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 alsoEdit

ReferencesEdit

  • "Tim Olmstead Memorial CP/M library". Archived from the original on 2017-08-20. Retrieved 2017-08-20. - in particular:

External linksEdit