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Questions and Answers
Which bits in the instruction code format are used to recognize the type of instruction?
Which bits in the instruction code format are used to recognize the type of instruction?
- Bits 12 through 15 (correct)
- Bits 8 through 11
- The first four bits
- The last four bits
What is the significance of the bit in position 15 when the operation code is equal to 111?
What is the significance of the bit in position 15 when the operation code is equal to 111?
- It is not used as a mode bit (correct)
- It signifies the end of the instruction
- It is a mode bit for register-reference instructions
- It is designated as 'I' for input-output instructions
For register-reference instructions, what are the leftmost four bits always set to?
For register-reference instructions, what are the leftmost four bits always set to?
- 1111
- 0101
- 0111 (correct)
- 0010
What do the other three hexadecimal digits represent in register-reference instructions?
What do the other three hexadecimal digits represent in register-reference instructions?
What do the last four bits represent in input-output instructions?
What do the last four bits represent in input-output instructions?
What is the purpose of the Fetch cycle in a computer?
What is the purpose of the Fetch cycle in a computer?
Which instruction is used to add a memory word to the Accumulator (AC)?
Which instruction is used to add a memory word to the Accumulator (AC)?
What happens during the Decode cycle in a computer?
What happens during the Decode cycle in a computer?
Which instruction is used to branch unconditionally in a computer program?
Which instruction is used to branch unconditionally in a computer program?
What is the function of the INP instruction in a computer?
What is the function of the INP instruction in a computer?
Which cycle in a computer simply executes the fetched and decoded instruction?
Which cycle in a computer simply executes the fetched and decoded instruction?
What is the outcome of executing the CLA instruction?
What is the outcome of executing the CLA instruction?
Which register holds the instruction to be executed in a computer?
Which register holds the instruction to be executed in a computer?
What does the CME instruction do in a computer?
What does the CME instruction do in a computer?
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Study Notes
Computer Instructions
- The Basic Computer has three instruction code formats, each with 16 bits.
- The type of instruction is recognized by the computer control from the four bits in positions 12 through 15 of the instruction.
Instruction Code Formats
- Register-reference instructions use 16 bits to specify an operation, with the leftmost four bits always being 0111 (equivalent to hexadecimal 7).
- Input-output instructions also use all 16 bits to specify an operation, with the last four bits always being 1111 (equivalent to hexadecimal F).
Instruction Set Completeness
- A computer's instruction set is complete if it includes a sufficient number of instructions in the following categories: arithmetic, logical, and shift instructions; moving information to and from memory and processor registers; program control instructions; and input/output instructions.
Basic Computer Instructions
- Hex code and symbol for each instruction are given, with examples including AND, ADD, LDA, STA, BUN, BSA, ISZ, CLA, CLE, CMA, CME, CIR, CIL, INC, SPA, SNA, SZA, SZE, HLT, INP, OUT, SKI, SKO, ION, and IOF.
Instruction Fetch Execute Cycle
- The cycle consists of three stages: Fetch, Decode, and Execute.
- The Fetch stage retrieves an instruction from memory and stores it in the Instruction Register (IR).
- The Decode stage interprets the instruction, and the Execute stage performs the instruction.
Fetch Cycle
- The Fetch cycle begins with retrieving the address stored in the Program Counter (PC).
- The address stored in the PC is used to fetch the instruction from memory, which is then transferred to the Instruction Register (IR).
- The Program Counter is incremented to point to the next address.
Fetch Process
- The fetch process uses the Memory Address Register (MAR) and the content of the instructions in memory to transfer the instruction to the Instruction Register (IR).
- The steps involved in the Fetch process are:
- MAR ← PC
- Address_Bus ← MAR
- RD: Data_Bus ← M[I]
- IR ← Data_Bus
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