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The SID (System IDentification) register is comprised of several fields.
The format of the SID varies from one class of VAX processors to another.
Not all processor classes have serial number fields.
In ALL cases, the left most byte of information is the CPU class number. 

Please note the importance of the last statement.
By examining the left most byte the processor type can be determined.
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        The CPU class numbers and the SID formats are:

                  Processor                  Number
                  ---------                  ------
                  uVAX-I                        7
                  uVAX-II                       8
                  VAX-730                       3
                  VAX-750                       2
                  VAX-780, VAX-782, & VAX-785   1
                  VAX-8600                      4

       31 23 23 16 15  8 7   0
      +-----+-----+-----+-----+
      |     |     |           |    The uVAX I and uVAX II SID format.
      +-----+-----+-----+-----+
         ^     ^        ^
         |     |        +------ Not Used
         |     +--------------- Micro Code Revision
         +--------------------- Cpu Type (7 for uVAX-I, 8 for uVAX-II)

       31 23 23 16 15  8 7   0
      +-----+-----+-----+-----+
      |     |     |     |     |    The VAX-730 and VAX-750 SID format.
      +-----+-----+-----+-----+
         ^     ^     ^     ^
         |     |     |     +--- Hardware Revision Level
         |     |     +--------- Micro Code Revision Level
         |     +--------------- Not Used
         +--------------------- Cpu Type (3 for VAX-730, 2 for VAX-750)

                          NOTE: The VAX-725 is a VAX-730 processor.
                                The VAX-751 (OEM) is a VAX-750 processor.

       31 23 23 15 14 12 11  0
      +-----+-----+-----+-----+
      |     |     |     |     |   The VAX-780 and VAX-8600 SID formats.
      +-----+-----+-----+-----+
         ^     ^     ^     ^
         |     |     |     +--- Serial Number
         |     |     +--------- Plant (where CPU was manufactured)
         |     +--------------- Hardware ECO Level
         +--------------------- Cpu Type (1 for VAX-780, 4 for VAX-8600)

                          NOTE: The VAX-782 is a VAX-780 processor.
                                The VAX-785 is a VAX-780 processor,
                                although the high bit of the ECO is
                                always ON for 785's.
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