b := (b and $7E) or (b shr 7) or (b shl 7);
First we keep the middle 6 bits, then swaps the MSB and the LSB bits in register shift operations.
Bit manipulation in Delphi is fun. There are many options, and this is a straight forward solution. David proposes a LUT solution which can be faster (only by a tiny bit though).
There was an interesting similar question about bit-reflecting a whole byte, see How can I bit-reflect a byte in Delphi?
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