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Vol. 2A 3-601
INSTRUCTION SET REFERENCE, A-M
MOV—Move
below). The segment descriptor data is obtained from the GDT or LDT entry for the
specified segment selector.
A NULL segment selector (values 0000-0003) can be loaded into the DS, ES, FS, and
GS registers without causing a protection exception. However, any subsequent
attempt to reference a segment whose corresponding segment register is loaded
with a NULL value causes a general protection exception (#GP) and no memory
reference occurs.
Loading the SS register with a MOV instruction inhibits all interrupts until after the
execution of the next instruction. This operation allows a stack pointer to be loaded
into the ESP register with the next instruction (MOV ESP, stack-pointer value)
before an interrupt occurs
1
. Be aware that the LSS instruction offers a more efficient
method of loading the SS and ESP registers.
When operating in 32-bit mode and moving data between a segment register and a
general-purpose register, the 32-bit IA-32 processors do not require the use of the
16-bit operand-size prefix (a byte with the value 66H) with this instruction, but most
assemblers will insert it if the standard form of the instruction is used (for example,
MOV DS, AX). The processor will execute this instruction correctly, but it will usually
require an extra clock. With most assemblers, using the instruction form MOV DS,
EAX will avoid this unneeded 66H prefix. When the processor executes the instruc-
tion with a 32-bit general-purpose register, it assumes that the 16 least-significant
bits of the general-purpose register are the destination or source operand. If the
register is a destination operand, the resulting value in the two high-order bytes of
the register is implementation dependent. For the Pentium 4, Intel Xeon, and P6
family processors, the two high-order bytes are filled with zeros; for earlier 32-bit
IA-32 processors, the two high order bytes are undefined.
In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R
prefix permits access to additional registers (R8-R15). Use of the REX.W prefix
promotes operation to 64 bits. See the summary chart at the beginning of this
section for encoding data and limits.
Operation
DEST ← SRC;
1. If a code instruction breakpoint (for debug) is placed on an instruction located immediately after
a MOV SS instruction, the breakpoint may not be triggered. However, in a sequence of instruc-
tions that load the SS register, only the first instruction in the sequence is guaranteed to delay
an interrupt.
In the following sequence, interrupts may be recognized before MOV ESP, EBP executes:
MOV SS, EDX
MOV SS, EAX
MOV ESP, EBP