Hardware Reference
In-Depth Information
Processors with 64-bit extension technology can run in real (8086) mode, IA-32 mode, or IA-32e
mode. IA-32 mode enables the processor to run in protected mode and virtual real mode. IA-32e
mode allows the processor to run in 64-bit mode and compatibility mode, which means you can run
both 64-bit and 32-bit applications simultaneously. IA-32e mode includes two submodes:
•
64-bit mode
—Enables a 64-bit OS to run 64-bit applications
•
Compatibility mode
—Enables a 64-bit OS to run most existing 32-bit software
IA-32e 64-bit mode is enabled by loading a 64-bit OS and is used by 64-bit applications. In the 64-
bit submode, the following new features are available:
• 64-bit linear memory addressing
• Physical memory support beyond 4GB (limited by the specific processor)
• Eight new general-purpose registers (GPRs)
• Eight new registers for streaming SIMD extensions (MMX, SSE, SSE2, and SSE3)
• 64-bit-wide GPRs and instruction pointers
IE-32e compatibility mode enables 32-bit and 16-bit applications to run under a 64-bit OS.
Unfortunately, legacy 16-bit programs that run in virtual real mode (that is, DOS programs) are not
supported and will not run, which is likely to be the biggest problem for many users, especially those
that rely on legacy business applications or like to run very old games. Similar to 64-bit mode,
compatibility mode is enabled by the OS on an individual code basis, which means 64-bit
applications running in 64-bit mode can operate simultaneously with 32-bit applications running in
compatibility mode.
What we need to make all this work is a 64-bit OS and, more importantly, 64-bit drivers for all our
hardware to work under that OS. Although Microsoft released a 64-bit version of Windows XP, few
companies released 64-bit XP drivers. It wasn't until Windows Vista and especially Windows 7 x64
versions were released that 64-bit drivers became plentiful enough that 64-bit hardware support was
considered mainstream.
Note that Microsoft uses the term
x64
to refer to processors that support either AMD64 or EM64T
because AMD and Intel's extensions to the standard IA32 architecture are practically identical and
can be supported with a single version of Windows.
Note
Early versions of EM64T-equipped processors from Intel lacked support for the LAHF and
SAHF instructions used in the AMD64 instruction set. However, Pentium 4 and Xeon DP
processors using core steppings G1 and higher completely support these instructions; a BIOS
update is also needed. Newer multicore processors with 64-bit support include these
instructions as well.
3.5
.
Table 3.5. Windows Physical Memory Limits
