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Processor Structure and Instruction Cycle
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Processor Structure and Instruction Cycle
Processor Structure and Instruction Cycle
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1
Question
What are the main requirements of a processor in executing instructions?
Answer
The processor must fetch instructions, interpret (decode) them, fetch data from memory or I/O if needed, process data by performing arithmetic or logical operations, write results back to memory or I/O, and store some data temporarily using internal memory.
2
Question
What are the major components of a processor?
Answer
The major components include the Arithmetic and Logic Unit (ALU) which performs computations, the Control Unit (CU) which manages data and instruction flow and controls the ALU, and registers which are small storage locations within the processor.
3
Question
How are data transferred within the CPU?
Answer
Data are transferred between various registers and the ALU using the internal processor bus, which connects these elements within the CPU's internal structure.
4
Question
Compare the internal structure of the computer as a whole to the internal structure of the processor.
Answer
Both have a small collection of major elements connected by data paths: the computer includes processor, I/O, and memory; the processor includes control unit, ALU, and registers.
5
Question
What is the role of registers within a processor?
Answer
Registers serve as a small, fast memory above main memory and cache, supporting temporary data storage; they perform control and status functions and can be user-visible for program execution.
6
Question
What are the two roles of registers in the processor?
Answer
They function as control and status registers, used by the control unit and privileged programs, and as user-visible registers utilized by assembly language programmers to optimize memory references.
7
Question
Describe categories of user-visible registers in a processor.
Answer
User-visible registers include general purpose registers (used for various functions), data registers (hold data only, not for calculating operand addresses), address registers (may be general or specialized for addressing modes, e.g., segment pointers, index registers, stack pointer), and condition code registers (flags set by hardware after operations).
8
Question
What are condition codes or flags in a processor?
Answer
They are bits set by the processor hardware as the result of operations to indicate conditions such as zero result, carry, overflow, or equality, used for controlling program flow and decisions.
9
Question
What are the four essential control and status registers for instruction execution?
Answer
Program Counter (PC) holds the address of the next instruction to fetch; Instruction Register (IR) holds the most recently fetched instruction; Memory Address Register (MAR) holds the address for memory access; Memory Buffer Register (MBR) holds data to be written to or read from memory.
10
Question
What is the purpose of the Program Counter (PC)?
Answer
The PC contains the address of the instruction to be fetched next from memory.
11
Question
What does the Instruction Register (IR) do?
Answer
The IR contains the instruction most recently fetched from memory, ready for decoding and execution.
12
Question
What is the function of the Memory Address Register (MAR)?
Answer
The MAR contains the address of a location in memory to be read from or written to.
13
Question
What is stored in the Memory Buffer Register (MBR)?
Answer
The MBR stores a word of data to be written to memory or the word most recently read from memory.
14
Question
What is the Program Status Word (PSW) and what does it contain?
Answer
The PSW is a register or set of registers holding status information including flags such as sign, zero, carry, equal, overflow, interrupt enable/disable, and supervisor mode flag.
15
Question
Explain some common flags contained in the Program Status Word (PSW).
Answer
- Sign flag: sign bit of last arithmetic result - Zero flag: set if result is zero - Carry flag: set if addition results in carry or subtraction in borrow - Equal flag: set if logical compare results in equality - Overflow flag: indicates arithmetic overflow - Interrupt Enable/Disable: controls interrupt processing - Supervisor mode flag: indicates if processor operates in supervisor or user mode.
16
Question
Why is the supervisor mode flag important in the PSW?
Answer
It indicates whether the processor is running in supervisor (privileged) mode or user mode, controlling access to certain instructions and memory areas reserved for system-level operations.
17
Question
What are the register types and sizes in the MC68000 microprocessor?
Answer
The MC68000 has 32-bit registers, including 8 data registers and 9 address registers. It also has a 32-bit program counter and a 16-bit status register.
18
Question
How are data registers used in the MC68000 processor?
Answer
Data registers are primarily used for data manipulation and also serve as index registers for addressing.
19
Question
What is unique about the register organization in the Intel 8086 microprocessor?
Answer
The Intel 8086 uses 16-bit registers, with every register being special purpose. It includes 4 data registers, 4 pointer and index registers, and 4 segment registers.
20
Question
How does the Intel 80386 differ from the Intel 8086 in terms of registers?
Answer
The Intel 80386 is a 32-bit extension of the 8086, but there is no universally accepted philosophy concerning the best way to organize processor registers.
21
Question
What are the usual stages included in the instruction cycle?
Answer
The instruction cycle includes Fetch, Execute, and Interrupt stages.
22
Question
What happens during the interrupt stage of the instruction cycle if an interrupt has occurred and interrupts are enabled?
Answer
The current process state is saved, and the interrupt is serviced.
23
Question
What must be done after fetching an instruction if indirect addressing is involved?
Answer
Operands required by indirect addressing are fetched by performing memory access operations to retrieve those operands from memory.
24
Question
Describe the general steps during the fetch cycle of the instruction cycle.
Answer
1. Program Counter (PC) contains the address of the next instruction. 2. Address moved to Memory Address Register (MAR). 3. Address placed on address bus to identify the memory location. 4. Control unit sends a memory read signal. 5. Data retrieved from memory is placed on the data bus, copied to Memory Buffer Register (MBR), then to Instruction Register (IR). 6. Meanwhile, the PC is incremented by 1 to prepare for the next fetch.
25
Question
What is the purpose of the indirect cycle following the fetch cycle?
Answer
The indirect cycle fetches the effective address of the operand when the instruction uses indirect addressing, ensuring the operand can be accessed for execution.
26
Question
Outline the steps involved in the indirect cycle.
Answer
1. The rightmost N bits of the MBR are transferred to the MAR and placed on the address bus to identify the memory location. 2. The control unit sends a memory read signal. 3. The retrieved result (address of the operand) is moved to the MBR.
27
Question
Why are fetch and indirect cycles considered predictable?
Answer
Because they follow a fixed sequence of well-defined steps, making their behavior consistent and reliable during instruction processing.
28
Question
What is the first step in processing an interrupt in a CPU according to the described interrupt cycle?
Answer
The contents of the Program Counter (PC) are copied to the Memory Buffer Register (MBR).
29
Question
Why is the current Program Counter (PC) value saved during an interrupt cycle?
Answer
To allow resumption of the program after the interrupt has been handled.
30
Question
Describe the steps involved in processing an interrupt as outlined in the interrupt cycle.
Answer
1. Contents of PC copied to MBR. 2. Special memory location (e.g., stack pointer) loaded to MAR to identify stack location. 3. MBR written to memory (saving the PC). 4. PC loaded with the address of the interrupt handling routine. Then, the next instruction fetched is the first of the interrupt handler routine.