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Definition: VT


(1) See Video Toaster.

(2) (Virtualization Technology) Circuits in Intel CPU chips that add virtual machine instructions (VT-x for x86; VT-i for Itanium). Introduced in Pentium chips in 2005, VT circuits eliminate the need to modify (paravirtualize) the guest OS to achieve maximum performance. See paravirtualization.

VT enables the virtual machine monitor (VMM) to run at the most privileged level, and it accelerates transitions between the VMM and guest OS. It traps the calls to the hardware from the guest OS, saves the CPU state and restores it after the VMM handles the event.

VT for Directed I/O (VT-d)
Introduced in 2008, VT for Directed I/O (VT-d) enables direct memory access (DMA) transfers between devices and guest OS memory without using the VMM as an interim stage. This allows network and graphics cards to be assigned to specific virtual machines for increased performance. For example, a second graphics card can be assigned to a VM running a CAD application, which must address the hardware directly, or a second network adapter can be assigned to a VM running a high-priority network application. See AMD-V, virtual machine and hardware virtualization.

(3) (Virtual Tributary) In SONET, a sub STS-1 rate, which is below 52 Mbps. A VT allows SONET to multiplex lower-capacity channels down to 64 Kbps, an important characteristic because SONET is used both as a carrier infrastructure and a customer-premises based service. A VT can operate at four speeds: 1.728 Mbps, 2.304 Mbps, 3.456 Mbps and 6.912 Mbps. A mixture of different capacity VTs can be combined into a VT Group and carried in a single STS-1. See OC.