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fix python code in SIGNEXTEND opcode #18

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fix python code in SIGNEXTEND opcode #18

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10 changes: 6 additions & 4 deletions 04_ArithmeticOp/readme.md
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Original file line number Diff line number Diff line change
Expand Up @@ -291,11 +291,13 @@ print(evm.stack)
raise Exception('Stack underflow')
b = self.stack.pop()
x = self.stack.pop()
if b < 32: # 如果b>=32，则不需要扩展
sign_bit = 1 << (8 * b - 1) # b 字节的最高位（符号位）对应的掩码值，将用来检测 x 的符号位是否为1
x = x & ((1 << (8 * b)) - 1) # 对 x 进行掩码操作，保留 x 的前 b+1 字节的值，其余字节全部置0
if b < 31: # 如果b>=31，则不需要扩展
sign_bit = 1 << (8 * (b + 1) - 1) # 第 b 字节的最高位（符号位）对应的掩码值，将用来检测 x 的符号位是否为1
mask = (1 << (8 * (b + 1))) - 1 # 前 b+1 字节对应的掩码
x = x & mask # 对 x 进行掩码操作，保留 x 的前 b+1 字节的值，其余字节全部置0
if x & sign_bit: # 检查 x 的符号位是否为1
x = x | ~((1 << (8 * b)) - 1) # 将 x 的剩余部分全部置1
x = x | ~mask # 将 x 的剩余部分全部置1
x = x %（2**256） # 置1后 x 为负数，取余变回非负数
self.stack.append(x)
```

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