28/06/2015CMPUT Functions (2)  Function calling convention  Various conventions available  One is specified by CMPUT229  Recursive functions

28/06/2015CMPUT Recursive functions  A function that calls itself  Terminating condition  Stack frame implementation

28/06/2015CMPUT Function calling convention  In summary, caller: 1.saves temporary registers by pushing their values on stack 2.pushes arguments (both “value” and “variable” on stack 3.calls the function with jal instruction  (function runs until it returns, then:) 4.clears function arguments by popping allocated space 5.restores saved temporary registers by popping their values off the stack 6.uses the return value found in both $v0 and popped “variable” arguments

28/06/2015CMPUT Function calling convention  In summary, callee: 1.saves $ra by pushing its value on stack 2.saves $fp by pushing its value on stack 3.copies $sp to $fp 4.allocates local variables  (body of function goes here, then:) 5.chooses return value by setting register $v0 6.deallocates local variables by popping allocated space 7.restores $fp by popping its saved value 8.restores $ra by popping its saved value 9.returns with jr $ra

28/06/2015CMPUT Recursion  Function calling convention works exactly the same for recursive functions  don’t need to do anything special  Each invocation of the function has its own stack frame  local variables and parameters with their current valueswith their current values  return address where to return towhere to return to

28/06/2015CMPUT Example  Fibonacci’s numbers fib(0) = 0 fib(1) = 1 fib(1) = 1 fib(n) = fib(n-1) + fib(n-2) for n > 1 fib(n) = fib(n-1) + fib(n-2) for n > 1

28/06/2015CMPUT Fibonacci Numbers

28/06/2015CMPUT Implementation of Recursive Functions  Different conventions can be used for writing recursive functions  MIPS  $a0 for taking input parameters  $v0 for returning results  Callers save $t0, $t1, …,  Callees save $s0, $s1, …,

28/06/2015CMPUT Implementation of Recursive Functions  Other conventions  Callees save all registers

28/06/2015CMPUT  More restricted conventions  All arguments are passed with the stack frame, including the value and variable parameters  Useful and important

28/06/2015CMPUT  More restricted conventions  All arguments are passed with the stack frame, including the value and variable parameters  Frame pointers  Used by compilers but difficult to understand

28/06/2015CMPUT  More restricted conventions  All arguments are passed with the stack frame, including the value and variable parameters  Frame pointers  Used by compilers but difficult to understand

28/06/2015CMPUT  Two different implementations of fibonacci numbers  Simple one (fib.a) Using $a0 and $v0 to pass argumentsUsing $a0 and $v0 to pass arguments Callees save all the registersCallees save all the registers  Stack frame (fib_stack.a) Using the stack frame for passing the arguments to and from the functionUsing the stack frame for passing the arguments to and from the function