1. Subroutines reasons for subroutines −repeat same code, or similar code with slightly different parameters −hide design decisions or design complexity −partition off code likely to change −provide for separate compilation −provide for reuse

2. Subroutines "open" subroutine = macro −resolved before assembly by textual substitution, −body of routine is placed in-line at call site with parameter substitution "closed" subroutine = standard notion −branch/execute/return at run time −one copy of body which accesses its formal parameters

3. Subroutines Three main concepts involved: Transferring control from calling program to a subroutine and back Passing parameter values to a subroutine and results back from the subroutine Writing subroutine code that is independent of the calling program In addition, sometimes a subroutine must allocate space for local variables.

4. Subroutines Transferring control from calling program to a subroutine and back Necessary to save the return address, then branch to the subroutine. This can be accomplished in ARM using the branch and link instruction bl bl subr_name To transfer control back to the calling program, can use the branch and exchange instruction bx bx lr Alternatively, you can pop the lr register to the pc

5. Subroutines Placing a return address in a link register works as long as there are no nested subroutines. For nested subroutines, it is necessary to save the return address to the stack subroutine calls and returns are LIFO, and older processors typically save the return address of a caller by pushing it onto a memory stack older processors typically pass parameters by pushing them onto the same memory stack that holds the return addresses

6. Subroutines Parameters: −actual parameters - values or addresses passed to subroutine −formal parameters - parameter names appearing in subroutine

7. General structure of a subroutine … … bl subr @ call subroutine ret_addr: add r0, r1, r2 … subr: … @ body of the subroutine … bx lr @ return to caller @ call @ return main:

8. C functions main() { int a,b,c;... c = sum(a,b); @ a,b,c:r0,r1,r2 } /* really dumb sum function */ int sum(int x, int y) { return x+y; } What information must compiler/programmer keep track of? What instructions can accomplish the return?

9. Function Call Bookkeeping Registers play a major role in keeping track of information for function calls Register conventions: – Return addresslr – Argumentsr0, r1, r2, r3 – Return valuer0, r1, r2, r3 – Local variablesr4, r5, …, r12 The stack is also used (more on this later)

10. Register Usage r8 r9/sb r10/sl r11 r12 r13/sp r14/lr r15/pc r0 r1 r2 r3 r4 r5 r6 r7 Register variables Must be preserved Arguments into function Result(s) from function otherwise corruptible (Additional parameters passed on stack) Scratch register (corruptible) Stack Pointer Link Register Program Counter Register - Stack base - Stack limit if software stack checking selected - R14 can be used as a temporary once value stacked - SP should always be 8-byte (2 word) aligned

11. Register Usage The compiler has a set of rules known as a Procedure Call Standard that determines how to pass parameters to a function (see AAPCS) CPSR flags may be corrupted by function call. Assembler code which links with compiled code must follow the AAPCS at external interfaces The AAPCS is part of the new Application Binary Interface (ABI) for the ARM Architecture

12. Register Conventions CalleR: the calling function CalleE: the function being called When callee returns from executing, the caller needs to know which registers may have changed and which are guaranteed to be unchanged. Register Conventions: A set of generally accepted rules as to which registers will be unchanged after a procedure call (BL) and which may be changed.

13. Register Conventions What do these conventions mean? – If function R calls function E, then function R must save any temporary registers that it may be using onto the stack before making a BL call (caller saved register values). – Function E must save any saved registers it intends to use before garbling up their values (callee saved register values) – Remember: caller/callee need to save only volatile/saved registers they are using, not all registers.

14. Saved Register Conventions r4-r11 (v1-v8): Restore if you change. Very important. If the callee changes these in any way, it must restore the original values before returning. sp: Restore if you change. The stack pointer must point to the same place before and after the BL call, or else the caller will not be able to restore values from the stack.

15. Volatile Register Conventions lr: Can Change. The BX call itself will change this register. Caller needs to save on stack if nested call. r0-r3 (a1-a4): Can change. These are volatile argument registers. Caller needs to save if they’ll need them after the call. e.g., r0 will change if there is a return value r12 (ip) may be used by a linker as a scratch register between a routine and any subroutine it calls. It can also be used within a routine to hold intermediate values between subroutine calls.

16. Rules for Subroutines Called with a BL instruction, returns with a BX lr (or MOV pc, lr) Accepts up to 4 arguments in r0, r1, r2 and r3 Return value is always in r0 (and if necessary in r1, r2, r3) Must follow register conventions (even in functions that only you will call)! What are the register conventions? 1)Save necessary values onto stack 2)Assign argument(s), if any 3)BL call 4)Restore values from stack

17. Instruction Support for Functions BL subroutine_name (Branch-and-Link) is the instruction to jump to a subroutine. It performs the following operations: – Step 1 (link): Save address of next instruction into lr (Why next instruction? Why not current one?) – Step 2 (branch): Branch to the given label (subroutine name) BL always uses r14 to store the return address. r14 is called the link register (lr)

18. Instruction Support for Functions BX - performs a branch by copying the contents of a general register, Rn, into the program counter, PC. The branch causes a pipeline flush and refill from the address specified by Rn. Instead of providing a label to jump to, the BX instruction provides a register which contains an address to jump to Only useful if we know the exact address to jump Very useful for function calls: – BL stores return address in register (lr) – BX lr jumps back to that address Syntax for BX (branch and exchange): BX register

19. Instruction Support for Functions main() {... sum(a,b); // a,b:r4,r5... } int sum(int x, int y) { return x + y; } address 1000 mov r0, r4 @ x = a 1004 mov r1, r5 @ y = b 1008 bl sum @ lr = 1012 branch to sum 1012... 2000 sum: ADD r0, r0, r1 2004 BX lr @ MOV pc, lr i.e., return Note: returns to address 1012 C ARMARM

20. C functions.text.global main.type main, %function main: push {lr} mov r0, #37 @ put x in r0 mov r1, #55 @ put y in r1 bl sum... mov r0, #0 pop {pc}.global sum.type main, %function sum: push {lr} add r0, r0, r1 pop {pc}

21. Nested Procedures int sumSquare(int x, int y) { return mult(x,x)+ y; } Some subroutine called sumSquare, and now sumSquare is calling mult. There’s a value in lr that sumSquare wants to jump back to, but this will be overwritten by the call to mult. Need to save sumSquare’s return address before the call to mult.

