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A Word or Computer Word, is a term for the natural unit of data used by a particular computer architecture. Modern computers usually have a word size to be a power of 2 multiple of the unit of address resolution, likely a Byte, that is two, four, or eight Bytes, which are 16, 32, or 64 bits. Many other sizes have been used in the past, including 8 (a Byte), 9, 12, 18, 24, 36, 39, 40, 48, and 60 bits. Some of the early computers were decimal rather than binary, having a word size of 10 or 12 decimal digits, and some of them had no fixed word length at all.

Short

On recent 32-bit and 64-bit processors the primitive C datatype short and unsigned short refers to 16-bit words by most compilers for those architectures. In Java, short is guaranteed to have 16-bit. Signed short in C is assumed to use Twos' Complement, but not strictly specified. A Word-type, explicitly type-defined in C, is therefor usually treated as unsigned, also to avoid arithmetical right shift issues:

typedef unsigned char  BYTE;
typedef unsigned short WORD;

Ranges

| language | type | min | max | | --- | --- | --- | --- | | C, C++ | unsigned short | 0 | 65535 | | hexadecimal | 0x0000 | 0xFFFF | | #include <limits.h> | | USHRT_MAX | | C, C++,Java | short | -32768 | 32767 | | hexadecimal | 0x8000 | 0x7FFF | | #include <limits.h> | SHRT_MIN | SHRT_MAX |

Alignment

Words stored in memory should be stored at even byte addresses. Otherwise at runtime it will cause a miss-alignment exception on some processors, or a huge penalty on others.

Endianness

Main article: Endianness. An issue with words consisting of two or more bytes, is the order, bytes may appear inside a word of memory. According to their usual arithmetical significance, there is a low and a high byte of a 16-bit word, which may either be stored at the lower or higher byte-address in memory. Intel processors were always so called little-endian machines, the least significant byte (LSB) is at the lowest address. Other processors, including the IBM 370 family, the PDP-10 (36 bit), the Motorola microprocessor families, and most of the various RISC designs are big-endian, and store the ‘big-end-first’.

Extracting Bytes

Following C union to extract or synthesize bytes from/in words, is not portable and should be avoided.

union {
   BYTE b[2];
   WORD s;
} u;

u.s = 0xaa55;
assert (u.b[0] == 0x55); // fails, if big-endian

The portable way in C can be done with inlined functions or C preprocessor macros, using arithmetical divide or modulo by 256, aka shift and mask by bitwise 'and' - or for the synthesis multiplication of high byte by 256 plus low byte:

BYTE lowByte (WORD s) {return (BYTE)(s & 255);} // mod 256
BYTE highByte(WORD s) {return (BYTE)(s >>  8);} // div 256

WORD makeWORD (BYTE high, BYTE low) {
   WORD  s = high;
   return (s << 8) + low; // high * 256 + low
}

See also

External Links

John McLaughlin, Billy Cobham, Rick Laird, Jan Hammer, Jerry Goodman

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