1. A programmable controller for executing a sequence program having sequential steps to produce corresponding execution results and for monitoring on a display the execution results of at least one of said steps, said programmable controller comprising: execution program storage means for storing at addressable storage locations a plurality of instruction execution programs used in executing corresponding sequence instructions included in the sequential steps constituting the sequence program, each of said instruction execution programs commencing at an associated head address corresponding to an addressable storage location in said execution program storage means; execution results storage means for storing at least one of the execution results of an executed sequence instruction; execution/monitor program storage means for storing in corresponding sequences of addressasble storage locations a plurality of instructions execution/monitor programs used in executing corresponding sequence instructions included in said sequence program and in storing a corresponding execution result of each sequence instruction into said execution result storage means, each of said instruction execution/monitor programs having associated therewith a head address corresponding to an address in said corresponding sequence of addressable storage locations; execution program address table storage means for storing said head addresses associated with at least some of said instruction execution programs to be executed in accordance with the steps of said sequence programs; monitor instruction storage means for storing a flag used to indicate that the execution result of a sequence instruction is to be monitored; monitored step position storage means for storing an index to indicate a step in said sequence program having a sequence instruction for which the corresponding execution result is to be monitored; detection means for detecting whether said monitor flag exists by referring to said monitor instruction storage means when said sequence program is executed; data insertion means, responsive to detection of said flag and to said index stored in said monitored step position storage means, for inserting in said execution program storage means, in place of the head address of sasid instruction execution program corresponding to the sequence instruction for which the execution result is to be monitored, the head address of the corresponding instruction execution/monitor program; processing means for processing said sequence program in accordance with the contents of said execution program address table storage means after said insertion in said execution program storage means of said heat address of the corresponding instruction execution/monitor program has been performed by said data insertion means; and display control means for causing the display to display the execution results store in said execution results storage means after said processing of said sequence instruction for which the execution result is to be monitored has been procoessed by said processing means.