This is like (vdkc) except upon memory traps the domain's instruction address {right four bytes of PSW} would be stored at locations 12 through 15 and the 4 bytes at location 8 would be loaded into the domain's instruction address. The 6 byte trap code would be placed at location 16 through 21 and the domain's trap code set to 0.

Because only the low 4 bytes of the PSW are exchanged, the code at the new instruction address would have to be prepared to use the high bytes of the PSW as they are left by the instruction{s} accessing the segment.

This facility allows a program to retain control when it accesses an untrusted segment without having to pay the cost of a private domain keeper until it is needed.

Implementation Note:

Although VDK's are already somewhat private since they are local to switchers, there is a possibility of an unprompt segment keeper at location 0. This would disable the shared VDK unless the following steps are adopted.

It is OK for the VDK to be shared until the occurrence of a memory exception. This event is presumably rare enough so that a private keeper can then be afforded and established.

But then there would have to be a convention about what bank to use to create the private keeper, and what to do if it runs out of space.

The private keeper can even delete itself and restore the shared keeper when it has done its thing.