Computer Architecture test (Intermediate)

Sample questions

5 sample questions from the Computer Architecture test, with the answers. Samples are taken out of the live test, so you will not meet these when you sit it.

1. A new instruction set extension is being developed for an existing processor architecture. Which of the following is a valid concern that must be addressed in the design of the extension?

   1. The extension adds new instructions without affecting existing ones.
   2. The extension increases the clock speed of the processor.
   3. The extension decreases the number of registers available for use.
   4. The instruction set extension is not backward compatible with existing software.

   Answer: D. The instruction set extension is not backward compatible with existing software.

   When extending an instruction set architecture, backward compatibility with existing software is a critical consideration. If the extension is not backward compatible, it can lead to incompatibility issues with existing programs and tools. The other options are not directly related to the principles of instruction set extension and are not accurate in this context.

2. Which of the following accurately describes the instruction encoding formats used in RISC and CISC architectures?

   1. RISC and CISC architectures both use fixed-length instructions for simplicity.
   2. RISC and CISC architectures both use variable-length instructions for flexibility.
   3. RISC architectures typically use fixed-length instructions, while CISC architectures use variable-length instructions.
   4. CISC architectures use fixed-length instructions, while RISC architectures use variable-length instructions.

   Answer: C. RISC architectures typically use fixed-length instructions, while CISC architectures use variable-length instructions.

   RISC architectures use fixed-length instructions for simplicity and faster decoding, while CISC architectures use variable-length instructions to support a wider range of operations. This distinction is a key factor in the design of instruction sets and influences performance characteristics.

3. In a little-endian system, if the hexadecimal value 0x12345678 is stored at memory address A, what will be the value of x1 if it is loaded from address A?

   1. The value of x1 will be 0x56
   2. The value of x1 will be 0x12
   3. The value of x1 will be 0x78
   4. The value of x1 will be 0x34

   Answer: C. The value of x1 will be 0x78

   In little-endian systems, the least significant byte is stored at the lowest memory address. For the hexadecimal value 0x12345678, the least significant byte is 0x78, which will be stored at address A.

4. When a variable is not aligned to a 4-byte boundary in assembly code, what happens to its storage address?

   1. The address of the variable is aligned to a 4-byte boundary, and the value is stored in the same address.
   2. The address of the variable is aligned to a 1-byte boundary, and the value is stored in the same address.
   3. The address of the variable is aligned to a 2-byte boundary, and the value is stored in the same address.
   4. The address of the variable is aligned to a 4-byte boundary, and the value is stored in the next available aligned address.

   Answer: D. The address of the variable is aligned to a 4-byte boundary, and the value is stored in the next available aligned address.

   Memory alignment ensures that data is stored at addresses that are multiples of their size. For example, a 4-byte integer must be stored at an address that is a multiple of 4. If the variable is not already aligned, the compiler or hardware may store it in the next available aligned address to ensure proper alignment.

5. Which of the following best describes the difference between operand forwarding and stalling in pipelining?

   1. Operand forwarding is only used in RISC architectures, while stalling is used in CISC architectures.
   2. Operand forwarding and stalling are both used to resolve control hazards in the pipeline.
   3. Operand forwarding is used to resolve data hazards by directly passing results from one stage to another, while stalling is used when forwarding is not possible or would not improve performance.
   4. Operand forwarding is used to resolve structural hazards, while stalling is used to resolve control hazards.

   Answer: C. Operand forwarding is used to resolve data hazards by directly passing results from one stage to another, while stalling is used when forwarding is not possible or would not improve performance.

   Operand forwarding is a technique used in pipelining to resolve data hazards by passing results from earlier stages of the pipeline to later stages, avoiding the need for stalls. Stalls are used when forwarding is not possible or would not improve performance, such as in the case of a load-use hazard in a classic pipeline where a single stall cycle is needed even with forwarding. This distinction is crucial for optimizing pipeline efficiency.