10.1 Processor State After Reset

The contents of EAX depend upon the results of the power-up self test. The self-test may be requested externally by assertion of BUSY# at the end of RESET. The EAX register holds zero if the 80386 passed the test. A nonzero value in EAX after self-test indicates that the particular 80386 unit is faulty. If the self-test is not requested, the contents of EAX after RESET is undefined.

DX holds a component identifier and revision number after RESET as Figure 10-1 illustrates. DH contains 3, which indicates an 80386 component. DL contains a unique identifier of the revision level.

Control register zero (CR0) contains the values shown in Figure 10-2. The ET bit of CR0 is set if an 80387 is present in the configuration (according to the state of the ERROR# pin after RESET). If ET is reset, the configuration either contains an 80287 or does not contain a coprocessor. A software test is required to distinguish between these latter two possibilities.

The remaining registers and flags are set as follows:

   EFLAGS             =00000002H
   IP                 =0000FFF0H
   CS selector        =000H
   DS selector        =0000H
   ES selector        =0000H
   SS selector        =0000H
   FS selector        =0000H
   GS selector        =0000H
   IDTR:
              base    =0
              limit   =03FFH

These settings imply that the processor begins in real-address mode with interrupts disabled.

Figure 10-1. Contents of EDX after RESET

                                 EDX REGISTER

      31               23               15               7              0
     +----------------+----------------+----------------+----------------+
     |#################################|       DH       |      DL        |
     |############UNDEFINED############|   DEVICE ID    |  STEPPING ID   |
     |#################################|       3        |   (UNIQUE)     |
     +----------------+----------------+----------------+----------------+

Figure 10-2. Initial Contents of CR0

                               CONTROL REGISTER ZERO

   31                23                15                  7     4 3   1  0
  +-+---------------+-----------------+-------------------+-----+-+-+-+-+-+
  |P|                                                           |E|T|E|M|P|
  | |                            UNDEFINED                      | | | | | |
  |G|                                                           |T|S|M|P|E|
  +++---------------+-----------------+-------------------+-----+++++++++++
   |                                                             | | | | |
   +-------------0 - PAGING DISABLED                             | | | | |
                 * - INDICATES PRESENCE OF 80387-----------------+ | | | |
                 0 - NO TASK SWITCH--------------------------------+ | | |
                 0 - DO NOT MONITOR COPROCESSOR----------------------+ | |
                 0 - COPROCESSOR NOT PRESENT---------------------------+ |
                 0 - PROTECTION NOT ENABLED (REAL ADDRESS MODE)----------+