Question

One of the most successful instruction sets is the x86 (including x86 and x86_64), which is used by processor products from both Intel and AMD. This instruction set began in the 1970s and has been updated many times since. A key design element enabling expansion has been its variable-length encoding of instructions. Instructions currently range from 1 to 17 bytes.

(a) Explain how variable-length instruction encoding complicates the steps of instruction execution (the downside).
(b) One additional advantage of variable-length encoding is that a clever design uses short encodings (a small num- ber of bytes) to represent frequent operations. Look up a description of the x86 instruction set on the Internet. What are some of the shortest instructions (one byte)?
(c) Suppose the shortest instructions accounted for 30 percent of the total Instructions, 2-byte Instructions ac- counted for 30 percent, 4-byte instructions for 30 percent, and 6-byte instructions for 10 percent. How would pro- gram size compare to a fixed 32-bit instruction format, such as used by RISC-V?

Answer 1

a) Variable-length instruction encoding adds complexity to the decoding process, slowing down instruction execution. b) Shortest x86 instructions include NOP, INC, and DEC. c) Smaller program size compared to a fixed 32-bit instruction format.

Step-by-step explanation:

a) Variable-length instruction encoding complicates the steps of instruction execution because the processor needs to determine the length of each instruction before it can be executed. This adds complexity to the decoding process and can slow down instruction execution.

b) In the x86 instruction set, some of the shortest instructions (one byte) include NOP (No Operation), INC (Increment), and DEC (Decrement).

c) If the shortest instructions accounted for 30% of the total instructions, then program size would be smaller compared to a fixed 32-bit instruction format like RISC-V. This is because shorter instructions require fewer bytes in memory, leading to lower memory usage and smaller programs.