From JVM Spec:

• JVM Instruction Summary
• Example instruction descriptions:
  • imul

What instructions operate on:

• operands that are part of instruction itself, following their op code (unique number for instruction)
• operand stack - used for computation
• memory managed by the garbage collector
• constant pool - used to store 'big' values instead of in instruction stream
  • e.g. string constants, method and field names