What is MAKEWORD used for?

Question

I have come across this macro MAKEWORD(2,2) in a piece of instructional code. I read in MSDN that it "Creates a WORD value by concatenating the specified values."

The question is, isn't a WORD something like an unsigned integer why would I ever need to do such a strange procedure such as using MAKEWORD()?

Answer 1

The macro expects two bytes as its parameters:

WORD MAKEWORD(
  BYTE bLow,
  BYTE bHigh
);

Its defined in Windef.h as :

#define MAKEWORD(a,b)   ((WORD)(((BYTE)(a))|(((WORD)((BYTE)(b)))<<8)))

It basically builds a 16 bits words from two 1 bytes word (and doesn't look very portable)

The binary representation of the number 2 with 1 byte (a WORD) is : | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |

If we take the concatenate two of those bytes as in MAKEWORD(2,2) , we get:

| 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |

Which is 512 + 2 = 514 : live demo.

The only real life example of this particular macro is in the Initialization of Winsock, to generate the versioning word expected by WSAStartup.

Answer 2

Roughly speaking, MAKEWORD(x,y) is equivalent to ((y) << 8 | (x)); this is useful when packing two byte-sized values into a single 16-bit field, as often happens with general-purpose message structures. The complementary operation is performed by the LOBYTE and HIBYTE macros, which extracts the low- or high-order byte from a WORD operand.

The macro saw considerable use during the 16-bit days of Windows, but its importance declined once 32-bit programs came to dominance. Another vestige of 16-bit Windows lies in the names of the MSG structure members wParam and lParam, which were originally typed WORD and LONG respectively; they're both LONG now.

Trememdous historical insight can be found in Charles Petzold's tome, Programming Windows, second edition.