cal-cpm

A Unix-like cal(endar) utility for CP/M

Version 0.1c, May 2023

What is this?

This is my first attempt in nearly 40 years to write something reasonably substantial, entirely in Z80 assembly language. It's a (somewhat cut down) implementation of the Unix cal utility, for CP/M. The usage is straightforward:

A> cal {month} {year}

For example

A> cal 2 2022
February 2022
Mo Tu We Th Fr Sa Su
    1  2  3  4  5  6 
 7  8  9 10 11 12 13 
14 15 16 17 18 19 20 
21 22 23 24 25 26 27 
28

I wrote a C version of this utility for the Manx Aztec C compiler, which results in an 11kB .com file. This assembly-language version is about 2kB.

CP/M does not have any standard system call to read a real-time clock, even if one is fitted. In fact, most CP/M machines never had one. The ROMWBW firmware does have a way to read an RTC so, if this program is run on a CP/M machine with this firmware, the current date will be used if none is given on the command line. Of course, this date can be overridden on the command line.

Building

I wrote this utility to be built on CP/M using the Microsoft Macro80 assembler and Link80 linker. These are available from here:

http://www.retroarchive.org/cpm/lang/m80.com http://www.retroarchive.org/cpm/lang/l80.com

Assemble all the .asm files to produce .rel files, then feed all the .rel files into the linker. See the Makefile (for Linux) for the syntax for these commands. There is no make for CP/M, so far as I know, so building is a bit of a tedious process.

Limitations

The most obvious limitation is that CP/M systems typically don't have a real-time clock, so the utility can't display the calendar for the current month if it is invoked without command-line arguments. It will use the RTC date on a z80 machine with ROMWBW firmware. Sometimes the RTC hardware will provide broken results, and there isn't much that can be done about this.

Unlike the Linux version, this cal only supports the Gregorian calendar, and can only display a single month.

Technical notes

Although cal is a simple utility by modern standards, it still has some technical complexity. It requires

Conversion of integers to strings, and vice versa
Integer multiplication and division in 16 bits
A rudimentary command-line parser
Date calculation routines

In case I should start feeling pleased with myself for fitting this all into 2k, I remind myself that the entire assembler is only 20k.

The screenshot in this source bundle shows cal running on a real Z80-based CP/M machine, just to prove it really works.

Revisions

0.1d July 2023

Fixed some bugs in command-line parsing
Tidied up linker invocation

0.1c May 2023

Added support for ROMWBW real-time clock

0.1b April 2023

Fixed some bugs

0.1a February 2022