[panic]

The idea is to multiplex, not abstract. To illustrate the difference, say you have an OS that runs only applications written in JVM bytecode. This is an abstraction: the OS is providing a different interface (bytecode) than the actual hardware interface (machine code).

Most OSes don't do anything like this. They allow applications to be written in raw machine code. The applications run as if they had full control over the CPU hardware. The OS then multiplexes the CPU by saving and restoring the program counter (and, typically, the rest of the registers and CPU state as well, though the exokernel design in this paper doesn't even do that). The idea is to (as much as possible) provide the same interface as the underlying hardware provides, then do a little extra work to make sure different applications aren't stepping on each other's toes.

[heinrich5991]

It's more like the CPU has a multiplexing interface and the OS is managing that. Applications are not run in the same environment as the OS is.