Zero page (CP/M)

The Zero Page (or Base Page) is a data structure used in CP/M systems for programs to communicate with the operating system. In 8-bit CP/M versions it is located in the first 256 bytes of memory, hence its name.

The equivalent structure in MS-DOS is the Program Segment Prefix.

In 8-bit CP/M, it has the following structure:

Offset Size    Contents
--------------------------------------------------------------------
00-02  Code    Exit program (jumps to the BIOS, and is also used
               to find BIOS entry points).
03     Byte    I/O byte, an optional feature allowing device 
               reassignment in CP/M 2.
04     Byte    Current command processor drive (low 4 bits) and user
               number (high 4 bits).
05-07  Code    Jump to CP/M BDOS entry - main system call entry 
               point. This is also the address of the first byte 
               of memory not usable by the program.
08-3A  Code     restart/interrupt vectors.
3B-4F  Bytes   Reserved
50     Byte    The drive from which the program was loaded (CP/M 3)
51-52  Word    Address of the password for the first FCB (CP/M 3)
53     Byte    Length of the password for the first FCB (CP/M 3)
54-55  Word    Address of the password for the second FCB (CP/M 3)
56     Byte    Length of the password for the second FCB (CP/M 3)
57-5B  Bytes   Reserved
5C-6B          Default FCB 1
6C-7F          Default FCB 2 (overwritten if FCB 1 is opened)
80     Byte    Number of characters in command tail.
81-FF  Bytes   Command tail (everything after the program name).

In CP/M-86, the structure is:

Offset Size    Contents
--------------------------------------------------------------------
00-02  Bytes   Length of code group in bytes
03-04  Word    Segment address of code group
05     Byte    8080 model flag - set if program only has one segment
06-08  Bytes   Length of data group in bytes
09-0A  Word    Segment address of data group
0B     Byte    Reserved
0C-11          Descriptor for extra group - same format as for data
12-17          Descriptor for stack group
18-1D          Descriptor for X1 group
1E-23          Descriptor for X2 group
24-29          Descriptor for X3 group
2A-2F          Descriptor for X4 group
30-4F  Bytes   Reserved
50     Byte    The drive from which the program was loaded (CP/M 3)
51-52  Word    Address of the password for the first FCB (CP/M 3)
53     Byte    Length of the password for the first FCB (CP/M 3)
54-55  Word    Address of the password for the second FCB (CP/M 3)
56     Byte    Length of the password for the second FCB (CP/M 3)
57-5B  Bytes   Reserved
5C-6B          Default FCB 1
6C-7F          Default FCB 2 (overwritten if FCB 1 is opened)
80     Byte    Number of characters in command tail.
81-FF  Bytes   Command tail (everything after the program name).

References

• Tim Olmstead Memorial CP/M library - in particular, the CP/M 3 Programmers' Manual and the CP/M-86 System Guide.

External links

• CP/M Internals - In-depth description of CP/M zero page functions