8.2: Runtime Environments

Compiler Task

Must correctly map the high-level constructs of the source language to the low-level target machine constructs.

OS Address mapping

Maps the logical addresses to physical addresses in memory

Virtual Memory Structure

• Stack

• Free Memory

• Heap

• Static

• Code

Code contents

Generated target code

Static contents

Global constants and some objects

Stack contents

Stores activation records during procedure calls and starts from a higher address, growing downwards

Heap contents

Stores dynamically allocated data and starts from a lower address before growing upwards

Objects Storage

Stored in consecutive bytes, assigned the address of the first byte. Have to be allocated enough memory to hold them.

Stack

Where local variables are stored inside when the procedure containing the variable is called, then removed from the stack when the procedure finishes

Heap

Variables are stored in here when they outlive a procedure call and are manually de-allocated/garbage collected.

Activation Records/Stack Frames

Pushed onto the stack every time a function is called, and popped when a function returns

Stack Frame Contents

• Local variables

• Parameters

• Return Addresses

• Other temporaries

Stack pointer

Identifies where the current stack frame is on the stack

Access Link

Stores a link to the closest function surrounding a given function.

Stack Frame Layout

• Data communicated between caller and callee placed at the bottom of the callee’s record

  • Parameters, return address, return value

• Fixed-length variables in the middle

• Variables for which the size is unknown are placed at the end

  • Dynamically sized array

In a calling sequence

• Caller can store the parameters and return addresses onto the stack - gives control to the callee

  • Value of the program counter, gives control to the callee.

• Callee saves register values and other status information

• Callee initialises its local data, begins execution

In a return sequence

• Callee can store the return value

• Callee restores old register values

• Callee jumps to the return address

Memory Manager

Keeps track of free memory

Memory Manager - Requesting Memory

• Tries to produce some contiguous chunk of free memory

• If not possible, requests more memory from the OS

• If not possible, an error is returned

Memory Manager - Program Freeing Memory

• Returns that memory to the pool of free memory

• If the OS increased the memory, it is not returned to the OS

Memory Manager - Desired Properties

• Space efficiency

• Program efficiency

• Low overhead

Further Consideration

• Exploiting locality

• Reduce fragmentation

• Garbage Collection

Mark and sweep

Mark all unreachable objects, sweep the entire heap to free them up

Mark and compact

Move reachable objects to eliminate memory fragmentation.