Computer System Having a Central Processing Unit Responsive to the Identity of an Upgrade Processor

1. An upgrade central processing unit (CPU) comprising: interprocessor communication circuitry configured to transmit information to and to receive information from an original equipment manufacturer (OEM) processor, including information to allow the upgrade CPU and the OEM processor to operate cooperatively in a computer system in a multi-processor mode, wherein the interprocessor communication circuitry comprises a handshake serializer/deserializer that processes handshake signals to the OEM processor and from the OEM processor, the serializer/deserializer comprising: a shift register that receives and transmits a data word in parallel and receives and transmits a data word in serial; and a bidirectional buffer coupled to the shift register, comprising: a first buffer that is enabled when the bi-directional buffer is in an input configuration; and a second buffer that is enabled when the bi-directional buffer is in an output configuration; the bi-directional buffer transmitting and receiving handshake information via a line of a private bus between the upgrade CPU and the OEM processor.

2. The upgrade CPU of claim 1, further comprising: an instruction fetch unit; an instruction decode unit; an instruction execution unit; interrupt control circuitry coupled to the instruction fetch unit, the instruction decode unit, the instruction execution unit and to an interrupt bus of the computer system; and memory bus interface circuitry coupled to the instruction fetch unit, the instruction decode unit, the instruction execution unit and to a memory bus of the computer system.

3. An upgrade central processing unit (CPU) comprising: interprocessor communication circuitry configured to transmit information to and to receive information from an original equipment manufacturer (OEM) processor, including information to allow the upgrade CPU and the OEM processor to operate cooperatively in a computer system in a multiprocessor mode, wherein the interprocessor communication circuitry comprises a handshake serializer/deserializer that processes handshake signals to the OEM processor and from the OEM processor, the serializer/deserializer comprising: a shift register that receives and transmits a data word in parallel and receives and transmits a data word in serial; and a bi-directional buffer coupled to the shift register, comprising: a first buffer that is enabled when the bidirectional buffer is in an input configuration; a second buffer that is enabled when the bi-directional buffer is in an output configuration, wherein the bidirectional buffer transmits and receives handshake information via a line of a private bus between the upgrade CPU and the OEM processor, and wherein the serializer/deserializer further comprises a selector that determines whether the line of the private bus conveys handshake information or other information.

4. An upgrade central processing unit (CPU) comprising: interprocessor communication circuitry configured to transmit information to and to receive information from an original equipment manufacturer (OEM) processor, including information to allow the upgrade CPU and the OEM processor to operate cooperatively in a computer system in a multiprocessor mode, wherein the interprocessor communication circuitry comprises a handshake serializer/deserializer that processes handshake signals to the OEM processor and from the OEM processor, the serializer/deserializer comprising: a shift register that receives and transmits a data word in parallel and receives and transmits a data word in serial; and a bidirectional buffer coupled to the shift register, comprising: a first buffer that is enabled when the bi-directional buffer is in an input configuration; a second buffer that is enabled when the bidirectional buffer is in an output configuration, wherein the bi-directional buffer transmits and receives handshake information via a line of a private bus between the upgrade CPU and the OEM processor, and wherein the serializer/deserializer further comprises a selector that determines whether the line of the private bus conveys handshake information or other information, wherein the serializer/deserializer further comprises an input for receiving a most recent master (MRM) signal that configures the bi-directional buffer according to which of the OEM processor and upgrade processor has ownership of a memory bus of the computer system.

5. An upgrade central processing unit (CPU) comprising: interprocessor communication circuitry configured to transmit information to and to receive information from an original equipment manufacturer (OEM) processor, including information to allow the upgrade CPU and the OEM processor to operate cooperatively in a computer system in a multiprocessor mode, wherein the interprocessor communication circuitry comprises a handshake serializer/deserializer that processes handshake signals to the OEM processor and from the OEM processor, the serializer/deserializer comprising: a shift register that receives and transmits a data word in parallel and receives and transmits a data word in serial; and a bi-directional buffer coupled to the shift register, comprising: a first buffer that is enabled when the bidirectional buffer is in an input configuration; a second buffer that is enabled when the bidirectional buffer is in an output configuration, wherein the bi-directional buffer transmits and receives handshake information via a line of a private bus between the upgrade CPU and the OEM processor, and wherein the serializer/deserializer further comprises a selector that determines whether the line of the private bus conveys handshake information or other information, wherein the other information includes a cache hit signal transmitted to maintain cache coherency in the multi-processor mode.