Zero page (CP/M)
The Zero 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, a 256-byte structure, which is by default located exactly before offset 0 of the program's load segment, rather than in segment 0. A segment register is initialised to 0x10 less than the code segment, in order to address it.
In 8-bit CP/M, it has the following structure:
| Offset | Size | Contents |
| 00–02 | Code | Exit program. |
| 03 | Byte | I/O byte, an optional feature allowing device reassignment in CP/M 2. |
| 04 | Byte | Current command processor drive and user number. |
| 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–3F | Bytes | Reserved |
| 40–4F | Bytes | Reserved for use by the BIOS |
| 50 | Byte | The drive from which the program was loaded |
| 51–52 | Word | Address of the password for the first FCB |
| 53 | Byte | Length of the password for the first FCB |
| 54–55 | Word | Address of the password for the second FCB |
| 56 | Byte | Length of the password for the second FCB |
| 57–5B | Bytes | Reserved |
| 5C–6B | Default FCB 1 | |
| 6C–7F | Default FCB 2 | |
| 80 | Byte | Number of characters in command tail. |
| 81–FF | Bytes | Command tail. |
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 |
| 51–52 | Word | Address of the password for the first FCB |
| 53 | Byte | Length of the password for the first FCB |
| 54–55 | Word | Address of the password for the second FCB |
| 56 | Byte | Length of the password for the second FCB |
| 57–5B | Bytes | Reserved |
| 5C–6B | Default FCB 1 | |
| 6C–7F | Default FCB 2 | |
| 80 | Byte | Number of characters in command tail. |
| 81–FF | Bytes | Command tail. |