call yourself

Write fact, a subroutine that computes a factorial by calling itself, and use it to print 10!.

Recursion is where the calling convention stops being theory. Each call to fact needs its own frame (lrlink register (x30 / lr): holds the return address set by bl. must survive the recursive blBranch with link; writes return address to X30.), and n has to outlive the recursive call, which means a callee-saved register that you save on entry and restore on exit. Base case first: when n is 1 or less, the answer is 1. Otherwise it is n * fact(n - 1).

what is checked

• 10! printed on its own line
• the program exits cleanly
• a real call chain does it: a frame is built (stpStore pair; mirrors LDP.) and the recursion happens (blBranch with link; writes return address to X30.), with no precomputed answer