Consider a more realistic alternative:
Suppose a small program is permanently stored in the last few mailbox locations. A BRANCH instruction at location 00, also permanent, will start this program. This program will accept input values and will store them at consecutive mailbox locations, starting with mailbox 01. You may assume that these values represent the instructions and data of a user’s program to be executed. When a 999 is received as input data, the program jumps to location 01 where it will proceed to execute the values just entered.

The small program described here is known as a program loader, or, under certain circumstances as a bootstrap. Write a Little Man program loader. (It may be useful to remember that instructions and data are indistinguishable. Thus, instructions could be treated as if they were data, if necessary.)