> For the complete documentation index, see [llms.txt](https://ret2basic.gitbook.io/ctfwriteup/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ret2basic.gitbook.io/ctfwriteup/rareskills-puzzles/yul-puzzles/01-returnbool.md).
# 01 - ReturnBool
**Goal:** Return a boolean value (`true` or `false`) from the Yul assembly block.
{% code title="ReturnBool.t.sol" %}
```solidity
// SPDX-License-Identifier: AGPL-3.0-or-later
pragma solidity ^0.8.13;
import {Test, console} from "forge-std/Test.sol";
import {ReturnBool} from "../src/ReturnBool.sol";
contract ReturnBoolTest is Test {
ReturnBool public c;
function setUp() public {
c = new ReturnBool();
}
function test_ReturnBool() public {
bool r = c.main();
assertEq(r, true);
}
}
```
{% endcode %}
In Yul, to return data we must explicitly store the value in memory and use the `return` opcode with a pointer and size. Booleans in the EVM are represented as 32-byte values (1 for true, 0 for false). To return `true`, we store 1 in memory and return 32 bytes; for `false`, we store 0.
{% code title="ReturnBool.sol" %}
```solidity
// SPDX-License-Identifier: AGPL-3.0-or-later
pragma solidity ^0.8.13;
contract ReturnBool {
function main() external pure {
assembly {
// Store boolean true (1) at memory position 0
mstore(0, 1)
// Return 32 bytes from memory position 0 (bool true)
return(0, 32)
}
}
}
```
{% endcode %}
**Explanation:** We use the `mstore` opcode to write to memory, and then `return(0, 32)` to output 32 bytes starting at memory position 0. The value `1` will be interpreted as `true` by Ethereum tools (and `0` as `false`). Memory is byte-addressable, so writing at offset 0 is convenient for returning data, although not memory safe (it overwrites the "scratch space for hashing methods", read more [here](https://docs.soliditylang.org/en/latest/internals/layout_in_memory.html)).
**Why not use free memory pointer?**
In this scenario we just overwrite the scratch space with a single byte, it will not overwritten important memory regions. In future levels we will use free memory pointer if the data is long.
Run test:
```bash
forge test --mp test/ReturnBool.t.sol -vvvv
```
