC-MAN
altairsim raiders.toml       # RAIDERS

A fifth file, trek80-ent.toml, loads TREK80 without the cassette. It pastes TREK80.ENT, a SOLOS ENTER script, at the keyboard and then starts the game with EX 0000:

altairsim trek80-ent.toml    # TREK80, from TREK80.ENT

More Sol-20 programs are at https://deramp.com/downloads/processor_technology/sol-20/software/.

The game loads itself. Each file mounts the cassette, types the SOLOS command that runs it, and boots — so after the tape finishes loading (about the minute a real cassette took, at the Sol's own speed) the game comes up. TREK80 asks for a speed factor and starts; the others run straight away.

You do not have to type anything — but on a real Sol-20 you would, and the machine files simply do it for you. At the SOLOS > prompt you would enter XE and the file's five-character name; a TYPE line in each startup queues exactly that, and SOLOS reads it as type-ahead at its first prompt, as if the keys had been pressed. The names, for when you XE one yourself after a REW:

RAIDERS needs two commands, because its tape names the wrong start address. A SOLOS tape header carries a load address and an execute address, and XE jumps to the execute address when the load ends. The RAIDERS header says load 0100, execute 0000 — but nothing on the tape loads at 0000, so a plain XE RAID loads the game correctly and then runs whatever RAM held there. SOLOS comes back with a bare > and no game. GE RAID loads the tape without running it, and EX 100 starts the game at its real entry point.

raiders.toml still types XE RAID for you: before it boots, it deposits JMP 0100 at address 0 (DEPOSIT 0000 C3 00 01), so the jump to 0000 lands on the game. That jump stays in memory, so XE RAID also works after a REW — until the game or a power cycle overwrites it. The two commands always work.

What you are looking at

The Sol-20 is not an Altair with a terminal on it — it is an integrated computer with a keyboard and a screen built in, and that changes what this looks like on your terminal. The screen is a VDM-1, memory-mapped video at 0CC00H–0CFFFH, and nothing it displays reaches your terminal: SOLOS signs on, XE echoes, and the game paints its starfield all into that video memory. With SDL3 you get a window, in the VDM-1's own character font, and you watch it there. Built without SDL the machine still runs perfectly and the console stays quiet — that quiet is correct. Your typing goes the other way and does work: keystrokes reach the Sol's keyboard port.

Headless, you can still read what the game painted: stop the machine with ^E and dump the VDM's memory. Each row is 64 bytes, and DUMP … WIDTH=64 prints the ASCII beside the hex, so one screen row is one line. For TREK80:

altairsim> DUMP CD00-CD3F WIDTH=64
CD00  43 4F 50 59 52 49 47 48 54 ... 43 4F 52 50 2E  COPYRIGHT (C) 1977  PROCESSOR TECHNOLOGY CORP.

altairsim> DUMP CFC0-CFFF WIDTH=64
CFC0  45 4E 54 45 52 20 53 50 ... 54 29 29           ENTER SPEED FACTOR (9(SLOW)-0(FAST))

That is the game's own copyright line off a 1977 tape, and the row where it stops to ask you something — read straight out of the screen it drew them on. RUN resumes, and the answer you type reaches the game. ^E (STOP) takes the keyboard back to the monitor at any point; the machine is not disturbed by stopping it.

The clock is 2.045 MHz, the stock Sol-20 — the 14.31818 MHz dot clock divided by 7 (Sol Systems Manual, Theory of Operation §VIII). Everywhere else in this simulator the default is flat out; here the real speed is set on purpose, because these are games and they were played at that speed. It is also why the tape takes a real minute to load. SET cpu0 clock_hz=0 puts it back to flat out.

To play a game a second time, rewind first — nothing inside the guest can wind a tape back:

REW sol0:tape1
XE TRK80

The files

The tapes are Philip Lord's real recordings, and reading them is the point. Each .WAV decodes to a valid SOLOS tape with zero framing errors — for TREK80 the header names TRK80, its SIZE is 1EA0, and its checksum is D9, all matching the archived ENTER script byte for byte. That did not used to work: an earlier decoder garbled these exact recordings (dozens of framing errors, most of the payload wrong), and the TREK80 example tape had to be synthesized from the .ENT instead. The demodulator now reads the real cassettes, so the real cassettes are what ship.

The synthesis is kept as make-trek80-tape.sh because it demonstrates the one thing a SOLOS tape cannot fake: a SOLOS cassette carries a header checksum and one after every 256-byte block, and a tape whose checksums are wrong is invisible — CA lists nothing and GE never finds the file. The script has SOLOS SAve the image so the checksums are the machine's own arithmetic; it writes to a separate TRK80-solos.* so it never overwrites the shipped recording, and it proves the two agree — SOLOS computes the same D9 header checksum the real tape carries.