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Recommended: Gigatron

a computer w/o a microprocessor


CP/M Internals

The reason most people are still drawn to CP/M is because it is so easy to fully understand the system, up from the tiniest detail. Yet, CP/M is the direct predecessor of MS-DOS (which was modeled very closely after CP/M) and has full functionality for normal use. If you know CP/M, you understand the low-level basics of any PC, and it gives you a level of understanding of the hardware that you'd never gain with, for instance, Linux. In short, understanding CP/M is relatively easy, and it gives you an insight in today's computers that is hard to obtain in any other way.

Page Contents:


  1. Introduction
  2. Memory Map
  3. Boot Process
  4. The Low Storage Area
  5. The Disk Drive
  6. Organising file storage
          through the Directory
  7. Using Files

Introduction: What CP/M provides

CP/M essentially provides programs with a set of function calls that allow them to communicate with the computer's I/O devices in a standardised manner. These system calls (the BDOS functions) ensure that user programs never have to bother with how the computer hardware stores a file or puts text on a screen - instead, the system calls can be relied upon to do the job. That concept is the basis for any operating system, and it enables computers that are very different in hardware to run the same programs. Here is a list with the CP/M BDOS calls - the operating system services.

The second service that CP/M provides is a command line interface and a number of small support programs that enable the user to maintain his computer and files. For CP/M, the list of support programs is minimal but complete: programs to edit files (ED, never to be used as it is truly bad. Instead, use VDE or WordStar) and copy (PIP) files, create assembler (ASM and LOAD) programs and debug (DDT) them.


Memory Map

CP/M requires a minimum of 20K RAM, although realistically, 48K is the bare minimum. Most systems have the maximum 64K. The chart below shows the memory map of a CP/M system: the first 256 bytes (100 hexadecimal) are used as a scratchpad by the operating system. It contains buffers, parameters and other transient data. This area is called Low Storage.

Moving up, the area where the user loads and runs his programs is called the TPA, Transient Program Area. The size of this area depends on the amount of RAM - its size is whatever is left after CP/M has taken what it needs. For a typical 64K system, the TPA stops at DC00h, but this number can vary.

Running a program from the command line does nothing more that load the file into memory, starting at address 0100h, and start executing the code at 0100h after loading.

The top segment of memory is used by CP/M. First, the CCP area contains the command line user interface. Then, the BDOS contains all primitive functions (available to CP/M and user programs) to deal with disk drives, keyboard and screen, etc. BDOS and CCP are the same across CP/M machines, they do not need any customisation - other than the need for them to be relocated (shifted up or down in memory) if the size of total memory changes after a user forked out to buy another 16K of RAM, for instance.

Lastly, the BIOS sits at the top end of memory. It normally is about 600h bytes in size, but can be larger if there is a lot of special hardware that needs to be managed. The BIOS is a function library that supports BDOS: anything that is hardware-specific needs to be done in the BIOS functions. BDOS, for instance, has a function for putting text on the screen, but will hand over the physical work of flipping bits in chips to the BIOS. BDOS has no idea whether the screen is a serial terminal or a graphics board.

The Boot Process

To start up a CP/M computer, some bootup code needs to put the CP/M code into memory, and all hardware must be initialised properly. The Z80 is hardwired to start executing program code at location 0000h when it is switched on. There are various schemes to ensure that proper startup code is stored there; but the most common method is to have a ROM at location 0000h that does all it needs to do, and then switch that ROM out for RAM memory after the initial start-up has been completed.

Usually, the ROM will load the CP/M boot sector from disk, store it in memory from DC00h onwards (in a typical 64K system), and start executing the BIOS code. CP/M is then in charge and the Low Storage area is properly set up.

The Low Storage Area


The first 256 bytes of memory play a crucial role in CP/M. The area is initialised by the BIOS during the boot process, and then handed over to the CCP for maintenance. User programs will use specific parts of Low Storage when they need to access disk drives, or change I/O assignments (using the IO Byte).

Low Storage - The First Eight Bytes of Memory

Starting at the bottom of the memory map, the first three bytes of code contain the machine language instruction JMP F200h (or whatever address is the start of the BIOS). Because the Z80 processor starts itself up by beginning to execute whatever it finds at memory location 00, these three bytes are a logical place for the system reset routine. Executing the code causes the BIOS to reinitialise the Low Storage Area and forget about anything.

The following byte is the IO Byte. In good CP/M implementations (many computer vendors were sloppy in using this functionality), you can re-assign the real devices to CP/M virtual devices. For instance, you can assign the keyboard hardware to what CP/M sees as the serial port. This byte serves as a switch between real and virtual devices and is a GREAT idea.

Byte 04h logs the current default drive (in bits 0-3, 0 being A: and 1 being B:, etc) and the current user (in bits 4-7). The user code is normally 0, but through the USER x command, it can be changed.

Bytes 05h-07h contain the machine language instruction JMP E400 (i.e., Jump to BDOS). Calling address 05h is the standard way in which programs use the services of CP/M. For instance, if a program wants to print the character 'A' to the screen:

it loads the value 65 (for A) in the Z80's accumulator,

-  loads the value 02h (for BDOS service #, print character)
    in the Z80's C register
-  and simply does a CALL 0005.
CP/M will put the character

   on the screen, and then return execution to the program.


