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  Function 33H (decimal 51) lets you test or update DOS's internal Ctrl-C
  flag. When you call function 33H with AL = 00H, DOS reports the current
  state of the Ctrl-C flag in DL:

  .  If the flag is clear (DL = 00H), DOS checks for Ctrl-C keystrokes only
     when transmitting a character to or from a character device (interrupt
     21H functions 00H through 0CH).

  .  If the flag is set (DL = 01H), DOS also checks for Ctrl-C when it
     responds to other service requests, such as file I/O operators.

  When you call function 33H with AL = 01H, DOS expects DL to contain the
  desired value for the break flag:

  .  DL = 00H disables the break check.

  .  DL = 01H enables the break check.