Computer system having an instruction interception and substitution circuit

A diagnostics system for diagnosing a fault in a computer system, comprising:

• a bus controller coupled between a first bus and a second bus and capable of managing communication of signals between said first bus and said second bus;
• a memory, coupled to said second bus, capable of storing boot instructions;
• a central processing unit (CPU), coupled to said first bus, capable of issuing a signal over said first bus to fetch a requested one of said boot instructions from said memory unit when said CPU is booted;
• and
• a circuit, transparent to said CPU, within said bus controller capable of intercepting said signal and providing, in lieu of said requested one of said instructions, a diagnostic instruction that provides an external indication of whether said CPU is functioning properly when said CPU executes said diagnostic instruction no hardware modifications to operate in conjunction with said circuit.

A diagnostics system for diagnosing a fault in a computer system, comprising:

• a host bus capable of transmitting address and data signals at a relatively high speed;
• an I/O bus capable of transmitting address and data signals at a relatively low speed;
• a bus controller coupled between said host bus and said I/O bus, said bus controller managing communication of address and data signals between said host bus and said I/O bus and including bus controller memory for storing bus management instructions;
• an indicator port coupled to said host bus, said indicator port allowing said host bus to provide signals to an externally-readable indicator;
• read-only memory (ROM) coupled to said I/O bus, said ROM storing power-on self-test (POST) and basic input/output system (BIOS) instructions;
• a microprocessor coupled to said host bus, said microprocessor capable of retrieving said POST instructions from said ROM via said host bus, said bus controller and said I/O bus in series;
• a reset circuit capable of causing said microprocessor to boot, said microprocessor sending an initial request to retrieve one of said POST instructions from ROM;
• and
• a diagnostic circuit, transparent to said microprocessor, within said bus controller capable of intercepting said initial request and providing to said microprocessor, in lieu of said one of said POST instructions, a diagnostic instruction stored in said bus controller memory, that, when said microprocessor executes said diagnostic instruction, indicates whether said microprocessor and said host bus are functioning properly via said indicator port and said externally-readable indicator, said diagnostic circuit dormant with respect to subsequent requests to thereby allow said computer system to boot, said microprocessor requiring hardware modifications to operate in conjunction with said diagnostic circuit.

A method of diagnosing a fault in a computer system, comprising the steps of:

• booting a microprocessor within a computer system;
• issuing a retrieve instruction command from said microprocessor to retrieve a POST instruction from a read-only memory (ROM) within said computer system, said microprocessor coupled serially to said ROM via a host bus, a bus controller and an I/O bus;
• intercepting said retrieve instruction command with a diagnostic circuit, transparent to said microprocessor, within said bus controller, said diagnostic circuit substituting, in lieu of said POST instruction, a diagnostic instruction stored in said bus controller memory;
• executing said diagnostic instruction in said microprocessor, said diagnostic instruction causing said microprocessor to externally indicate whether said microprocessor and said host bus are functioning properly via an externally-readable indicator coupled to said host bus via an indicator port;
• and
• rendering said diagnostic circuit dormant with respect to subsequent requests to thereby allow said computer system to boot by retrieving said POST instructions, said microporcessor requiring no hardware modifications to operate in conjunction with said diagnostic circuit.