text: The initial version of x86-64 (AMD64) did not allow for a software-only full virtualization due to the lack of segmentation support in long mode, which made the protection of the hypervisor's memory impossible, in particular, the protection of the trap handler that runs in the guest kernel address space. Revision D and later 64-bit AMD processors (as a rule of thumb, those manufactured in 90 nm or less) added basic support for segmentation in long mode, making it possible to run 64-bit guests in 64-bit hosts via binary translation. Intel did not add segmentation support to its x86-64 implementation (Intel 64), making 64-bit software-only virtualization impossible on Intel CPUs, but Intel VT-x support makes 64-bit hardware assisted virtualization possible on the Intel platform.
  On some platforms, it is possible to run a 64-bit guest on a 32-bit host OS if the underlying processor is 64-bit and supports the necessary virtualization extensions.
  Hardware-assisted virtualization
  In 2005 and 2006, Intel and AMD (working independently) created new processor extensions to the x86 architecture. The first generation of x86 hardware virtualization addressed the issue of privileged instructions. The issue of low performance of virtualized system memory was addressed with MMU virtualization that was added to the chipset later.
  Central processing unit
  Virtual 8086 mode
  Based on painful experiences with the 80286 protected mode, which by itself was not suitable enough to run concurrent DOS applications well, Intel introduced the virtual 8086 mode in their 80386 chip, which offered virtualized 8086 processors on the 386 and later chips. Hardware support for virtualizing the protected mode itself, however, became available 20 years later.
  AMD virtualization (AMD-V)
  AMD developed its first generation virtualization extensions under the code name "Pacifica", and initially published them as AMD Secure Virtual Machine (SVM), but later marketed them under the trademark AMD Virtualization, abbreviated AMD-V.