minor 1000× error: 'CP/M systems rarely had more than 64Mb of RAM' should read 'CP/M systems rarely had more than 64 kibibytes of RAM' (because memory addresses were 16 bits and there wasn't much demand for bank-switching in cp/m's heyday, though later 8-bit machines like the nes and the msx did use bank-switching extensively)

(disclaimer, i never programmed in c on cp/m, and although i used to use cp/m daily, i haven't used it for about 35 years)

he's using aztec c, but anyone who's considering this needs to know that aztec c isn't under a free-software license. bds c is a properly open-source alternative which seemed to be more popular at the time (though it wasn't open source then)

https://www.aztecmuseum.ca/docs/az80106d.txt says

> This compiler is both the MS-DOS cross-compiler and the native mode CP/M 80 Aztec CZ80 Version 1.06d (C) Copyright Manx Software Systems, Inc. and also includes the earlier Aztec CZ80 Version 1.05 for native mode CP/M 80. I cannot provide you with a legally licenced copy.

> I herewith grant you a non-exclusive conditional licence to use any and all of my work included with this compiler for whatever use you deem fit, provided you do not take credit for my work, and that you leave my copyright notices intact in all of it.

> I believe everything I have written to be correct. Regardless, I, Bill Buckels...

but https://en.wikipedia.org/wiki/Aztec_C explains that manx software 'was started by Harry Suckow, with partners Thomas Fenwick, and James Goodnow II, the two principal developers (...) Suckow is still the copyright holder for Aztec C.'

so it's not just that the source code has been lost; the licensing situation is basically 'don't ask, don't tell'

bds c comes with some integration with an open-source (?) cp/m text editor whose name i forget, so you can quickly jump to compiler errors even though you don't have enough ram to have both the compiler and the editor in memory at once. other ides for cp/m such as turbo pascal and the f83 forth system do manage this. f83 also has multithreading, virtual memory, and 'go to definition' but it's even more untyped than k&r c

bds c is not quite a subset of k&r c, and i doubt boone's claim that aztec c is a strict subset of k&r c as implemented by gcc

sdcc is another free-software compiler that can generate z80 code https://sdcc.sourceforge.net/doc/sdccman.pdf#subsection.3.3.... but it can't run on a z80 itself; it's purely a cross-compiler

a thing that might not be apparent if you're using a modernized system is how constraining floppy disks are. the data transfer rate was about 2 kilobytes per second, the drive was obtrusively loud, and the total disk capacity was typically 90 kilobytes (up to over a megabyte for some 8-inchers). this means that if a person needed data from the disk, such as wordstar's printing overlay, you had to request it and then wait for the disk to find it. so it wasn't a good idea to do this for no user-apparent reason

with respect to

  int elems[5][300];
  ...
  int i, j;
  for (i = 0; i < m; i++)
    {
    for (j = 0; j < n; j++)
      {
      int elem = elems[i][j];
      ... process the value ...
      }
    }

if i wanted efficiency on a compiler that didn't do the strength-reduction for me, i would write it as

  int elems[5][300];
  ...
  int i, *p, *end, elem;
  for (i = 0; i < m; i++) {
    end = elems[i+1];
    for (p = elems[i]; p != end; p++) {
      elem = *p;
      ... process the value ...
    }
  }

this avoids any multiplications in the inner loop while obscuring the structure of the program less than boone's version

cp/m machines are interesting to me as being a good approximation of the weakest computers on which self-hosted development is tolerable. as boone points out, you don't have valgrind, you don't have type-checking for subroutine arguments (in k&r c; you do in pascal), the cpu is slow, the fcb interface is bletcherous, and, as i said, floppy disks are very limited; but the machine is big enough and fast enough to support high-level languages, a filesystem, and full-screen tuis like wordstar, supercalc, turbo pascal, the ucsd p-system, etc.

(second disclaimer: i say 'tolerable' but i also wrote a c program in ed on my cellphone last night; your liver may vary)

on the other hand, if you want to develop on a (logically) small computer, there are many interesting logically small computers available today, including the popular and easy-to-use atmega328p; the astounding rp2350; the popular and astonishing arm stm32f103c8t6 (and its improved chinese clones such as the gd32f103); the ultra-low-power ambiq apollo3; the 1.5¢ cy8c4045fni-ds400t, a 48-megahertz arm with 32 kibibytes of flash and 4 kibibytes of sram; and the tiny and simple 1.8¢ pic12f-like ny8a051h. the avr and arm instruction sets are much nicer than the z80 (though the ny8a051h isn't), and the hardware is vastly cheaper, lower power, physically smaller, and faster. and flash memory is also vastly cheaper, lower power, physically smaller, and faster than a floppy disk