An apparatus and method of speculatively decoding non-memory read commands. A command register and decoder, within the apparatus, compares high-order command bits provided on a serial bus with corresponding bits of recognized non-memory read commands. An early non-memory read command is asserted when incoming command bits match a non-memory read command. Early responsive data is prepared speculatively during the time the remainder of command bits is received and decoded. A determination of command speculation correctness is made after receipt of the full command. If the full command received is not the speculated non-memory read command, the prepared data is discarded. Earlier prepared data is produced as the subsystem response if the full command matches the speculative non-memory read command. For incoming commands with operands, such as an address, the same speculative determination based on high-order operand bits is performed.