Why is memory used by RETURN misaligned compared to how it was allocated?

Question

I am experimenting with evmone and evmc to execute some simple assembly code snippets on EVM. I wrote a test that is supposed to return 0x42 as output data. The code goes as follows:

PUSH 0x42
PUSH 0x70
MSTORE
PUSH 0x01
PUSH 0x70
RETURN

However, this code returns 0, which I don't understand. I did some more scanning of the memory laying around and I found that if I fix the lookup position to 0x8f then it returns 0x42 as expected:

PUSH 0x42
PUSH 0x70
MSTORE
PUSH 0x01
PUSH 0x8f
RETURN

What is going on here?

---

My testing code:

#include <evmc/evmc.hpp>
#include <evmc/loader.h>
#include <evmone/evmone.h>

#include <iostream>
#include <vector>

const evmc_revision REVISION = EVMC_SHANGHAI;

uint8_t parse_byte(char c) {
  if(c >= 'a' && c <= 'f') {
    return c - 'a' + 10;
  }
  if(c >= 'A' && c <= 'F') {
    return c - 'A' + 10;
  }
  return c - '0';
}

std::vector<uint8_t> parse_bytes(std::string in) {
  std::vector<uint8_t> out;
  for(int i = 0; i + 1 < in.size(); i += 2) {
    uint8_t x = parse_byte(in[i]) * 0x10 + parse_byte(in[i + 1]);
    out.push_back(x);
  }
  return out;
}


int main(int argc, char** argv) {
  std::vector<uint8_t> code = parse_bytes("60426070526001608ff3"); // EVM code in hex

  auto vm = evmc::VM{evmc_create_evmone()};

  auto msg = evmc_message{};
  auto dst = evmc_address{};
  dst.bytes[19] = 0xaa;
  auto val = evmc_uint256be{};
  msg.kind = EVMC_CALL;
  msg.flags = int32_t{EVMC_STATIC};
  msg.recipient = dst;
  msg.code_address = dst;
  msg.sender = dst;
  msg.value = val;
  msg.gas = 10000;

  auto res = vm.execute(REVISION, msg, code.data(), code.size());

  std::cout << "OUTPUT: ";
  for(int i = 0; i < res.output_size; ++i) {
    std::cout << std::hex << int(res.output_data[i]);
  }
  std::cout << std::endl;
}

Answer 1

Because 0x42 is interpreted as integer so it is stored in memory as 0x0000.....0042.

If you want to return 0x42 you have to store

PUSH 0x4200...0000    // so the first byte is 0x42 an the rest are zeros
PUSH 0x70
MSTORE