An electronic system such as an imaging system may include processing circuitry and memory circuitry. Data replacement circuitry may be interposed between the processing circuitry and the memory circuitry. In some implementations, the memory circuitry may be a read-only memory, and data replacement circuitry may be used to selectively replace executable firmware instructions stored on the read-only memory. The selective replacement operations may be based on an address that processing circuitry provides to access the memory circuitry. The data replacement circuitry may be implemented separately from the processing circuitry and the memory circuitry and may include a comparator block, registers, and switching circuitry.