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> Есть дока на интеле (ссылку не помню) 253665.pdf
> "Intel® 64 and IA-32 Architectures Software Developer’s Manual"
> Там много чего написано про 64 бита.
> в частности:
> "3.1.1 Intel® 64 Architecture
> Intel 64 architecture adds IA-32e mode. IA-32e mode has two sub-modes. These
> are:
> • Compatibility mode (sub-mode of IA-32e mode) — Compatibility mode
> permits most legacy 16-bit and 32-bit applications to run without re-compilation
> under a 64-bit operating system. For brevity, the compatibility sub-mode is
> referred to as compatibility mode in IA-32 architecture. The execution
> environment of compatibility mode is the same as described in Section 3.2.
> Compatibility mode also supports all of the privilege levels that are supported in
> 64-bit and protected modes. Legacy applications that run in Virtual 8086 mode or
> use hardware task management will not work in this mode.
> Compatibility mode is enabled by the operating system (OS) on a code segment
> basis. This means that a single 64-bit OS can support 64-bit applications running
> in 64-bit mode and support legacy 32-bit applications (not recompiled for
> 64-bits) running in compatibility mode.
> Compatibility mode is similar to 32-bit protected mode. Applications access only
> the first 4 GByte of linear-address space. Compatibility mode uses 16-bit and 32-
> bit address and operand sizes. Like protected mode, this mode allows applications
> to access physical memory greater than 4 GByte using PAE (Physical
> Address Extensions).
> • 64-bit mode (sub-mode of IA-32e mode) — This mode enables a 64-bit
> operating system to run applications written to access 64-bit linear address
> space. For brevity, the 64-bit sub-mode is referred to as 64-bit mode in IA-32
> architecture.
> 64-bit mode extends the number of general purpose registers and SIMD
> extension registers from 8 to 16. General purpose registers are widened to 64
> bits. The mode also introduces a new opcode prefix (REX) to access the register
> extensions. See Section 3.2.1 for a detailed description.
> 64-bit mode is enabled by the operating system on a code-segment basis. Its
> default address size is 64 bits and its default operand size is 32 bits. The default
> operand size can be overridden on an instruction-by-instruction basis using a REX
> opcode prefix in conjunction with an operand size override prefix.
> REX prefixes allow a 64-bit operand to be specified when operating in 64-bit
> mode. By using this mechanism, many existing instructions have been promoted
> to allow the use of 64-bit registers and 64-bit addresses."

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