A Unix-like cal(endar) utility for CP/M
Version 0.1c, May 2023
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.
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.
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.
Although cal is a simple utility by modern standards, it still has
some technical complexity. It requires
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.
0.1d July 2023
0.1c May 2023
0.1b April 2023
0.1a February 2022