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 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).Famous quotes containing the word page:
“There is then creative reading as well as creative writing. When the mind is braced by labor and invention, the page of whatever book we read becomes luminous with manifold allusion. Every sentence is doubly significant, and the sense of our author is as broad as the world.”
—Ralph Waldo Emerson (1803–1882)