Position independent code location system

Abstract

Disclosed is a position independent code system that enables a programmable digital data processing system to invoke and execute procedures and access data that are loaded into system memory at addresses that are not specified by the program code and are not determined until the program code is compiled, linked and loaded into system memory. The coded call sequence instructions that are provided by the disclosed position independent code allow a procedure to call another procedure and are based upon the value that is stored in a dedicated register (or equivalent memory space) of the system and memory offset pointers, which are determined during system compilation and linking of program code and which indicate the memory location of the calling procedure and the call sequence of the calling procedure relative to the memory location at which the system stores the address of the procedure being called. When a call sequence is executed, the address of the called procedure is transferred to the dedicated register. Similar position independent code instructions are utilized for accessing a data set that is stored in memory external to the procedure, without transferring the address of the accessed data to the dedicated register. Compatibility between procedures that employ the disclosed position independent code and programmed code that is written for use with prior art position dependent code can be established by supplementing and modifying the position dependent code procedures during compilation and linking.