Low Storage - the middle part​

Moving further up in the memory map, the restart vectors starting at byte 08h can be initialised by the BIOS or a user program. Each interrupt level of the Z80 has a vector here. If an interrupt occurs, the Z80 will stop with whatever it is doing, and jump execution to one of these vectors depending on the level of the interrupt (0 to 6). Interrupt 7 is reserved for a debugger. If a debugger like DDT is loaded, it'll put the instruction JMP XXXX in bytes 38h-3Ah, where XXXX is the address where the debugger resides. Many earlier computers had a debug button, which when pressed would simply create a hardware interrupt 7 signal to land the user in the debugger. CP/M doesn't use interrupts itself, so these vectors can be used for whatever interrupt-generating hardware is built into the system.

The area from 40h upwards is used as a variable scratchpad by the BIOS for normally undocumented purposes. The area above, starting at 50h, is unused by CP/M 2.2 (not true for MP/M). It is a great place to store small programs that can remain resident in the computer.

Low Storage - the upper part

The File Control Block resides in bytes 60h-80h. This is a group of parameters that needs to be filled in before a program calls a BDOS function that has anything to do with disk drives. In the FCB, the program stores the drive it wants to look at, its file name, and any other relevant details. The FCB is explained further down in this text.

The Disk Drive

The main benefit of CP/M is that it offers an easy way to handle disk drives to the user and his programs. A CP/M disk is a nothing more than a series of sectors that store 128 bytes each. The first sectors on the disk are reserved for CP/M, and hold a copy of the CCP, BDOS and BIOS. These are read off the disk during the boot procedure by the boot loader.

Normally, the sectors on the first two tracks of a disk (the system tracks) are reserved

for this purpose.

Directly after the system tracks, a fixed number of sectors are reserved for use as the

directory of the disk. These are used by CP/M to store file names, and to keep track

of which sectors are allocated to store the data of each file the user has created.

The Disk Drive - BIOS versus BDOS

Before going into the inner workings of file management on the disk, it is useful to separate

responsibilities between the different parts of CP/M. Because disk drives come in a wide

variety of types, it is the BIOS that knows about the physical layout of the drive. It commands

the read/write head to move to a certain track on the disk, and read or write a certain sector.

All this is programming code specific to the hardware of a particular computer.

The BDOS knows nothing of this all. Through the BIOS, is sees the disk drive as an entity with a number of tracks, each containing a number of 128-byte records. In the early days, a record equalled a sector - both being 128 bytes of storage. But later on, sectors became larger and disks got more double-sided. Hard disks even got up to 4 platters with 8 sides to write on. The BIOS hides this physical reality from the rest of CP/M and translates standard tracks and records into whatever is the physical layout of the disk.

Organising file storage through the Directory

When a disk is initialised, CP/M assumes that there will be a maximum of 64 potential files on the disk, and it thus creates 64 empty slots of 32 bytes each in its disk directory. These slots - directory entries - are the things that are changed by commands like ERA and REN. 64*32bytes occupies 2048 bytes, or 16 sectors. The chart below shows how a directory entry is made up.

The first byte in a directory entry simply states whether the entry is used or not (if it holds E5h, it's empty). This is actually how the ERA (file delete) command works: it looks up which directory entry matches the filename it should delete, then places the value E5h in the first byte of that directory entry.

The following bytes hold the filename and extension of the file. ASCII uses only the first 7 bits of each byte. The eighth, upper bit of the three bytes that hold the extension are used to signal whether a file is read-only, invisible, or backed-up (archived).

After that follow three bytes that deal with files that are over 16K in length. We'll skip their workings here (suffice it to say that a 32K file simply occupies two linked directory entries), but the reason for the 16K file limit is explained by the RC byte and the 16 Disk Map bytes that are explained below.

The Disk Map (16 bytes in size) holds a table with the consecutive numbers of all sectors used by this file. The logical thing would be to hold the number of each sector in this table. But then the file size would be limited to 16 sectors, each referenced by one consecutive byte in the Disk Map. As a byte can only hold numbers between 0 and 255, there could only be 255 sectors on the disk. Which is not enough.

The solution is the introduction of the concept of a block. A block is a unit of eight 128-byte sectors, or 1K in storage. By counting a disk in blocks (each consisting of 8 sectors) instead of sectors, the number stored in a single byte in the Disk Map can cover a sufficiently large disk drive. One of 255K in fact. As disk drives quickly grew past this size, CP/M allows different block sizes to be used. A normal 360K disk uses a block size of 16 sectors, or 2K.

    Obviously, the problem with using blocks rather than sectors is that a file of 20 bytes in size doesn't just use up an entire sector of 128 bytes, but an entire block of 1K. This problem is still with us: a file of 20 bytes on a modern PC can easily occupy a block of 16K bytes.

Anyway, suffice it to say the the RC byte contains the number of sectors used by the file, and the Disk Map contains the list of blocks in which they can be found.

Using Files

With the knowledge above, it is easy to trace what happens if a user, through a program like a word processor, creates a new file. A program creates a new file through the use of a BDOS command, #15. Before calling that command, the program must fill in the FCB (File Control Block). The default FCB is a block of memory in Low Storage, starting at address 60h specifically reserved for this purpose.

For opening a file so its data can be read, the FCB needs to be initialised as follows:


  •     Enter the drive number in byte 60h, the first byte. 0 = current default drive, 1 = A:, 2 = B:.

  •     Enter the filename in the following 11 bytes.

  •     Assuming the file is smaller than 16K, the ex/s1/s2 bytes should be left to 0

Useful Links


The Unofficial CP/M web site holds all the original source code from Digital Research as well as all CP/M releases and manuals. Thomas Scherrer's Z80 page is a good place to start for learning Z80 assembler.

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