Increased systems level integration has meant the movement of many traditionally off chip components onto a single chip including a processor, instruction storage, data path, and local memory. The design of these systems are driven by two conflicting goals, the need for reduced area and the need for rapid development times. The two current design options for instruction storage, ROM and Flash, are each highly optimized to one of these two goals but provide little compromise between them. ROM is used for highly area optimized instruction memory to minimize area per instruction, although this comes at a price of lengthy integration time because of it's need to be correct before the chip is sent for fabrication. Flash is an alternative instruction memory that can significantly reduce the time to market by allowing embedded software to be upgraded after fabrication, which means that software test and fabrication can be overlapped. Unfortunately Flash takes over a factor of 2 times the area of the equivalent ROM based storage. In this paper we present the Patchable Instruction ROM as an architecture for instruction storage that can provide the best of both worlds -- reduced area and faster time to market. With area efficiency similar to a standard ROM and support for limited post fabrication software patching, Patchable Instruction ROM provides a new set of design points to consider when building embedded systems. For the programs we examine, we show that our hardware/software technique can achieve an area only 10% larger than ROM with only an 11% inflation in design time over a Flash based approach.
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