Computer Instructions: Number & Length of Operands

      Opcode             Operand #1               Operand #2           Operand #3
+---+---+---+---+    +---+---+---+---+    +---+---+---+---+---+---+    +---+---+
| 1 | 0 | 1 | 1 |    | 0 | 1 | 1 | 0 |    | 0 | 0 | 0 | 1 | 1 | 0 |    | 1 | 0 |
+---+---+---+---+    +---+---+---+---+    +---+---+---+---+---+---+    +---+---+

Most architectures today support instructions that contain between $0$ to $3$ operands. The operands don't even have to be all of the same length and don't have to have fixed length:

• Instructions whose operands have fixed length (e.g., each operand must contain exactly 4 bits) are fast and result in better execution performace. The cons are that they waste memory and disk space (if certain operands are short, we still must use that fixed size.)
• Instructions whose operands have variable length result in space savings but will take longer time to decode and execute.

In reality, a modern computer will support $2-3$ instruction patterns, e.g., one 64-bit instruction with 1 operand of size 32 bits, another 64-bit instruction with 2 16-bit operands , and a 3rd, 86-bit instruction with 1 64-bit operand.