Implement Subprograms Flashcards by Larissa Perara

Subroutine linkage

Subroutine call and return operations

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Implementation of subroutines must be based on ….

Semantics of the subroutine linkage

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Semantics of a subroutine call (4)

(1) save the execution status of the current Program unit
(2) pass the Parameters
(3) pass the return address to the callee
(4) transfer control to the callee

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Subroutine return:

If there are pass-by-value-result or out-mode parameters, the current values of those parameters are move to …

The corresponding actual parameters

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Subroutine return:

Move the return value to…

A place accessible to the caller

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Subroutine return:

Execution status of the caller is …

Returned

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Subroutine return:

Control is transferred …

Back to the caller

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Storage:

Call and return actions require storage for … (5)

Status information about the caller, parameters, return address, return value for functions, and local variables

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“Simple” subroutines are

Those that cannot be nested and all local variables are static.

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A simple subroutine consists of two parts: (give def for each to/size)

Code and data
Code: constant (instruction space)
Data: can change when the subroutine is executed (data space)
Both have fixed size

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Activation record is

The format or layout of the data part because the data is relevant to an activation, execution, of the subroutine

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The formation of an activation record is

Static

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An activation record instance is a

Concrete example of an activation record corresponding to one execution

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An activation record for a simple subroutine

What does the stack look like, size, allocation type, side note

Local var
Params
Return Add

Fixed size (statically allocated)

Could be attached to the code part of the sub Program

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When each programming unit is compiling is it

Machine code along with a list of references to external sub programs are written to a file

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A linker is called for …

MAIN to create an executable program

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What does a linker do? (4)

Is a part of the OS
Also called loader, linker/loader, loader, link editor
Finds and loads all referenced subroutines including code and activation records into memory
Sets the target addresses of calls to those subroutines entry addresses

Call stack is

A stack data structure that stores information about the active subroutines of a Program

Also known as execution stack, control stack, runtime stack, machine stack.

Frowns downward in memory towards lower addresses and shrinks upwards

The activation record on stack is also called

Stack frame

stack pointer (sp) points to

The frame top

Frame pointer (to) points

To the frame base

Important advantage of stack dynamic local variables

Support for recursion

Comple activation record stack contents

Local variables 
Parameters 
Dynamic link 
Return address 
Stacktop

Dynamic link (control links) =

Previous sp

What is dynamic link used for?

In destruction of the current activation record instance when the procedure completes its execution

What does the dynamic link restore?

The sp in the previous frame (caller)

What is a collection of dynamic links in the stack at a given time called? And what do the represent?

Called dynamic chain (call chain) Represents the dynamic history of how execution got to its current position

What is the purpose of a dynamic link?

Required when there are allocations from the stack by a subroutine by one it’s activation record (temporaries) Access biblical variables in dynamic scoped languages

Local scalar variables are bound to

Storage within an activation record instance

Local variables that are structures are sometimes allocate elsewhere and only leave their ...

Descriptors and a pointer to the storage as a part of their activation record

Recursion adds the possibility of multiple ...

Simultaneous activations of a subroutine at a given time with atleast one call from outside the subroutine and one or more recursive calls.

Each recursion activation requires

It’s own activation record instance

What static scoped languages use stack dynamic local variables and allow subroutine to be nested?

Fortran, ada, Python, JavaScript

Finding access to non local variables in nested subroutines of static scoped Lang but stack dynamic local variables

(1) find activation record instance on the stack where variable is allocation (2) local-offset of the variables to access it

Local offset describes the ...

Offset from the beginning/bottom of an activation record

Finding activation record instance: Static chaining (3)

(1) new ptr (static link aka static scope pointer/access links) point to bottom of activation record instance of the static parent (2) used to access non local varicela (3) static link appears below parameters on activation record

Activation record instance with static link stack

``` Local variables Parameters Dynamic link Static link Return address ```

A static chain is a chain of

Static links that connect the activation record instances of all static ancestors for an executing subroutine

Static chain can be used to implement

No local variable access

Static_depth and nesting_depth difference

Static depth: how deeply it is nested in the outermost scope for a non local reference Static depth of a variable definition is the nesting_depth/chain_depth of the reference