The Program Segment Prefix (PSP) is a data structure used in DOS systems to store the state of a program. It has the following structure:
| Offset | Size | Contents |
| 00-01 | Code | CP/M exit (always contain INT 20) |
| 02-03 | Word | Memory size in paragraphs |
| 04 | Reserved | |
| 05-09 | Code | Far call to CP/M compatibility code within DOS |
| 0A-0D | DWord | Terminate address of previous program (old INT 22) |
| 0E-11 | DWord | Break address of previous program (old INT 23) |
| 12-15 | DWord | Critical error address of previous program (old INT 24) |
| 16-17 | Word | Parent psp segment (PSP of caller - usually command.com - internal) |
| 18-2B | Bytes | Job File Table(JFT) (internal) |
| 2C-2D | Word | Environment segment |
| 2E-31 | DWord | SS:SP on entry to last INT 21 call (Internal) |
| 32-33 | Word | Max open files (Internal - see below) |
| 34-37 | DWord | Handle-entries address (Internal - see below) |
| 38-4F | Reserved | |
| 50-52 | Code | Far call to DOS (always contain INT 21 + RETF) |
| 53-5B | Reserved | |
| 5C-6B | Unopened Standard FCB 1 | |
| 6C-7F | Unopened Standard FCB 2 (overwritten if FCB 1 is opened) | |
| 80 | Byte | Number of characters on command-line (excl. filename and trailing blanks but including leading blanks) |
| 81-FF | Bytes | Command-line (terminated by a 0Dh) |