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 DOS is the Program Segment Prefix (PSP), a 256-byte structure, which, however, is by default located at offset 0 in the program's load segment (rather than in segment 0) immediately preceding a loaded program.

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	8080 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).

See also

• Zero page (processor property)
• Page boundary relocation

References

• "Tim Olmstead Memorial CP/M library". Archived from the original on 2017-08-20. Retrieved 2017-08-20. - in particular:
  • "CP/M 3 Programmers' Manual" (PDF). Archived (PDF) from the original on 2017-08-20. Retrieved 2017-08-20.
  • "CP/M-86 System Guide" (PDF). Archived (PDF) from the original on 2017-08-20. Retrieved 2017-08-20.

External links

• "CP/M Internals". Archived from the original on 2017-08-20. Retrieved 2017-08-20. - In-depth description of CP/M zero page functions