Early Chapter 2

Notes

• This is strange for me, you know quite a bit of this chapter
• But we need to look at the ARM/LEG specific portions.
• Word size
  • The word size on the ARM architecture is 32 bits,
  • But the machines we are using are 64 bits.
  • 64 bits is a double word.
  • Note, the ARM 8 can operate in 32 or 64 bit mode.
• Design Priciples:
  • In this chapter they give us some design priciples
  • The first Design Principle 1: Simplicity favors regularity
  • You see this by looking at Figure 2.1 page 64-65
    • Almost all of the instructions they present are
    • opcode destination source1, source2
    • I don't know ARM well enough to know that this is completely true
      • Remember the assembler may support pseudo ops
        • Or things in assembly that do not exist in machine code.
      • So, for example, the mov dest src instruction is not a machine instruction. (see below)
• Registers
  • The ARM/LEG architecture has 32 general purpose registers
  • X0-X31
    • But x31 is the zero register
    • also called xzr
    • It is always 0
      • You can copy anything into it and it stays 0
      • You can read from it, but it is always 0
    • Why?
      • We often need a source of zeros
      • Also, we may be able to eliminate instructions
        • Consider mov rd rs
        • With a zero register, is this needed?
        • add rd, rs, xzr
        • What other instructions might we be able to eliminate?
          • Any comparison to zero, which we do quite a bit
          • We will see more as time goes by.
      • Design Principal 2: Smaller is faster
        • In this case we are trying to reduce the number of op-codes needed.
        • How can this make things faster, we will see!
        • Patterson said in a paper "Dedicating a register to zero is surprisingly a large factor in simplifying the RISC-V ISA"
      • ARM Added the zero register in the 64 bit version.
  • See the table on page 109 for usage
    • x0-x7 are for parameters, scratch
    • x8 is for the assembler/compiler
    • x9-x15 are temporaries, scratch
    • x16-x18 are either temps or can be used by assembler/compiler, scratch
    • x19-27 are preserved
    • x28 is also sp, the stack pointer.
    • x29 is the fp
    • x30 is the link register
    • x31 is also xzr, the zero register.