Intel 64 and IA-32 Architectures Software Developers Manual Volume 2A, Instruction Set Reference, A-M

Vol. 2 1-5
ABOUT THIS MANUAL
• When loading a register, always load the reserved bits with the values indicated
in the documentation, if any, or reload them with values previously read from the
same register.
NOTE
Avoid any software dependence upon the state of reserved bits in
IA-32 registers. Depending upon the values of reserved register bits
will make software dependent upon the unspecified manner in which
the processor handles these bits. Programs that depend upon
reserved values risk incompatibility with future processors.
1.3.3 Instruction Operands
When instructions are represented symbolically, a subset of the IA-32 assembly
language is used. In this subset, an instruction has the following format:
label: mnemonic argument1, argument2, argument3
where:
• A label is an identifier which is followed by a colon.
• A mnemonic is a reserved name for a class of instruction opcodes which have
the same function.
• The operands argument1, argument2, and argument3 are optional. There may
be from zero to three operands, depending on the opcode. When present, they
take the form of either literals or identifiers for data items. Operand identifiers
are either reserved names of registers or are assumed to be assigned to data
items declared in another part of the program (which may not be shown in the
example).
When two operands are present in an arithmetic or logical instruction, the right
operand is the source and the left operand is the destination.
For example:
LOADREG: MOV EAX, SUBTOTAL
In this example, LOADREG is a label, MOV is the mnemonic identifier of an opcode,
EAX is the destination operand, and SUBTOTAL is the source operand. Some
assembly languages put the source and destination in reverse order.
1.3.4 Hexadecimal and Binary Numbers
Base 16 (hexadecimal) numbers are represented by a string of hexadecimal digits
followed by the character H (for example, F82EH). A hexadecimal digit is a character
from the following set: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F.