MITS Programming System II

The MITS Programming System II monitor loads from a cassette through an 88-ACR. The package has a monitor, an editor, two assemblers and a debugger. Each one is on its own tape.

cd examples/ps2int
altairsim ps2int.toml

The monitor prompt is two spaces and a question mark:

  ?

Do not type a word to see what it does. A word that the monitor does not know is a program name. The monitor then loads that program from the device that ABS is open to, and waits. When the monitor starts, ABS is open to the terminal. Ctrl-C does not stop a load from the terminal. To get the prompt back:

1. Press Ctrl-E.
2. Type RUN 0x40. 0x40 (100 octal) is the restart address of the monitor.

After OPN ABS,AC (step 1 of "Load the editor and the assembler"), the monitor searches the cassette, and Ctrl-C gets the prompt back.

What is different

The built-in ps2int machine is the same hardware with 16K of RAM. This directory adds the tapes, the loader and the steps to edit and assemble a program.

• An 88-VI/RTC board and sense switch A9 down turn on serial input interrupts. Then Ctrl-C returns you to the monitor, but not during a load from the terminal. The RTC interrupt output of the 88-VI/RTC is not connected, because the monitor has no handler for it.
• The console is Port A of an 88-2SIO at 0x10, strapped to vector 7.
• The machine has 32K of RAM. The steps below move the edit buffer to 0x4000-0x5FFF.
• The sense switches are 0x8C. The loader is the 4K BASIC 3.2 bootstrap, not the 8K one.

Load the editor and the assembler

1. Set the cassette as the program load device. At the ? prompt, type:

OPN ABS,AC

2. Put the editor tape in. Press Ctrl-E, then type:

UNMOUNT acr0 MOUNT acr0:tape "PS2-EDT.BIN" RUN

3. Type EDT to load the editor. The editor prompt is *. Type E to go back to the monitor.

4. Move the edit buffer to 0x4000-0x5FFF, above the assembler. The addresses are octal. Type each line, and end each DEP with Ctrl-Z:

DEP 5124 0 100

DEP 5530 0 140

5. Put the assembler tape in. Press Ctrl-E, then type:

UNMOUNT acr0 MOUNT acr0:tape "PS2-AM2.BIN" RUN

6. Type AM2 to load the assembler. The prompt is *ASM*. Type EOA to go back to the monitor.

Edit and assemble

1. Type EDT to start the editor with an empty buffer. EDT(R) keeps the text that is already in the buffer.
2. In the editor: - I inserts text. Ctrl-Z ends the insert. - P shows the text with line numbers. W shows it without line numbers. - D line[,line] deletes lines. R line replaces a line. I line inserts after a line. - E goes back to the monitor.
3. End each program with these three lines. start label is the entry point. name is a program name of three characters.

BEG start label END name EOA

4. Tell the assembler to read the edit buffer as a text file. Do this one time in a session:

OPN FIL,EB,A

5. Type AM2, then FILE. The assembler shows the errors and the undefined symbols, then returns to the monitor. The "Undefined Symbols" heading comes even when there are none.
6. Type JMP address to run the program. The address is octal.
7. To run the program by its name: after a good assembly, type AM2(P). This keeps the symbols. Then type RUN name. After that, the name alone runs the program.

Save and load source files on tape

1. Type NUL 4. This puts four nulls after each line that is written to tape.
2. Type OPN FIL,AC,A. This sets the cassette as the file device, for text files.
3. In the editor, type L to load a source file, or S to save one.
4. After a load, type OPN FIL,EB,A again, so that the assembler reads the edit buffer.
5. Before you save with S, set the file device back to the cassette with OPN FIL,AC,A.

To save to a new tape, mount a new file for recording. See "Tapes" in the altairsim manual.

Files

The processor runs at full speed, and each tape loads in a few seconds. To run at the 2 MHz speed of the real machine, type SET cpu0 clock_hz=2000000. To load a tape at the real 300 baud, type SET acr0:tape rate=real.

Ctrl-E returns you to the altairsim> monitor at any time.