22. Nested Procedures In general, it may be necessary to save some other info in addition to lr. When a C program is run, there are 3 important memory areas allocated: – Static: Variables declared once per program, cease to exist only after execution completes. e.g., C globals – Heap: Variables declared dynamically – Stack: Space to be used by subroutines during execution; this is where we can save register values

23. Using the Stack We have a register sp which always points to the last used space in the stack. To use the stack, we decrement this pointer by the amount of space we need and then fill it with info. Consider the following C function: int sumSquare(int x, int y) { return mult(x, x) + y; }

24. Using the Stack sumSquare: add sp,sp,#-8 @ space on stack str lr, [sp,#4] @ save ret addr str r1, [sp] @ save y mov r1, r0 @ mult(x,x) bl mult @ call mult ldr r1, [sp] @ restore y add r0,r0,r1 @ mult()+y ldr lr, [sp, #4] @ get ret addr add sp,sp,#8 @ restore stack bx lr mult:... int sumSquare(int x, int y) { return mult(x, x)+ y; } "push" "pop"

25. C functions x86 example of swap routine in C void main() { main: void swap(); pushl %ebp ! save old bp int a,b; movl %esp,%ebp ! set new bp as ! current sp a = 5; b = 44; subl $8,%esp ! sub for a and b swap(&a,&b); movl $5,-4(%ebp) ! initialize a } movl $44,-8(%ebp) ! initialize b leal -8(%ebp),%eax ! form addr of b pushl %eax ! push onto stack leal -4(%ebp),%eax ! form addr of a pushl %eax ! push onto stack call swap ! call addl $8,%esp ! clean parms off ! stack leave ret

26. C functions x86 example output for swap routine in C (sp called esp, fp called ebp) void swap(x,y) swap: int *x,*y; pushl %ebp ! save old bp { movl %esp,%ebp ! set new bp as current sp int temp; subl $4,%esp ! sub for temp temp = *x; movl 8(%ebp),%eax ! move addr x into eax *x = *y; movl (%eax),%edx ! indirectly get value in a *y = temp; movl %edx,-4(%ebp) ! store into temp return; movl 8(%ebp),%eax ! move addr x into eax } movl 12(%ebp),%edx ! move addr y into edx movl (%edx),%ecx ! indirectly get value in b movl %ecx,(%eax) ! store indirectly into a movl 12(%ebp),%eax ! move addr y into eax movl -4(%ebp),%edx ! move temp into edx movl %edx,(%eax) ! store indirectly into b leave ret

27. C functions ARM code for swap() void swap(int *x, int *y) { swap: @ addr x is in r0 int temp; @ addr y is in r1 temp = *x; push {lr} @ save return address *x = *y; ldr r2, [r0] @ move x into r2 *y = temp; ldr r3, [r1] @ move y into r3 return; str r3, [r0] @ store indirectly into x } str r2, [r1] @ store into y mov r0, #0 pop {pc}

28. Basic Structure of a Function entry_label: add sp,sp, -framesize str lr, [sp, #framesize-4] @ save lr @ save other regs if necessary... restore other regs if necessary ldr lr, [sp, framesize-4] @ restore lr add sp,sp, #framesize bx lr Epilogue Prologue Body (call other functions…)

29. Example main() { int x, y; /* x: r0, y: r1 */... m = mult(y,y);... } int mult (int multiplicand, int multiplier) { int product; product = 0; while (multiplier > 0) { product += multiplicand; multiplier -= 1; } return product; }

30. Example.global main.type main, %function x.req r4 y.req r5 prod.req r3 main: push {r4, r5, lr} @ save registers mov x, #7 @ put x in r4 mov y, #5 @ put y in r5 /* set up registers for call to mult */ mov r0, x mov r1, y bl mult @ call mult mov r3,r0 @ m = mult(x, y) done: … mov r0, #0 pop {r4, r5, pc}

31. Example.global mult.type mult, %function mult: push {lr} product.req r2 multiplicand.req r0 multiplier.req r1 mov product, #0 cmp multiplier, #0 ble finished loop: add product, product, multiplicand sub multiplier, multiplier, #1 cmp multiplier, #0 bgt loop finished: mov r0, product pop {pc}

32. Passing Parameters to Subroutines in ARM Parameters can be passed to subroutines in three ways: Through registers Through memory Via the stack

33. Passing Parameters in registers This is what we have done so far Fast way of transferring data

34. Passing Parameters by Reference Parameters can easily be stored in program memory and then loaded as they are used Send the address of the data to the subroutine More efficient in terms of register usage for some types of data, e.g. a long string of characters or an array of int values

35. Passing Parameters on the stack Similar to passing parameters in memory The subroutine uses a dedicated register for a pointer into memory – the stack pointer, register r13. Data is pushed onto the stack before the subroutine call The subroutine gets the data off the stack. The results are then stored back onto the stack to be retrieved by the calling routine.