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 |
| 81-FF | Bytes | Command-line (terminated by a 0Dh) |
The PSP is most often used to get the command line arguments of a DOS program, for example the command "foo.exe -a -f" executes foo.exe with the arguments '-a' and '-f'.
Code Samples
The segment address of the PSP can be determined by using interrupt 21 subfunction 62. This interrupt will return the PSP address in register BX.1
Alternatively, in COM programs, one can address the PSP directly just by using the offsets listed above. 00h points to the beginning of the PSP, FFh points to the end, etc. For example, the following code gets the length (in bytes) of the command line arguments:
org 100h mov al, byte [80h] ; al now contains the number of characters on the command-line (byte 80h of the PSP)
1 http://www.htl-steyr.ac.at/~morg/pcinfo/hardware/interrupts/inte8fjk.htm