Process VM - OS Emulation

When guest and host OS are the same, the problem of OS call emulation is primarily one of matching the OS interface syntax. The OS functions required by the guest are available in the host, but it may be necessary to move and format arguments and return values, possibly forming some data covnersion in the process. For example, OS running on platform with few register may pass parameter in stack while OS with many registers may pass arguments in registers. In this case, the call must be set up by copying arguments from stack to registes when emulating a system call. This is call wrapper.

Some call may be handled by the runtime code instead of transalation. For example, a call to establish signal. In this case, the runtime code will own all signal and so the request from the guest will be marked in a side table instead. Runtime will pass on the signal if it matches the side table. Another example is memory management call.

Pracitically, if the guest and host OS are different, there is relatively little likelihood that a complete compatible OS emulation can be performed. Some compromise and approximations will be required. A compromise often used is to restrict the application supported (thus limiting the system calls).