Here is the specification for the instructions that work off runtime and operand stack.

LoadEnv$($identifier$)$: The instruction pops one value v off the operand stack and pushes the pair $($identifier$,$ v$)$ on the runtime stack.

StoreEnv$($identifier$)$: The instruction scans the runtime stack starting from the top until it finds the first value of the form $($identifier$,$ v$).$ It pushes v onto the operand stack. If in this process, it reaches the end of the stack without finding an entry corresponding to identifier, throw an appropriate exception.

PopEnv: pop the top element off the runtime stack.