Differences
This shows you the differences between two versions of the page.
| Both sides previous revisionPrevious revisionNext revision | Previous revisionNext revisionBoth sides next revision | ||
| user:embedded_c_code_doesn_t_have_to_be_ugly [2020/05/11 16:41] – [1. Use *YOUR* favorite IDE] Igor Yefmov | user:embedded_c_code_doesn_t_have_to_be_ugly [2020/05/11 17:18] – [4. Bit manipulations vs. structured data] Igor Yefmov | ||
|---|---|---|---|
| Line 98: | Line 98: | ||
| ==== 4. Bit manipulations vs. structured data ==== | ==== 4. Bit manipulations vs. structured data ==== | ||
| + | Working with the low-level code((which is, basically, pretty much all the embedded code)) quite often requires direct bit manipulation. And as much as every '' | ||
| + | |||
| + | Here's a more concrete example: working with 16/32 bit integers, where depending on the context it is either a CPU-addressable word, or a stream of bytes on the wire (be it USB or I²C). Traditionally that would result in the 1970's style code like this: | ||
| + | <code C> | ||
| + | uint8_t location[4]; | ||
| + | location[1] = (byteAddress >> 16) & 0xFF; /* MS byte */ | ||
| + | location[2] = (byteAddress >> 8) & 0xFF; | ||
| + | location[3] = byteAddress & 0xFF; /* LS byte */ | ||
| + | </ | ||
| + | |||
| + | Do you see an **obvious bug** in that code((yes, this is what was in the code! An uninitialized first byte of the '' | ||
| + | |||
| + | Here's a much more modernized version of this, which also (automagically!!!) takes care of that idiotic bug: | ||
| + | <code C> | ||
| + | uint16_t v; | ||
| + | struct{ uint8_t l, h; }; | ||
| + | uint8_t a[2]; | ||
| + | } PackedUint16; | ||
| + | typedef union PackedInt16{ | ||
| + | int16_t v; | ||
| + | struct{ uint8_t l, h; }; | ||
| + | uint8_t a[2]; | ||
| + | } PackedInt16; | ||
| + | typedef union PackedUint32{ | ||
| + | uint32_t v; | ||
| + | struct{ PackedUint16 l, h; }; | ||
| + | uint8_t a[4]; | ||
| + | } PackedUint32; | ||
| + | |||
| + | /////////////////////////////////// | ||
| + | |||
| + | PackedUint32 byteAddress = {pageAddress * glSpiPageSize}; | ||
| + | uint8_t location[4] = {}; /* uninitialized memory is evil */ | ||
| + | location[1] = byteAddress.h.l; | ||
| + | location[2] = byteAddress.l.h; | ||
| + | location[3] = byteAddress.l.l; | ||
| + | |||
| + | /* alternatively the code above can be written much more to-the-point, | ||
| + | const uint8_t * const location_arr = pageAddress.a;</ | ||
| + | |||
| ==== 5. Globals ==== | ==== 5. Globals ==== | ||