Nibbles trade places

A nibble is four bits: half a byte, and exactly one hex digit. Swapping the two nibbles of 0xa7 gives 0x7a.

The starter reads a value typed in hex into value_r. Keep only its low byte in byte_r, then build swap_r from byte_r with the two nibbles traded. lsrLogical shift right (unsigned). by 4 brings the high nibble down, lslLogical shift left (Rd = Rn << amount). by 4 moves the low nibble up, and orrBitwise OR; register or bitmask immediate. joins the halves. After the left shift the old high nibble sits above bit 7, so mask it away with andBitwise AND; register or bitmask immediate. to keep the answer inside one byte.

With the sample input a7 the program prints 0xa7 -> 0x7a.

What is checked

• 0xa7 -> 0x7a for the sample input
• the program exits cleanly
• the nibbles move with lslLogical shift left (Rd = Rn << amount). and lsrLogical shift right (unsigned).
• the right answer for other values, including 0, ff, and values wider than one byte