1. A control system for controlling the operation of a storage/retrieval (S/R) machine in a material handling system, the S/R machine being directed by the control system to perform an ordered sequence of movements between addressable stations in response to a control command for the stacking and/or retrieving of material units, the control system comprising: input means for operator communication of a control command specifying an S/R machine operation: programmable processing means operatively coupled with the input means and responsive to an operator control command for directing the S/R machine to perform an ordered sequence of movements corresponding to the specific control command, the programmable processing means being controlled by a stored program and including, memory means for storing program instructions, and central processing means operatively coupled with the memory means and responsive to the stored program for executing the instructions to cause the S/R machine to perform the specified S/R machine operation, the memory means comprising a first memory block having a host control program of instructions encoded in an, elementary program language, and a second memory block having a user application program of instructions encoded in a non-elementary program, the central processing means including first means operatively coupled with the first memory block and the second memory block and being responsive to a selected instruction in the first memory block for accessing selected instructions in the second memory block, the first memory block including second means operatively coupled with the first means for emulating the selected instructions from the second memory block, the central processing means including third means operatively coupled with the second means for producing control signals for the S/R machine in accordance with the selected instructions from the second memory block, said first memory block being physically separate from the second memory block and being a read only memory.