# 2.59 Bit Expressions

**Problem**:

Write a C expression that will yield a word consisting of the least significant byte of `x` and the remaining bytes of `y`. For operands `x=0x89abcdef` and `y=0x76543210`, this would give `0x765432ef`.

expression：

```c
(x & 0xff) | (y & ~0xff)
```

test：

```c
#include <stdio.h>
#include <assert.h>

int main() {
    size_t mask = 0xff;
    size_t x = 0x89abcdef;
    size_t y = 0x76543210;
    size_t result = (x & mask) | (y & ~mask);
    assert(result==0x765432ef);
}
```

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