Skate · Book overview

A first program

A purchase costs 120 cents and postage costs another 15 cents. We can calculate the total with a small program and print it at the CP/M terminal. This gives us a complete edit, compile and run session with an answer we can check ourselves.

Your work disk

Use a writable CP/M disk containing EDIT.COM, SKATE.COM and SKATE.RT. The Skate browser starter provides an older release with the facilities used here. On that setup, B is the writable work disk. At the CP/M prompt, enter:

B:
DIR

B: selects the drive. DIR lists its files so you can check that the three tools are present. The B> prompt identifies the selected drive. CP/M prompts shown in transcripts are printed by the operating system and are not part of the command you type.

Write the source

To create the program, enter:

EDIT POSTAGE.SK8

Type these three expressions in the editor:

(display "Total: ")
(write (+ 120 15))
(newline)

The first expression prints the text inside the quotation marks. In the second expression, (+ 120 15) adds the two numbers. write prints the resulting 135. The last expression prints a newline so the next terminal output begins on a fresh line.

A procedure call places its procedure first, followed by its arguments inside parentheses. + takes the two numbers as arguments. The outer call to write takes the result of that addition. The parentheses show which calculation belongs inside which call.

Press Ctrl-S to save the source and Ctrl-Q to return to CP/M. These are Control key combinations on a Mac as well. The source file is POSTAGE.SK8; its name has seven letters and its extension has three, within CP/M's filename limits.

Compile and run

Compile the saved source with:

SKATE POSTAGE.SK8

After successful compilation, run the generated program by entering its name:

POSTAGE

The program output is:

Total: 135

CP/M then prints its prompt again. Skate has produced POSTAGE.COM, which contains Z80 machine code and runtime support. It also writes POSTAGE.NOB, the object file. CP/M runs the .COM file when you enter POSTAGE.

SKATE.RT must be available during compilation because the compiler copies runtime support into the executable. Once compiled, POSTAGE.COM can run without that separate provider file.

Change the calculation

Run EDIT POSTAGE.SK8 again and change the postage from 15 to 20. Save, quit and compile the source again before running POSTAGE. The new total is 140. Saving the source alone leaves the previous executable in place. Recompilation produces the program that performs the changed calculation.

The editor saves into the emulated disk. Browser persistence and an external backup are separate operations. Keep a downloaded copy of your work before clearing browser storage or moving to another computer. Reopening POSTAGE.SK8 lets you check that the saved source contains the change.