Data Movement

Objectives

1. Formalize moving data about in an assembly program.

Notes

• This is Jorgensen 7.2 (pdf absolute page 87)
• mov dest, src
  • The dest and src arguments must be the same size.
  • And the destination can not be an immediate.
  • You can not move from memory to memory.
    • You must move into a register from memory address 1,
    • Then move into memory address 2 from that register.
  • You can use the following words to specify types
    • byte
    • word
    • dword
    • qword
  • These are used when moving from or to memory
    • When moving labels
    • label copies the address
    • [label] dereferences the address
  • See mover.asm.
• We can also use lea to load an address
  • Load effective address
  • lea reg, mem
• We will discuss memory access further in the future.
• As we have to be careful (as we see in mover) accessing different parts of the register
• Narrowing
  • You narrow a value when you move a larger value into a smaller location

  • mov rax, something_bigger_than_28
    mov some_byte_location, al
           

  • This will result in the upper bits of the number being truncated.
• Widening is when you move a smaller value into a larger location
  • We see this in mover.asm
  • And depening on the operation, the upper bits might not be set right.
• Both narrowing and widening can lead to error.
• Fixing this
  • Positive Widening 1
    • Fill the 64 bit register with the proper sign
    • Move the lower bits into the proper location.
  • Positive Widening 2
    • movzx
    • This only works for 8 and 16 bit,
    • 32 bit already does this.
    • And there is no need in the 64 bit case.