18.3: Data Movement
- Page ID
- 19976
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Typically, data must be moved into a CPU floating-point register in order to be operated upon. Once the calculations are completed, the result may be copied from the register and placed into a variable. There are a number of simple formulas in the example program that perform these steps. This basic data movement operations are performed with the move instruction.
The general form of the move instruction is:
movss <dest>, <src>
movsd <dest>, <src>
For the movss instruction, a single 32-bit source operand is copied into the destination operand. For the movsd instruction, a single 64-bit source operand is copied into the destination operand. The value of the source operand is unchanged. The destination and source operand must be of the correct size for the instruction (32 or 64-bits). Neither operand can be an immediate value. Both operands, cannot be memory, however one can be. If a memory to memory operation is required, two instructions must be used.
The move instruction loads one value, using the lower 32 or 64-bits, into or out of the register. Other move instructions are required to load multiple values.
The floating-point move instructions are summarized as follows:
|
Instruction |
Explanation |
movss <dest>, <src> |
Copy 32-bit source operand to the 32-bit destination operand. |
|
Examples: |
movss xmm0, dword [x] movss dword [fltSVar], xmm1 movss xmm3, xmm2 |
movsd <dest>, <src> |
Copy 64-bit source operand to the 64-bit destination operand. |
|
Examples: |
movsd xmm0, qword [y] movsd qword [fltDVar], xmm1 movsd xmm3, xmm2 |
A more complete list of the instructions is located in Appendix B. For example, assuming the following data declarations:
fSVar1 dd 3.14
fSVar2 dd 0.0
fDVar1 dq 6.28
fDVar2 dq 0.0
To perform the basic operations of:
fSVar2 = fSVar2 ; single precision variables
fDVar2 = fDVar1 ; double precision variables
The following instructions could be used:
movss xmm0, dword [fSVar1]
movss dword [fSVar2], xmm0 ; fSVar2 = fSVar1
movsd xmm1, qword [fDVar1]
movsd qword [fDVar2], xmm1 ; fDVar2 = fDVar1
movss xmm2, xmm0 ; xmm2 = xmm0 (32-bit)
movsd xmm3, xmm1 ; xmm3 = xmm1 (64-bit)
For some instructions, including those above, the explicit type specification (e.g., byte, word, dword, qword) can be omitted as the other operand will clearly define the size. It is included for consistency and good programming practices.