Writing Altair cassettes — the MITS Tapes disk

A CP/M disk full of the MITS distribution tapes — every 4K, 8K, Extended and Disk BASIC, and the Programming System II monitor — as .TAP images, together with WRTAPE.COM, the utility that writes one of them back out through the cassette port the way a real Altair did.

Boot it, put a blank tape in the recorder, and record a bootable cassette you can then load on the basic4k / basic8k / ps2 machines — or on real hardware, from the .WAV it makes.

The disk and WRTAPE are Mike Douglas's, from deramp.com. This directory adds a machine file that plugs an 88-ACR into the bus alongside the disk, and the host-bridge utilities (HDIR, R, W) so you can move .TAP files in and out of your working directory.

From this directory (acr):

altairsim mitstapes.toml

48K CP/M
Version 2.2mits (07/28/80)
A>

TYPE CONTENTS.TXT at the A> prompt is the disk's own annotated tape list; it is also reproduced below.

Recording a tape

WRTAPE reads a .TAP off the disk and writes it, byte by byte, to the 88-ACR. On a real Altair the operator held PLAY+RECORD on the deck first; here that is one MOUNT with mode=record, and CREATE makes the output file since it does not exist yet. MOUNT is a monitor command, so you reach it with ^E:

The A> prompt is CP/M's; the altairsim> prompt is the monitor's. ^E is a keypress, not a typed command — it toggles from CP/M to the monitor.

$ altairsim mitstapes.toml                                  # boots to A>

A>^E                                                        # press ^E -> the monitor
altairsim> MOUNT acr0:tape "8kbas40.wav" CREATE mode=record # blank tape in, PLAY+RECORD down
altairsim> RUN                                              # resume CP/M -> back at A>

A>WRTAPE 8KBAS40.TAP                                         # press RETURN; wait for:
                                                            #   File Transfer Complete

A>^E                                                        # press ^E -> the monitor
altairsim> UNMOUNT acr0:tape                                # flush the recording to disk

That leaves 8kbas40.wav in this directory — a 300-baud FSK audio cassette, 44.1 kHz, that decodes back with zero framing errors and plays into a real Altair's 88-ACR. Name the file .tap instead of .wav and you get the raw byte image rather than audio; everything else is the same.

• A fresh tape does not need a REWIND. CREATE starts an empty tape with the head at the beginning. (You only REWIND when you are recording over a tape that already has something on it.)
• The transfer is instant at the default clock. On real hardware it runs at 30 cps — about two minutes for 4K BASIC, four and a half for 8K. SET acr0:tape rate=real before you record buys back that true speed, and the console tape counter climbs while it runs.
• A second cassette port. WRTAPE writes through the 88-SIO at 6/7 (the Altair Cassette port, which is where the acr0 in this machine lives) by default. Following the file name with a 2 — WRTAPE 8KBAS40.TAP 2 — writes through the second port of an 88-2SIO instead, for a machine strapped that way.

Cassette and paper-tape images are identical, so the exact same steps write a paper tape.

Moving tapes in and out — the host bridge

The default machine this builds on carries an 88-hostbridge (hb0), and the disk has its three guest utilities installed:

These are ordinary CP/M programs, typed at the A> prompt:

A>HDIR               # list your working directory (the host)
A>R MYPROG.TAP       # host -> CP/M: copy a .TAP onto drive A:
A>W 8KBAS40.TAP      # CP/M -> host: copy a .TAP back out to your working directory

So the full loop is: R a tape image you got from somewhere onto the disk, then WRTAPE it to a cassette — or W one of the disk's own tapes out to keep it.

What is on the disk

Boot tapes — each loads and runs on its own through the bootstrap MITS shipped:

Boot tapes strapped for port B of an 88-2SIO as the cassette input:

Programming System II files:

The disk also holds WRTAPE.COM and its source WRTAPE.ASM, CONTENTS.TXT, the host-bridge utilities R.COM / W.COM / HDIR.COM, and the usual CP/M tools (PIP, STAT, DDT, ASM, SUBMIT, FORMAT, and so on).

The files in this directory

The recorder starts empty — a deck with no cassette in it — so a plain boot writes nothing; you MOUNT a tape when you are ready to record, as above.