Control Abstraction (Section 8.1-8.2)

Control Abstraction(Section 8.1-8.2) CSCI 431 Programming Languages Fall 2003 A compilation of material developed by Felix Hernandez-Campos and Michael Scott

Abstraction • Programming languages support the binding of names with potentially complex program fragments that can be used through an interface • Programmers only need to know about the purpose of the fragment rather than its implementationAbstraction • A control abstraction performs a well-defined operation • Subroutines • A data abstraction represents information • Data structures • Most data structures include some number of control abstractions

Subroutines • Execute an operation on behalf of a calling program unit • Subroutines can be parameterized • The parameters in the definition of the function are known as formal parameters • The parameters passed in the subroutine call are known as actual parameters or arguments • At the time of the call, actual parameters are mapped to formal parameters • Functions are subroutines that return a value, while procedures are subroutines that do not return a value

Actual Parameters Formal Parameters Subroutine Frames • Each subroutines requires a subroutine frame (a.k.a activation record) to keep track of • Arguments and return values • Local variables and temporaries • Bookkeeping information • When a subroutine returns, its frame is removed 1001: A(3) … 2001: int A(int n) { int m = n * n; return m + A(n-1); }

Call StackRecursive Subroutine A A A A A A

Static chain - composed of static links: • Static link - from a subroutine to the lexically surrounding subroutine • Disadvantage - Stack Layout • In a language with nested subroutines - how can we access non-local objects? • static chain • display to access an object k levels deeper → need to dereference k pointers

Stack Layout • Display: • Element j in display - reference to most recently called subroutine at lexical nesting level j • From a subroutine at lexical level i, to access an object k levels outwards: • follow only one pointer, stored in element i-k in display • constant access time

C Example • Stack pointer sp • Top of the frame stack • Frame pointer fp • Access to arguments and locals via offset of fp • They differ if temporary space is allocated in the stack

Stack Maintenance • Calling sequence (by caller) and prologue (by callee) are executed in the way into the subroutine: • Pass parameters • Save return address • Change program counter • Change stack pointer to allocate stack space • Save registers (including frame pointer) • Change frame pointer to new frame • Initialization code • Separation of tasks? • As much as possible in the callee (only once in the program)

Stack Maintenance • Epilogue (by callee) is executed in the way out of the subroutine: • Pass return value • Execute finalization code • Deallocate stack frame (restore stack pointer) • Restore registers

C on MIPS Calling sequence by the caller: • Caller saves registers in local variables and temporaries space • 4 scalar arguments in registers • Rest of the arguments at the top of the current frame • Return address in register ra and jump to target address

C on MIPS Prologue by the callee: • Subtract the frame size from the sp • Save registers at the beginning of the new frame using sp as the base for displacement • Copy the sp into the fp

C on MIPS Callee epilogue: • Set the function return value • Copy fp to sp to deallocate any dynamically allocated space • Restore registers including ra (based on sp) • Add frame size to sp • Return (jump to ra) • Caller (after call): • Restores caller-saves registers lazily • over time, as their values are needed

Pascal Example

Pascal on 680x0 • Significant differences from MIPS: • all arguments passed on stack (not in registers) • all arguments pushed and popped (not assembled in a build area) • sp and fp are not the same • special subroutine-calling instructions • save and update frame pointers, save registers, jump, allocate space for frame • all in one or two instructions • special push and pop operations • load/store and update sp in one instruction

Pascal on 680 • Calling sequence: • Caller (before call) • creates space for a small return value (or address for a large one) • pushes arguments • pushes the static link • performs a jsr instruction • pushes the return address, updates sp, and jumps to subroutine • Callee (in its prologue) • executes a link instruction • pushes fp on stack • copies sp into fp • allocates space - subtracts a constant from sp • pushes registers on stack • copies large arguments

Pascal on 680 • Calling sequence (cont.): • Callee (in its epilogue) • sets return value • pops saved registers • executes an unlk instruction • reverse of link • returns (executes a rts instruction) • pops return address, and updates sp • Caller (after call) • moves return value from stack • restores caller-saves registers lazily over time, as their values are needed

Control Abstraction (Section 8.1-8.2)