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 , 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/ 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  (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......

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