Intel ARCHITECTURE IA-32 - Manual (Page 35)

IA-32 Intel® Architecture Processor Family Overview
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Intel ® Extended Memory 64 Technology
(Intel ® EM64T)
Intel EM64T is an extension of the IA-32 Intel architecture. Intel
EM64T increases the linear address space for software to 64 bits and
supports physical address space up to 40 bits. The technology also
introduces a new operating mode referred to as IA-32e mode.
IA-32e mode consists of two sub-modes: (1) compatibility mode
enables a 64-bit operating system to run most legacy 32-bit software
unmodified, (2) 64-bit mode enables a 64-bit operating system to run
applications written to access 64-bit linear address space.
In the 64-bit mode of Intel EM64T, software may access:
• 64-bit flat linear addressing
• 8 additional general-purpose registers (GPRs)
• 8 additional registers for streaming SIMD extensions (SSE, SSE2
and SSE3)
• 64-bit-wide GPRs and instruction pointers
• uniform byte-register addressing
• fast interrupt-prioritization mechanism
• a new instruction-pointer relative-addressing mode
For optimizing 64-bit applications, the features that impact software
optimizations include:
• using a set of prefixes to access new registers or 64-bit register
operand
• pointer size increases from 32 bits to 64 bits
• instruction-specific usages