CS162B: Assembly and C

Jacob T. Chan

Objectives

▪ System calls

▪ Relation of System calls to Assembly and C

▪ Special System Calls (exit, write, print, etc.)

▪ Lab 5

Processing…

▪ Some programs take too long to run– Running from console terminal will prevent you from running

other programs in the same terminal (until program exits)– Issue when system complains for running too much consoles

running▪ Program vs process?

– One solution: put &▪ Makes program run in background▪ BUT it does not make the program a daemon (more on this later on)

Exit Code

▪ In C/C++, main() method returns 0 (unless there was something wrong)– Same goes for Java’s System.exit() that takes in an int as an

argument▪ This int is usually 0 (unless specified otherwise due to errors)

▪ Exit code or return value is tested by whatever called the program (either by main() or by exit())– Zero (0) = normal program termination– Non-zero = abnormal termination of program▪ Cause: ERRORS▪ Return value may give a hint of what type of error it is

System Calls

▪ Layer between user and kernel

▪ Lowest interaction level with O/S

▪ Entry point to the kernel– Should be provided by O/S

▪ Everything else is built on top of the system call

▪ Normally uses C/C++– Not Java (because system calls only relay signals to JVM)– Java system calls result in OVERHEAD– And besides, JVM is a VIRTUAL MACHINE

Categories of System Calls

▪ Process Control

▪ File Manipulation

▪ Signals (obsolete; system calls rely on signals)

▪ Device Management

▪ Information Maintenance

▪ Communications

Categories of System Calls

▪ System calls may fall on more than one category– chmod is both file manipulation and system maintenance– abort is process control and signal system call

In relation to Assembly and C…

▪ Function calls behave similarly for Assembly code– CS152a format of function calls is same in Assembly– GNU Assembler format will be used for this class– AT&T Syntax will be followed– Show assembly code in C programs– We will assume a 32-bit architecture (not 64-bit)▪ Might not work. Why?

GAS

▪ GNU Assembler files end with .S

▪ Programs in C are converted to assembly then to binary during compiling

Registers

▪ ESP, EAX, EDP, etc. are the registers– Remember R0 to R31?

▪ What is the register name for the stack pointer? The base pointer?

▪ GAS code follows Beta assembly code (BSIM)

Defining Functions in GAS...

▪ This is just crash course Assembly (it’s gonna take a separate semester if so)

▪ Defining a function myFunc

.text

.globl myFunc

.type myFunc, @function

myFunc:

# code follows

# this is a comment

System calls in GAS

▪ Some functions provide kernel-mode only functionality– Just like sudo

▪ To obtain such features, function must issue signal to O/S via interrupt

▪ In Linux, this is achieved by typing

int $0x80

Example: Write function

▪ Check out the mySysWrite.S (uploaded in Moodle)

▪ Then add it to C code (and use it like a regular function)

extern int myFunc(int fildes,const void *buf, int nbyte);

▪ Compiling the program

gcc -o demoProg myProg.c myFunc.S

Required: Knowledge

▪ In making system calls, you need:– The number code to be inserted in the eax register before calling

interrupt– Arguments to be placed in ebx, ecx, etc.

Lab 5: Making Assembly Sys Calls

▪ Implement system calls for opening, closing, reading, writing, and exiting using assembly in C– NOTE: writing is already demonstrated here in C

▪ Write a C program copy.c that makes use of these system calls– This copies specified input file to a specified output file– DO NOT USE the stdio.h library functions for this one

(printf/scanf)– Output file is to be named “output.txt” but for input files, assume

it’s a .txt file

Lab 5: Making Assembly Sys Calls

▪ Include a Certificate of Authorship, your copy.c, and your .S files (open.S, write.S, read.S, close.S, exit.S)

▪ Filename Convention:

CS162B_Lab5_<Section>_<Surname>_<ID#>.tar

▪ Deadline: 3 Days after today