title: What do we mean by instruction size?
body: <p>I am really confused and the question may sound dumb but I'm really not sure what it means when we say an instruction size is 32-bit or 16-bit.</p>
  <p>Is it if operand size is 32-bit then we say it's an 32-bit instruction and if operand size is 16-bit instruction.</p>
  <p>Or every instruction has opcode and if opcode address is 32-bit then we say instruction is 32-bit.</p>
  <p>I am also going through ARM architecture where I read all thumb instruction are of 16 bit but ARM mode instruction are 32-bit.</p>
  <p>Does instruction size totally depend upon the architecture in use or it depends on the modes in which the CPU operates?</p>
  <h2>EDIT:</h2>
  <p><a href="http://www.xgc-tek.com/manuals/m1750-ada/m1750/x524.html" rel="nofollow noreferrer">http://www.xgc-tek.com/manuals/m1750-ada/m1750/x524.html</a></p>
  <p>The given link suggests that 16-bit instruction is composed of 8 bits of opcode and 4+4 bits of two General Purpose register.</p>
  <p>How is it true for ARM thumb instruction set??</p>