How is the implicit segment register of a near pointer determined?
In section 4.3 of Intel 64® and IA-32 Architectures Software Developer's Manual. Volume 1: Basic Architecture, it says:
A near pointer is a 32-bit offset ... within a segment. Near pointers are used for all memory references in a flat memory model or for references in a segmented model where the identity of the segment being accessed is implie开发者_运维百科d.
This leads me to wondering: how is the implied segment register determined?
I know that (%eip)
and displaced (%eip)
(e.g. -4(%eip)
) addresses use %cs
by default, and that (%esp)
and displaced (%esp)
addresses use %ss
, but what about (%eax)
, (%edx)
, (%edi)
, (%ebp)
etc., and can the implicit segment register depend also on the instruction that the memory address operand appears in?
Other than CS
and SS
register accesses that you mentioned, most other accesses will use the DS
segment register. A couple exceptions that I can recall:
- the string-oriented instructions will use the
ES
for the 'destination' (implicitly addressed by theEDI
register) segment register - memory accesses using the
EBP
register as a 'base address' register will use theSS
segment register. However, if theEBP
register is used as a scaled index register, the segment used will beDS
(unless the base address register isESP
)
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