1. Registers as Hardware “Bricks”

• Arithmetic operands in LEGv8 (and most architectures) must come from a limited set of registers, not directly from memory.
• Registers are hardware primitives—fast, directly built into the CPU, and also visible to programmers.
• You can think of registers as the “bricks” of computer construction.
• In LEGv8, each register is 64 bits wide, which is called a doubleword.
  • A word is 32 bits (another common size).
  • A doubleword = 64 bits = the size of each LEGv8 register.

2. Limited Number of Registers

• Most current architectures (including LEGv8) have 32 general-purpose registers.
• Arithmetic instructions in LEGv8 must choose all three operands from one of the 64-bit registers.

Design Principle 2: “Smaller is Faster.”

• A very large number of registers can slow down the clock because the clock signal takes longer to reach every register.
• Designers must balance:
  • Programs’ desire for more registers.
  • The designer’s desire to keep the clock cycle fast.
• Also, more registers means more bits needed in the instruction format.

Example from textbook:

• 5 registers → clock 1.4 GHz.
• 10 registers → clock 1.2 GHz.

3. Register Naming Convention in LEGv8

• Although you could refer to registers as numbers 0–31, LEGv8 uses X0–X30 plus XZR.
• XZR (register 31) is hardwired to zero.