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57 lines
1.5 KiB
Markdown
57 lines
1.5 KiB
Markdown
# Section 2 / Bit Fields / With Bit Fields
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Given how long the previous chapter, describing life without bit fields, was
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this chapter will be a let down.
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Recall:
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```c
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struct BF {
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unsigned char a : 1;
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unsigned char b : 2;
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unsigned char c : 5;
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};
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```
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With bit fields, assigning values to `a`, `b` and `c` are just:
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```c
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bf.a = 1;
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bf.b = 2;
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bf.c = 3;
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```
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What about ensuring the values to be assigned to the bit fields are
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within the right range? For example:
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```c
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bf.c = 345;
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```
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Clearly, 345 cannot fit in 5 bits. Depending upon the compiler you
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should get a warning or an error. For example:
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```text
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test.c:61:12: warning: unsigned conversion from ‘int’ to ‘volatile unsigned char:5’ changes value from ‘345’ to ‘25’ [-Woverflow]
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61 | bf.c = 345;
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^~~
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```
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As for the assembly language that bit field will produce, it depends
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upon optimization level. Unoptimized, the code produced will be much
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longer and cumbersome than the "sophisticated" assembly language
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in the previous chapter.
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Turning on full optimization, you're get pretty much the same
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assembly language generated as the sophisticated version in the
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previous chapter.
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One remaining difference is that the optimized bit field instructions
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will not require the initial `ldr` nor the final `str` to load and
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store the byte containing our fields.
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Note that the C version of the bit field setters could be declared
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to be `inline` so that would be closer in performance to equivalent
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code written to use C / C++ bit fields.
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