If you look in System.pas, you won’t find a High() or Low()  function that is declared anything like what you would expect.  These functions and procedures are called compiler “magic” functions.

This list enumerate the intrinsic routines exposed by the System unit. These intrinsic routines are actually handled by the compiler rather then the run-time library.

Original list http://docwiki.embarcadero.com/RADStudio/en/Delphi_Intrinsic_Routines

System.Abs	Returns an absolute value.
System.Addr	Returns a pointer to a specified object.
System.Append	Prepares an existing file for adding text to the end.
System.Assert	Tests whether a Boolean expression is true.
System.Assigned	Tests for a nil (unassigned) pointer or procedural variable.
System.Assign	Associates the name of an external file with a file variable.
System.AssignFile	Associates the name of an external file with a file variable.
System.BlockRead	Reads one or more records from an open file into a variable.
System.BlockWrite	Writes one or more records from a variable to an open file.
System.Break	Causes the flow of control to exit a for, while, or repeat statement.
System.Chr	Returns the character for a specified ASCII value.
System.Close	Terminates the association between a file variable and an external file.
System.CloseFile	Terminates the association between file variable and an external disk file.
System.Concat	Concatenates two or more strings into one.
System.Continue	Allows the flow of control to proceed to the next iteration of for, while, or repeat statements.
System.Copy	Returns a substring of a string or a segment of a dynamic array.
System.Dec	Decrements a variable by 1 or N.
System.Default	Returns the default value for generic type.
System.Delete	Removes a substring from a string.
System.Dispose	Releases memory allocated for a dynamic variable.
System.Eof	Tests whether the file position is at the end of a file.
System.Eoln	Tests whether the file pointer is at the end of a line.
System.Erase	Deletes an external file.
System.Exclude	Removes an element from a Delphi set.
System.Exit	Exits from the current procedure.
System.FilePos	Returns the current file position.
System.FileSize	Returns the number of records in a file.
System.FillChar	Fills contiguous bytes with a specified value.
System.Finalize	Uninitializes a dynamically allocated variable.
System.Flush	Empties the buffer of a text file opened for output.
System.FreeMem	FreeMem frees a memory block.
System.GetMem	GetMem allocates a memory block.
System.Halt	Initiates abnormal termination of a program.
System.Hi	Returns the high-order byte of X as an unsigned value.
System.High	Returns the highest value in the range of an argument.
System.Inc	Increments an ordinal value by one or N.
System.Include	Adds an element to a Delphi set.
System.Initialize	Initializes a dynamically allocated variable.
System.Insert	Inserts a substring into a string beginning at a specified point.
System.Length	Returns the number of characters in a string or elements in an array.
System.Lo	Returns the low order Byte of argument X.
System.Low	Returns the lowest value in a range.
System.New	Creates a new dynamic variable and sets P to point to it.
System.Odd	Returns true if argument is an odd number.
System.Pi	Returns 3.1415926535897932385.
System.Pred	Returns the predecessor of the argument.
System.Ptr	Converts a specified address to a pointer.
System.Read	Read reads data from a file.
System.ReadLn	Reads a line of text from a file.
System.ReallocMem	ReallocMem reallocates a memory block.
System.Rename	Changes the name of an external file.
System.Reset	Opens an existing file.
System.Rewrite	Creates a new file and opens it.
System.Round	Returns the value of X rounded to the nearest whole number.
System.RunError	Stops execution and generates a runtime error.
System.Seek	Moves the current position of a file to a specified component.
System.SeekEof	Returns the end-of-file status of a file, ignoring whitespace.
System.SeekEoln	Returns the end-of-line status of a file, ignoring whitespace.
System.SetLength	Sets the length of a string or dynamic-array variable.
System.SetString	Sets the contents and length of the given string.
System.SizeOf	Returns the number of bytes occupied by a variable or type.
System.Slice	Returns a sub-section of an array.
System.Sqr	Returns the square of a number.
System.Str	Formats a string and returns it to a variable.
System.Succ	Returns the successor of an argument.
System.Swap	Exchanges high order byte with the low order byte of an integer or word.
System.Trunc	Truncates a real number to an integer.
System.TypeHandle	Returns the RTTI information for a given type.
System.TypeInfo	Returns the RTTI information for a given type.
System.TypeOf	Deprecated routine.
System.Val	Converts a string to a numeric representation.
System.VarCast	Converts a variant to specified type.
System.VarCopy	Copies a Variant to another Variant.
System.Write	Writes to either a typed file or a text file
System.WriteLn	Writes to a text file and adds an end-of-line marker.