Operation Code Field

M68000 (including MC68882)

• Many M68000 machine instructions can operate on byte, word, or long word data. The desired size is indicated as part of the instruction mnemonic by adding a trailing b, w, or l:

Mnumonic	Description
b	byte (8-bit) data
w	word (16-bit) data
l	long word (32-bit) data

For instance, a movb instruction moves a byte of data, but a movw instruction moves a 16-bit word of data. In general, the default size for data manipulation instructions on the 68030 and 68040 processors is 16-bit word.

• Many 68882 instructions (as well as built-in floating-point instructions on the 68040) can operate on other types of data besides byte, word, or long word integer data. Again, the size required is specified as part of the instruction mnemonic by a trailing letter:

Mnemonic	Description
s	single-precision (32-bit) floating-point data
d	double-precision (64-bit) floating-point data
x	extended-precision (96-bit) floating-point data
p	packed decimal (96-bit) floating-point data (note that the assembler currently doesn't support packed immediate formats)

Intel i386 Architecture

• As with the Motorola 68000 family, i386 instructions can operate on byte, word, or long word data (the last is called "double word" by Intel). The size can be indicated in the same way as it is for the MC68000. If no size is specified, the assembler attempts to determine the size from the operands. For example, if the 16-bit names for registers are used as operands, a 16-bit operation will be performed. When both a size specifier and a size-specific register name are given, the size specifier is used. Thus, the following are all correct and result in the same operation:

movw %bx,%cx
mov %bx,%cx
movw %ebx,%ecx

• An i386 operation code can also contain optional prefixes, which are separated from the operation code by a slash (`/') character. The prefix mnemonics are:

Prefix	Description
data16	operation uses 16-bit data
addr16	operation uses 16-bit addresses
lock	exclusive memory lock
wait	wait for pending numeric exceptions
cs , ds , es , fs , gs , ss	segment register override
rep , repe , repne	repeat prefixes for string instructions

More than one prefix may be specified for some operation codes. For example:

lock/fs/xchgl %ebx,4(%ebp)

Segment register overrides and the 16-bit data specifications are usually given as part of the operation code itself or of its operands. For example, the following two lines of assembly generate the same instructions:

movw %bx,%fs:4(%ebp)
data16/fs/movl %bx,4(%ebp)

Not all prefixes are allowed with all instructions. The assembler does check that the repeat prefixes for strings instructions are used correctly, but doesn't otherwise check for correct usage.