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80386 Microprocessor State After Reset

Last Updated : 03 Mar, 2023
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80386 microprocessor, often known as the i386, was a popular x86-based microprocessor. It is important to note that the state of the processor after reset is not only critical for the processor itself, but also for the entire system. The contents of the EAX, DX, and CR0 registers provide information about the status of the 80386 processor and the presence of coprocessors. This information is used by the system to determine its configuration and ensure proper operation.

It is important to understand the processor state after reset in order to properly configure and use the 80386 processor in a system. A well-designed system will take into account the specific characteristics of the 80386 processor state after resetting and use this information to ensure proper operation.

EAX Register: The contents of EAX depend upon the results of the power-up self-test. If the self-test is requested externally by asserting the BUSY# signal at the end of the RESET process, the EAX register holds a value of zero if the 80386 has passed the test. If a non-zero value is present in EAX after the self-test, it indicates that the 80386 unit is faulty. If the self-test is not requested, the contents of EAX after RESET are undefined.

Contents of EDX after Reset


DX Register: DX holds a component identifier and revision number after RESET. DH contains the value 3, which indicates that the component is 80386. DL contains a unique identifier of the revision level.

Initial contents of CR0


CR0 Register: Control register zero (CR0) contains specific values after reset. The ET bit of CR0 is set if an 80387 coprocessor is present in the configuration. The state of the ERROR# pin after RESET determines the presence of the 80387. If the ET bit is reset, the configuration either contains an 80287 coprocessor or does not contain a coprocessor at all. A software test is required to distinguish between these two possibilities

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

All registers not mentioned 

above are undefined.

The state of the processor after reset is critical to its operation and understanding the contents of specific registers after reset is important for system designers and software developers. The 80386 processor has specific characteristics in terms of its register contents after reset.

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