x86 - how is machine-virtualization achieved without hardware support -

this reference machine virtualization. going through virtualization , got know hardware assisted virtualization technique, privileged instructions identified trap-fault method , replaced equivalent user-level instructions on fly. how is/was virtualization achieved in absence of hardware support? prior intel vti or amd-v, how privileged instrutions trapped on fly software itself? everywhere :"binary translation" term used fine far replacing privileged instruction user instructions concerned how privileged instructions ran guest os identified virtualization tool(hypervisor/vmm)

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edit: people thinking question not show research effort , down-voting. these of papers went through

overview : https://www.vmware.com/pdf/virtualization.pdf

intel doc: https://software.intel.com/sites/default/files/m/d/4/1/d/8/an_introduction_to_virtualization.pdf

intorduction: http://www.kernelthread.com/publications/virtualization/

x86 virtualization http://en.wikipedia.org/wiki/x86_virtualization

comparison of hw sf virtualization: http://www.vmware.com/pdf/asplos235_adams.pdf

nuts , bolts: http://www.anandtech.com/show/2480

paravirtualization: http://en.wikipedia.org/wiki/paravirtualization

if has paper/source can answer question asked above , might have missed kindly respond.

in absence of hardware support, paravirtualization can used. guest operating systems modified instead of accessing hardware resources directly, calls made virtual machine manager (vmm) or hypervisor.

for example, guest operating system on x86 cannot allowed disable interrupts on actual cpu. instead, guest os makes call vmm simulate disabling interrupts.

another alternative native virtualization. in native virtualization, instructions of guest os , processes emulated. emulation layer allows privileged instructions cli handled virtualization software. native virtualization requires neither hardware support nor modifying guest os.