fibonacci goes large

Compute the fiftieth Fibonacci number. It is 12,586,269,025, which is the point: that does not fit in 32 bits, so this loop is why the x registers exist.

Keep the classic sliding pair: a and bUnconditional branch to label (±128 MiB). start as 0 and 1, and fifty times over, the new value is a + b, then the pair slides forward. When the loop ends, a holds the answer.

what is checked

• fib(50) printed on its own line
• the program exits cleanly
• the loop builds it (answer not written in)