EVM Puzzle 5 solution

This is Part 5 of the “Let’s play EVM Puzzles” series, where I will explain how to solve each puzzle challenge.

EVM Puzzles is a project developed by Franco Victorio (@fvictorio_nan) that a perfect fit if you are in the process of learning how the Ethereum EVM works and you want to apply some of the knowledge you have just acquired.

EVM Puzzle 5

00      34          CALLVALUE
01      80          DUP1
02      02          MUL
03      610100      PUSH2 0100
06      14          EQ
07      600C        PUSH1 0C
09      57          JUMPI
0A      FD          REVERT
0B      FD          REVERT
0C      5B          JUMPDEST
0D      00          STOP
0E      FD          REVERT
0F      FD          REVERT

This challenge is a little different compared to the previous one. Instead of using JUMP it uses the opcode JUMPI.

The JUMPI instruction may alter the program counter, thus breaking the linear path of the execution to another point in the deployed code. It is used to implement functionalities like loops and conditions.

When the JUMPI is executed, it pops 2 values from the Stack. The first value will be the new Program Counter to jump to (as always, it must be a valid JUMPDEST instruction). The second value instead is a boolean flag (0 or 1) to evaluate to know if it must jump or not. If the value is 1 it will jump; otherwise it will continue to the next instruction.

Let's review each opcode before the JUMPI:

CALLVALUE push in the stack the msg.value in wei passed along the transaction
DUP1: duplicate the first value in the stack and push it to the first position of the stack
MUL: pop the first two values of the stack and multiply them. The result is pushed back to the stack
PUSH2: push 2 bytes input into the stack
EQ: pop 2 values from the stack, if those are equal push 1 to the stack, otherwise push 0.
PUSH: push 1 byte input into the stack

Let's review the stack after each operation

CALLVALUE is executed