The CP/M operating system was the first commercially successful disk operating system for microcomputers. As such, it recently received designation by the IEEE as a Milestone in Electrical Engineering and Computing. See the article at http://theinstitute.ieee.org/tech-history/technology-history/groundbreaking-operating-system-is-named-an-ieee-milestone. This operating system was designed by Gary Kindall in 1974, to run on microcomputers with an 8080 processor and 8-inch IBM floppy disks. However, it was designed to be portable to many different machine architectures, by having a machine-dependent, customizable basic input-output system (CBIOS) that had the software to operate the disks, console and other peripheral hardware, and a machine-independent basic disk operating system (BDOS) and console command processor (CCP), to process commands and create and use a disk file system. Since the 8080 processor uses a subset of the same machine code as the Z80, CP/M could be used on both 8080 and Z80 machines. CP/M use spread to a wide variety of machines using a wide variety of disk drives and peripherals. Eventually, the introduction of 16-bit microcomputers using MS-DOS made 8-bit microcomputers (and CP/M) obsolete, but it is still used and enjoyed by hobbyists and educators using 8-bit Z80 or 8080 systems.
As mentioned in the disk and memory expansion kit instructions, CP/M itself is not yet in the public domain, and must be downloaded from the site licensed to distribute the source code, and assembled by you. Then you need a number of programs to test the memory expansion, and to load CP/M onto the disk for use in the Z80 computer. Here are the programs for download:memory_test.bin
Listings of these files can be found in the Disk and Memory Expansion kit instructions.