FUNCTION FREE_BYTES(UNIT : CHAR) : REAL;

TYPE REG_PACK = RECORD
                  AX,BX,CX,DX,BO,SI,DI,DS,ES,FLAGS : INTEGER
                END;

VAR REGISTERS : REG_PACK;
    UNIT_BYTE : BYTE;
    R,A,B,C   : REAL;

BEGIN
  UNIT := UPCASE(UNIT);
  IF UNIT IN ['A'..'D'] THEN   { A=1; B=2; C=3; D=4 else 0 }
    UNIT_BYTE := ORD(UNIT)-64
  ELSE UNIT_BYTE := 0;
  REGISTERS.DX := UNIT_BYTE;
  REGISTERS.AX := $36 * 256;   { Call # must go in hi byte }
  MSDOS(REGISTERS);
  { Call $36 returns # of bytes per sector in CX; }
  { # of sectors per cluster in AX, and # of free }
  { clusters in BX, so free space in bytes is     }
  { AX * BX * CX.  This must be done in reals to  }
  { avoid integer overflow. }
  A := REGISTERS.AX; B := REGISTERS.BX; C := REGISTERS.CX;
  R := A * B * C;
  FREE_BYTES := R
END;