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▀ Before a program receives control from MS-DOS, its program segment pre-
  fix is set up to contain certain vital information, such as:
  - the segment address of the program's environment block;
  - the command line originally entered by the user;
  - the original contents of the terminate, CTRL+C, and critical-error
    handler vectors;
  - and the top address of available RAM.
 
▀ The segment address of the PSP is normally passed to the program in reg-
  isters DS and ES when it initially receives control from MS-DOS. This
  function allows a program to conveniently recover the PSP address at any
  point during its execution, without having to save it at program entry.
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