AltairSim

Quick start: CP/M in one command

This chapter boots CP/M 2.2 on a 56K Altair with an 8″ floppy disk. It also shows you the three commands that you use every time: Ctrl-E, RUN and QUIT.

Type the commands in the folder where you unzipped the package.

On macOS, if you got the package with a browser, type this command first. Without it, macOS does not run the program:

$ xattr -dr com.apple.quarantine ./altairsim

If macOS shows a message about the program, press Done, not Move to Trash. The chapter Running it tells you more.

Boot CP/M

  1. Start the machine:

$ ./altairsim examples/cpm/cpm22-buffered.toml

On Windows, the program is altairsim.exe, and the path uses backslashes:

```

altairsim.exe examples\cpm\cpm22-buffered.toml ```

The machine boots CP/M:

``` 56K track-buffered CP/M 2.2b, booting off the 8" floppy in drive A. You land at the A> prompt -- type DIR to see what is on the disk.

Drives B: through D: are empty. FORMAT one from CP/M, or MOUNT an image onto dsk0. AltairSim X.Y.Z -- 8080, full speed. machine: cpm22-buffered. HELP for commands. startup> RUN FF00 [console -- ^E returns to the monitor]

56K CP/M 2.2b v2.3 For Altair 8" Floppy

A> ```

  1. Type DIR to see what is on the disk:

A>DIR A: L80 COM : LADDER COM : ED COM : ASM COM A: DUMP COM : XSUB COM : PCGET COM : LS COM A: SUBMIT COM : LOAD COM : SURVEY COM : VIEW COM A: LADDER DAT : LUNAR BAS : M80 COM : MAC COM A: MBASIC COM : PIP COM : STAT COM : DDT COM A: MOVCPM8 COM : NSWP COM : SYSGEN COM : W COM A: ACOPY COM : VI DOC : STARTRK BAS : TICTAK BAS A: VI COM : CRC COM : PCPUT COM : AFORMAT COM A: STARINS BAS : IOBYTE TXT : R COM : HDIR COM A>

The disk has an assembler, a debugger, Microsoft BASIC, a text editor and Star Trek.

  1. Type MBASIC to start Microsoft BASIC, and give it a line to run:

A>MBASIC BASIC-80 Rev. 5.21 [CP/M Version] Copyright 1977-1981 (C) by Microsoft Created: 28-Jul-81 21816 Bytes free Ok PRINT 2+2 4 Ok

  1. Type SYSTEM to go back to the A> prompt.

Stop, continue and leave

  1. Press Ctrl-E. The machine stops, and you get the monitor:

A> STOP -- the machine is still at CA9C. RUN resumes. C0Z1M0E1I0 A=00 BC=007F DE=CA01 HL=BC0E SP=BC37 IE=1 PC=CA9C CALL CA78 altairsim>

  1. Type RUN. The machine continues from the instruction where it stopped. Your A> prompt is where you left it:

altairsim> RUN

  1. Press Ctrl-E again, and type QUIT to leave the program:

altairsim> QUIT

These are the three commands to remember:
Ctrl-E Stop the machine, and go to the monitor. The machine loses nothing.
RUN Start the machine again, from where it stopped.
QUIT Leave the program. Q is enough. There is no EXIT command.

What Ctrl-E does

Ctrl-E does what the STOP switch on the front panel does. The machine stops at the instruction where it was, and it loses nothing. STOP is not RESET, and it is not POWER. The registers, the memory, the disk in the drive and the program counter do not change. While the machine is stopped, you can read memory, step one instruction at a time and set breakpoints.

Ctrl-C is not the stop key, because Ctrl-C belongs to the guest. CP/M does a warm boot when you press Ctrl-C, and BASIC stops the program. The program reads Ctrl-E before the guest can get it, and every other key goes to the guest. If a guest needs Ctrl-E, type SET CONSOLE stop=1D at the monitor. The stop key is then Ctrl-].

Before you change the disk

Copy the folder before you write to the disk. The machine mounts the disk image read/write. Everything that you do in CP/M changes the file on your computer, and the program keeps no copy. The folder has all that the machine needs, and the copy boots from any place:

$ cp -R examples/cpm my-cpm
$ ./altairsim my-cpm/cpm22-buffered.toml

Go back to the A> prompt before you quit or copy the image. This BIOS keeps a full track in memory. It writes the track to the disk the next time it reads the console. If you quit before that, the last write is lost.

To look at a disk that you do not want to change, write-protect it at the monitor:

altairsim> MOUNT dsk0:drive0 cpm22b23-56k.dsk WP

The controller then refuses every write. CP/M does not know that the disk is write-protected, and a program that tries to write can fail. Do not use WP for a disk that you save work on. The disks chapter tells you more.

What happened when it booted

The machine file describes a 56K Altair with an 8″ floppy disk controller and a boot PROM at FF00. It puts the disk image in drive 0, and it types RUN FF00 for you. The startup> line shows this command.

There is no BOOT command. To boot a disk on a real Altair, you set the address switches to FF00, pressed EXAMINE, and then pressed RUN. RUN FF00 does the same thing. A machine file can do only what you can type at the monitor.

The rest of the boot is real software. The PROM loads a boot loader from the disk. The boot loader loads CP/M into memory, and the BIOS of CP/M prints its banner.

The machine file names its disk cpm22b23-56k.dsk, with no folder. The program looks for the disk in the folder of the machine file. For this reason, the copy in my-cpm finds its own disk.

Where to go next

  • Other machines. Type ./altairsim --list to see the built-in machines. The machines chapter describes them.
  • A machine with no disk. The tapes chapter loads Altair 4K BASIC from a cassette. You enter the bootstrap by hand, from the listing that MITS printed in its manual.
  • Complete sessions. The worked examples chapter has more, from start to end.