CP/M Features and    Facilities

       1.1 Introduction

     CPM is a monitor control program for microcomputer system development that uses
 floppy disks or Winchester hard disks for backup storage. Using a computer system based
 upon lntel's 8080 microcomputer, CP/M provides a general environment for program
 construction, storage, and editing, along with assembly and program check-out facilities.
 An important feature of CP/M is that it can be easily altered to execute with any
 computer configuration that uses an Intel 8080 (or Zilog Z-80) Central Processing Unit
 and has at least 20K bytes of main memory with up to 16 diskette drives. A detailed
 discussion of the modifications required for any particular hardware environment is
 given in Chapter 6. Although the standard Digital Research version operates on a
 single-density Intel MDS 800, several different hardware manufacturers support their
 own input-output drivers for CP/M.
     The CP/M monitor provides rapid access to programs through a comprehensive file
 management package. The file subsystem supports a named file structure, allowing
 dynamic allocation of file space as well as sequential and random file access. Using this file
 system, a large number of programs can be stored in both source and machine-
 executable form.
     CP/M 2 is a high-performance, single-console operating system that uses table-driven
 techniques to allow field reconfiguration to match a wide variety of disk capacities. AU
 fundamental file restrictions are removed, maintaining upward compatibility from pre-
 vious versions of release 1. Features of CP/M 2 include field specification of one to sixteen
 logical drives, each containing up to eight megabytes. Any particular file can reach the full
 drive size with the capability of expanding to thirty-two megabytes in future releases.
 The directory size can be field-configured to contain any reasonable number of entries,
 and each file is optionally tagged with read/only and system attributes. Users of CP/M 2
 are physically separated by user numbers, with facilities for file copy operations from one
 user area to another. Powerful relative-record random access functions are present in
 CP/M 2 that provide direct access to any of the 65536 records of an eight-megabyte file.
     CP/M also supports a powerful context editor, Intel-compatible assembler, and
 debugger subsystems. Optional software includes a powerful lntel-compatible macro
 assembler, symbolic debugger, along with various high-level languages. When coupled      
 With CP/M's Console Command Processor, the resulting facilities equal or excel similar
 large computer facilities.
  CP/M is logically divided into several distinct parts: "

 BIOS     Basic IIO System (hardware-dependent)

 BDOS     Basic Disk Operating System

 CCP      Console Command Processor


 TPA      Transient Program Area

     The BIOS provides the primitive operations necessary to access the diskette drives
 and to interface standard peripherals (teletype, CRT, paper tape reader/punch, and
 user-defined peripherals). They can be tailored by the user for any particular hardware
 environment by "patching" this portion of CP/M. The BDOS provides disk management
 by controlling one or more disk drives containing independent file directories. The BDOS
 implements disk allocation strategies that provide fully dynamic file construction while
 minimizing head movement across the disk during access. The BDOS has entry points
 that include the following primitive operations, which can be programmatically accessed:

 SEARCH      Look for a particular disk file by name.
 OPEN        Open a file for further operations.
 CLOSE       Close a file after processing.
 RENAME      Change the name of a particular file.
 READ        Read a record from'a particular file.
 WRITE       Write a record to a particular file.
 SELECT      Select a particular disk drive for further operations.

       The CCP provides a symbolic interface between the user's console and the remainder
 of the CP/M system. The CCP reads the console device and processes commands, which
 include listing the file directory, printing the contents of files, and controlling the
 operation of transient programs, such as assemblers, editors, and debuggers. The stand-
 ard commands that are available in the CCP are listed in Section 1.2,1.
     The last segment of CP/M is the area called the Transient Program Area (TPA). The
 TPA holds programs that are loaded from the disk under command of the CCP. During
 program editing, for example, the TPA holds the CP/M text editor machine code and data
 areas. Similarly, programs created under CP/M can be checked out by loading and
 executing these programs in the TPA.
     Any or all of the CPIM component subsystems can be "overlaid" by an executing
 program. That is, once a user's program is loaded into the TPA, tl~ CCP, BDOS, and
 BIOS areas can be used as the program's data area. A "bootstrap" loader is programmati-
 cally accessible whenever the BIOS portion is not overlaid; thus, the user program need
 only branch to the bootstrap loader at the end of execution and the complete CP/M
 monitor is reloaded from disk.
     TheCP/M operating system is partitioned into distinct modules, including the BIOS
 portion that defines the hardware environment in which CP/M is executing. Thus, the
 standard system is easily modified to any nonstandard environment by changing the
 peripheral drivers to handle the custom system.

               ALL INFORMATION PRESENTED HERE IS PROPRIETARY TO DIGITAL RESEARCH