The Abstraction Address Spaces

The AbstractionAddress Spaces Sung Gon Kim (skim@dcslab.snu.ac.kr) Yeong Ouk Kim (kim331@snu.ac.kr) Hwa Jung Kim (hkim@dcslab.snu.ac.kr) School of Computer Science and Engineering Seoul National University

Table of Contents • Motivation • Early System and Multiprogramming • Time Sharing • Address Spaces • Causation • Definition • Virtual Memory • Goal • Summary

Motivation • Early System • No abstraction • One process occupies entire memory at a time Expensive machines, BUT low utilization 0 KB Operating System - code, data, etc. - 64 KB Current Program - code, data, etc. - max Physical Memory

Motivation Long program-debug cycles 0 KB Operating System - code, data, etc. - 64 KB Process A Process B I/O Operation! Process A Process B max Disk Physical Memory • Multiprogramming • Switching multi processes between I/O operations • Improve utilization of the CPU  Efficient!

Motivation Saving state is Too Slow 0 KB Operating System - code, data, etc. - 64 KB Process A Process B Time’s UP! Process A Process B max Disk Physical Memory • Time Sharing Approach #1 • Granting time-slice for each process

Motivation Protection between multiple states 0 KB Operating System - code, data, etc. - 64 KB free 128 KB Process C - code, data, etc. - 192 KB Process B - code, data, etc. - 256 KB free 320 KB Process B - code, data, etc. - 384 KB free 448 KB free 512 KB Physical Memory • Time Sharing Approach #2 • Granting time-slice for each process • Leaving state of multiple processes in physical memory

Address Spaces 0 KB Program Code 1 KB Heap 2 KB free 63 KB Stack 64 KB Address Space • Protection from read and write from other processes • Address Space: Abstraction of physical memory provided by OS • Address Space of a process • Code: Code of the program • Stack: Track function call, parameters, return values • Heap: Dynamically-allocated, user-managed memory • But… • In reality, a process does NOT occupy entire memory • Hence the abstraction is required for OS to virtualize memory

Address Spaces in C #include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) { printf("location of code: %p\n", (void *)main); printf("location of head: %p\n", (void *)malloc(1)); int x = 3; printf("location of stack: %p\n", (void *)&x); return x; } 0x40057d Program Code 0x1548010 Heap location of code: 0x40057d location of code: 0x1548010 location of code: 0x7ffdc614670c free Stack 0x7ffdc614670c Address Space 64-bit Linux Machine

Virtual Memory Operating System …… …… Process A’s Address Space Physical Memory Process B’s Address Space Disk • Hide all physical aspects of memory from users • Memory is logically unbounded • Only portions of virtual address space are in physical memory at any one time

Goal • Transparency • The program is NOT aware that the memory is virtualized • Efficiency • Time: Does not affect the performance negatively • Space: Occupy minimal space for virtualization • For such efficiency, OS need hardware support • E.g., TLB for time efficiency • Protection • Protect a process from another process or even from the OS • Resulting in isolation among processes

Summary • Motivation • From early system to time sharing • Address Spaces • An abstraction of physical memory from the OS • To provide protection • Memory Virtualization • With the abstraction, OS can virtualize the memory • Goal • Transparency: Virtualization itself is invisible • Efficiency: Time and Space • Protection: From other processes and OS

Looking Forward • Mechanisms needed to virtualize memory • Address Translation • Segmentation • Paging • Policies about how to manage memory space • Free Space Management

Discussions in the Class • Motivation: From Multiprogramming to Time Sharing • Long program-debug cycle • Lack of Interaction • Although the book mentioned “long program-debug cycle” as the main issue, we believe that “lack of interaction” cannot be overlooked • Space efficiency issue to virtualize memory • Similar to TLB which was presented as hardware support for time efficiency, Multi-Level Page Table is one example of hardware support for space efficiency

The Abstraction Address Spaces