zkevm-docs/7-copy.markdown

+## 简介
+Copy类操作指的是在EVM中进行不定长的一段数据数据拷贝，例如CALLDATACOPY是一种将calldata中一段数据拷贝到memory中的操作。拷贝以byte作为数据长度单位，注意和栈的U256不同。
+
+由于其长度的不定性，即无法在编写电路时就获知数据拷贝的长度，我们难以在不引入新子电路、子表格的情况下处理此类操作。
+
+我们的做法是，定义了copy子电路，对于Copy类操作，假设其长度为`len`，在此子电路中，使用`len`行来处理。每一行都要加上相应的约束来证明是从来源拷贝到去向。对于每次操作，生成的Witness就会包含`len`行的copy子电路的Row。
+
+具体的，如下操作是Copy类操作：
+
+- CODECOPY
+- CALLDATACOPY
+- RETURN
+- RETURNDATACOPY
+- LOG
+- 调用开始（开始处理一笔交易或者CALL）时，CALLDATA要被写入STATE子电路所维持的状态中。分为两种：
+  - CALLDATA_FROMPUBLIC 外界（交易、公开数据）的CALLDATA输入
+  - CALLDATA_FROMCALL 合约调用另一个合约的CALLDATA输入
+
+## 设计
+
+### Witness、Column设计
+
+共使用9列，参见代码。
+
+```rust
+pub struct Row {
+    /// The byte value that is copied
+    pub byte: U256,
+    /// The source type, one of PublicCalldata, Memory, Bytecode, Calldata, Returndata
+    pub src_type: Type,
+    /// The source id, tx_idx for PublicCalldata, contract_addr for Bytecode, call_id for Memory, Calldata, Returndata
+    pub src_id: U256,
+    /// The source pointer, for PublicCalldata, Bytecode, Calldata, Returndata means the index, for Memory means the address
+    pub src_pointer: U256,
+    /// The source stamp, state stamp for Memory, Calldata, Returndata. None for PublicCalldata and Bytecode
+    pub src_stamp: Option<U256>,
+    /// The destination type, one of Memory, Calldata, Returndata, PublicLog
+    pub dst_type: Type,
+    /// The destination id, tx_idx for PublicLog, call_id for Memory, Calldata, Returndata
+    pub dst_id: U256,
+    /// The destination pointer, for Calldata, Returndata, PublicLog means the index, for Memory means the address
+    pub dst_pointer: U256,
+    /// The destination stamp, state stamp for Memory, Calldata, Returndata. As for PublicLog it means the log_stamp
+    pub dst_stamp: U256,
+    /// The counter for one copy operation
+    pub cnt: U256,
+    /// The length for one copy operation
+    pub len: U256,
+}
+```
+
+例子：CODECOPY，从bytecode拷贝到memory。例子里长度为8，被拷贝的数据为0xabcd......见下表。
+| byte | src type   | dst type | src id        | src pointer | src stamp | dst id | dst pointer   | dst stamp  | cnt | len |
+|------|------------|----------|---------------|-------------|-----------|--------|---------------|------------|-----|-----|
+| 0xab | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 0   | 8   |
+| 0xcd | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 1   | 8   |
+| ...  | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 2   | 8   |
+|      | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 3   | 8   |
+|      | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 4   | 8   |
+|      | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 5   | 8   |
+|      | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 6   | 8   |
+|      | bytecode=5 | memory=0 | contract addr | some offset | nil       | callid | some mem addr | some stamp | 7   | 8   |
+
+### 门约束
+
+TODO
+
+### Lookup
+
+每一行，都要进行两个约束，向Copy的来源和Copy的去向进行查找表。查找表的来源都是此子表格的一行（的某些列），去向是“Copy的来源或Copy的去向”。
+
+以上面为例，每一行，要进行：
+- 去向为bytecode的查找表，含义是确定拷贝的数据没错
+- 去向为state（具体tag为memory）的查找表，含义是确定拷贝的数据确实写进去了
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