1. A compiler system for converting an application source program into an executable program according to a predetermined instruction set architecture (ISA) that is executable by a general purpose processor implemented according to the predetermined ISA, in which the processor includes a programmable execution unit (PEU) that is programmable to execute a specified user-defined instruction (UDI) received by the processor for execution, said compiler system comprising: a PEU programming tool that converts a functional description of a processing operation to be performed by the PEU of the processor into programming information for programming the PEU to perform the processing operation in response to the specified UDI; and a compiler that converts the application source program into the executable program, wherein said compiler comprises at least one optimization routine that represents a portion of the application source program with the specified UDI and that inserts the UDI into the executable program, and that further inserts into the executable program a UDI load instruction that specifies the UDI and said programming information, wherein the programming information comprises a latency statement indicating the latency of the processing operation by a number of clock cycles to complete the processing operation, wherein the processor further includes a UDI loader and an ISA table, the UDI loader programming the PEU for the specified UDI in response to the UDI load instruction by accessing the programming information, wherein the UDI loader accesses the programming information by a programming information address field corresponding to the specified UDI in the ISA table that points to a location of the programming information in memory.