Abstract: A microcomputer system providing high performance access to external Special Function Registers (SFRs), has an 8051 architecture microcontroller modified such that the instruction stream can be externally examined and decoded by an external expansion decoder. The instruction stream can be examined and decoded regardless of whether the microcontroller fetches instructions from an internal program memory or an external program memory. Every State 6, Phase 2, data on the internal bus of the modified microcontroller is transferred to the PORT2 pins and is available to the external expansion decoder. During reset the microcontroller latches the state of the EA pin to internally determine whether to operate in ROM or ROMless mode.