Notes on CP/M 3 Password Protection

by A.G.H. Sibley

Introduction

The CP/M 3, or CP/M-Plus, operating system for 8080/Z80 machines provides two levels of password protection, namely a disk restriction and a file protection system. Exactly how the passwords are stored in an encrypted form and how the programmer may access these areas to decode the password, in the event of a loss of memory faculties, is explained below.

In order to use password and date time stamping of files, a CP/M 3 utility, INITDIR.COM, is run to reorganise the directory area. This creates space in the disk directory for the date/time stamping and passwords but at the expense of halving the number of directory entries. The area which is created is known as the Extended File Control Block, XFCB.

Label XFCB construction

If the directory area on a disk (which has been label stamped) is examined using a suitable utility like DU.COM, then the following should be seen:

DISK UTILITY v8.6
Universal Version under CP/M 3.1

Type ? for help
Type X to exit