> For the complete documentation index, see [llms.txt](https://calnix.gitbook.io/aave-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://calnix.gitbook.io/aave-book/functions/withdraw/collateral-check.md).
# collateral check
## Overview
* [x] cache
* [x] updateState
* [x] get user's AToken balance + amount to withdraw
* [x] validateWithdraw: confirm user has sufficient balance and asset isActive
* [x] updateInterestRates
* [ ] collateral check
* [ ] burn ATokens
* [ ] Ensure existing loans are collateralized
{% hint style="info" %}
Interest rates are updated to account for the withdrawal of deposits, and therefore the reduction of supply. See [.updateInterestRates](/aave-book/functions/common-functions/.updateinterestrates.md)
{% endhint %}
## isUsingCollateral
* Transaction reverts on invalid `reserveIndex`
Let's examine the bitwise manipulations with an example. In this example, we assume six bits for simplicity, w\.r.t to `self.data`
#### **Example: Check if the user has been using the reserve at index 2 as collateral:**
It the result of the bitwise operations is not equal to 0, the function would return `true`. This indicates that the user has been using the reserve at `reserveIndex = 2` as collateral.
## setUsingAsCollateral
Set the collateral bit for a specific asset to either `0` or `1`, depending on bool `usingAsCollateral`.
#### `(reserveIndex << 1) + 1`
* This bitwise operation doubles the index and increments it by 1
* Creates the necessary offset to manipulate the relevant bits to the asset of specified index
* For more in-depth explanation see: [isUsingCollateral](/aave-book/functions/withdraw/collateral-check.md#example-check-if-the-user-has-been-using-the-reserve-at-index-2-as-collateral)
#### **`bit = 1 << offset`**
If `usingAsCollateral` is **true**, indicating that the reserve should be marked as used for collateral,
* bitwise OR operation between `data` and the `bit` value (`self.data |= bit`).
* sets the corresponding bit in the `self.data` variable to `1`
If `usingAsCollateral` is **false**, indicating that the reserve should NOT be marked as collateral
* bitwise AND operation between `data` and the `bit` value (`self.data &= bit`).
* sets the corresponding bit in the `self.data` variable to `0`
## isUsingCollateral
* Transaction reverts on invalid `reserveIndex`
If `usingAsCollateral` is **true**, indicating that the reserve should be marked as used for collateral,
* bitwise OR operation between `data` and the `bit` value (`self.data |= bit`).
* sets the corresponding bit in the `self.data` variable to `1`
If `usingAsCollateral` is **false**, indicating that the reserve should NOT be marked as collateral
* bitwise AND operation between `data` and the `bit` value (`self.data &= bit`).
* sets the corresponding bit in the `self.data` variable to `0`