direct INT 20h instruction or else calling INT 21h function 0. However, the programmer still had to ensure that the CS register contained the segment address of the PSP at program termination. Thus, jmp start mess1 db 'Hello world!$' start: mov dx,mess1 mov ah,9 int 21h int 20h In DOS 2.x and higher, program termination was accomplished instead with INT 21h function 4Ch which did not require the CS register to contain the segment value of the PSP. See also Zero page (CP/M) CALL 5 (DOS) Stack frame (Unix) Process directory (Multics) Process identifier (PID) this (computer programming) Self-reference References Further reading (41 pages) (1123+v pages, foldout, 5.25"-floppy) External links Accessing Command Line Arguments (Microsoft.com) DOS technology