How do you compare the performance of superscalar and VLIW architectures?

1 Superscalar Architecture

A superscalar processor has multiple execution units that can process different types of instructions simultaneously. For example, it can perform arithmetic, logic, memory, and branch operations in parallel. The processor also has a complex hardware logic that dynamically schedules and dispatches the instructions to the available execution units based on their dependencies and availability. This allows the processor to exploit the instruction-level parallelism (ILP) in the code and increase the throughput.

3 Advantages of Superscalar

One of the main advantages of superscalar architecture is that it can adapt to the dynamic changes in the code and the data at runtime. For example, it can handle unpredictable branches, data dependencies, and cache misses by reordering or canceling the instructions as needed. It can also exploit the ILP in different types of code, such as loops, conditional statements, and function calls, without requiring any special compiler optimizations or annotations.

4 Advantages of VLIW

One of the main advantages of VLIW architecture is that it can achieve higher performance with less hardware complexity and power consumption. Since the compiler does the scheduling and dispatching of the instructions, the processor does not need any hardware logic for that purpose, which reduces the chip area, the clock cycles, and the energy consumption. It can also execute more instructions per cycle than a superscalar processor, as long as the compiler can find enough parallelism in the code.

5 Disadvantages of Superscalar

One of the main disadvantages of superscalar architecture is that it requires a lot of hardware resources and design efforts to implement the dynamic scheduling and dispatching logic. This increases the cost, the latency, and the power consumption of the processor. It also limits the scalability of the processor, as adding more execution units and instruction slots increases the complexity and the overhead of the hardware logic exponentially.

6 Disadvantages of VLIW

One of the main disadvantages of VLIW architecture is that it depends heavily on the compiler to find and exploit the ILP in the code. This means that the performance of the processor is sensitive to the quality and the compatibility of the compiler. It also means that the processor cannot adapt to the runtime variations in the code and the data, such as branches, dependencies, and cache misses, which may reduce the efficiency and the utilization of the execution units.