Chapter 7 – Subroutines

1. Chapter 7 – Subroutines These are lecture notes to accompany the book SPARC Architecture, Assembly Language Programming, and C, by Richard P. Paul, 2nd edition, 2000. By Anu G. Bourgeois Richard P. Paul, SPARC Architecture, Assembly Language Programming, and C

2. Subroutines allow us to either to repeat a computation or to repeat the computation with different arguments. Subroutines can be used in such situations Subroutines may be either open or closed Open subroutine inserts code whenever it is needed in the program --- macros arguments are passed in the registers that are given as arguments to the subroutine. Closed subroutine code appears only once in the program; whenever it is needed, a jump to the code is executed, and when it completes, a return is made to the instruction occurring after the call instruction (after the delay) arguments may be placed in registers or on the stack

3. A subroutine also allows you to debug code once and then ensure that all future instantiations of the code will be correct • Any register that the subroutine uses must first be saved and then restored after the subroutine completes execution • Arguments to subroutines are normally considered to be local variables of the subroutine, and the subroutine is free to change them • However, this is not always the case, for e.g., in multiplication, multiplicand is not changed

4. Open Subroutines are very efficient with no wasted instructions Open Subroutines are very flexible and can be as general as the program wishes to make them Every time open subroutine referenced, the code is expanded, resulting in long code So it is better to write code once as a closed subroutine and to branch to the code, whenever needed

5. Register Saving Almost any computation will involve the use of registers Usually when subroutines are called, registers are pushed onto the stack and popped from, when it returns To avoid the execution time involved, in CISC, sometimes a special register save mask is used, that would indicate, by bits that were set, which registers were to be saved

6. SPARC Registers SPARC architecture provides a register file with a mapping register that indicates the active registers It provides 128 registers, with the programmer having access to the eight global registers, and only 24 of the mapped registers at a time save instruction changes the register mapping so that new registers are provided restore instruction restores the register mapping on subroutine return

7. The 32 registers are divided into four groups : in, local, out and general The eight general register %g0 to %g8 are not mapped and are global to all subroutines “in” & “out” register are used to pass arguments to closed subroutine “local” registers are used for subroutine’s local variables When save instruction is executed the out register become the in register, and a new set of local and out registers is provided

8. Register Saving • When the save instruction is executed • the out registers become the in registers, and • a new set of local and out registers is provided. • The mapping pointer into the register file is changed by 16 registers

9. REGISTER FILE REGISTER FILE 8-Global 8-Global

11. Current register window

12. After branch to subroutine

13. After the return back to the calling portion of the code

14. If a further five subroutine calls are made without any returns, window overflow will occur saves and restores can be made in a range of six without window overflows or underflows (it is expensive if recursive subroutine calls are frequently made)

15. Restore Instruction • restore instruction restores the register window set. On doing this, a register window can underflow if the cwp is moved to the wim. When this happens the window trap routine restores the registers from the stack and resets the pointers • restore is also an add instruction and is used as the final add instruction in a subroutine

16. Subroutine Linkage • The SPARC architecture supports two instructions, call and jmpl, for linking to subroutines • The address of instruction which called the subroutine is stored in %o7 • The return from subroutine is to %o7 + 8, which is the address of the next instruction to be executed in the main program • If a save instruction is executed at the beginning of the subroutine, the contents of %o7 will become %i7, and the return will have to be to %i7 + 8

17. Call Instruction • If the subroutine name is known at assembly time, the call instruction may be used • call instruction has a target address label • It stores %pc contents to %o7 • always followed by a delay slot instruction

18. jmpl Instruction • Used when the address of the subroutine is computed and not known – address is loaded into a register • subroutine address is the sum of the source arguments, and the address of the jmpl instruction is stored in the destination register • always followed by a delay slot instruction • to call a subroutine whose address is in register %o0 and to store the return address into %o7, we would write: jmpl %o0, %o7

19. Call vs jmpl The assembler recognizes call %o0 as jmpl %o0, %07 The return from a subroutine also makes use of the jmpl instruction We need to return to %i7 + 8 Assembler recognizes ret for: jmpl %i7 + 8, %g0

20. The call to subroutine is: call subr nop And at the entry of the subroutine subr: save %sp, … %sp with the return ret restore The restore instruction is normally used to fill the delay slot of the ret instruction The ret is expanded to: jmpl %i7 + 8, %g0 restore

21. Arguments to Subroutines • 1. Arguments follow in-line after the call instruction: • For example, a Fortran routine to add two numbers, 3 and 4, together would be called by: • and handled by the following subroutine code: • Note that the return is to %i7 + 16 jumping over the arguments. • This type of argument passing is very efficient but is limited. Recursive calls are not possible, nor is it possible to compute any of the arguments.

22. Return Values Functions are subroutines which return a value In SPARC, the return value is always returned in an “out” register, e.g. %o0, i.e. %i0 of called program We have to put the return value in the corresponding “in”register before executing restore instruction Consider the .mul routine – we pass arguments to the routine using %o0 and %o1 and then expect the result to be passed back into %o0 – this means the routine places the result into %i0 before returning