GATE EC 2014 Set 4 — Question 52
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Digital Circuits → Computer Organization → Machine Instructions & Addressing
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Question
An 8085 microprocessor executes “STA 1234H” with starting address location 1FFEH (STA copies the contents of the Accumulator to the 16-bit address location). While the instruction is fetched and executed, the sequence of values written at the address pins is
Correct answer
(A) 1FH, 1FH, 20H, 12H
Solution
The instruction
STA 1234H is a 3-byte instruction. Its execution involves 4 machine cycles:1.Opcode Fetch: The microprocessor fetches the opcode from the starting address
1FFEH. The higher-order address pins will carry 1FH.2.Memory Read (Low Byte of Address): The microprocessor reads the lower byte of the target address (
34H) from the next memory location 1FFFH. The higher-order address pins will carry 1FH.3.Memory Read (High Byte of Address): The microprocessor reads the higher byte of the target address (
12H) from the next memory location 2000H. The higher-order address pins will carry 20H.4.Memory Write: The microprocessor writes the contents of the Accumulator to the target address
Thus, the sequence of values on is 1FH, 1FH, 20H, 12H.1234H. The higher-order address pins will carry 12H.Continue learning with Success Tracker
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