What is Assembly Language?

What is Assembly Language? Introduction to the GNU/Linux assembler and linker for Intel Pentium processors

High-Level Language • Most programming nowdays is done using so-called “high-level” languages (such as FORTRAN, BASIC, COBOL, PASCAL, C, C++, JAVA, SCHEME, Lisp, ADA, etc.) • These languages deliberately “hide” from a programmer many details concerning HOW his problem actually will be solved by the underlying computing machinery

The BASIC language • Some languages allow programmers to forget about the computer completely! • The language can express a computing problem with a few words of English, plus formulas familiar from high-school algebra • EXAMPLE PROBLEM: Compute 4 plus 5

The example in BASIC • LET X = 4 • LET Y = 5 • LET Z = X + Y • PRINT X, “+”, Y, “=“, Z • END Output: 4 + 5 = 9

The C language • Other high-level languages do require a small amount of awareness by the program-author of how a computation is going to be processed • For example, that: - the main program will get “linked” with a “library” of other special-purpose subroutines - instructions and data will get placed into separate sections of the machine’s memory - variables and constants get treated differently - data items have specific space requirements

Same example: rewritten in C #include <stdio.h> // needed for printf() int x = 4, y = 5; // initialized variables int z; // unitialized variable int main() { z = x + y; printf( “%d + %d = %d \n”, x, y, z ); }

“ends” versus “means” • Key point: high-level languages let programmers focus attention on the problem to be solved, and not exspend effort thinking about details of “how” a particular piece of electrical machiney is going to carry out the pieces of a desired computation • Key benefit: their problem gets solved sooner (because their program can be written faster) • Programmers don’t have to know very much about how a digital computer actually works

computer scientist vs. programmer • But computer scientists DO want to know how computers actually work: -- so we can diagnose computer errors -- so we can employ optimum algorithms -- so we can predict computer behavior -- so we can devise faster computers -- so we can build cheaper computers -- so we can pick one suited to a problem

A machine’s own language • For understanding how computers work, we need familiarity with the computer’s own language (called “machine language”) • It’s LOW-LEVEL language (very detailed) • It is specific to a machine’s “architecture” • It is a language “spoken” using voltages • Humans represent it with zeros and ones

Example of machine-language Here’s what a program-fragment looks like: 10100001 10111100 10010011 00000100 00001000 00000011 00000101 11000000 10010011 00000100 00001000 10100011 11000000 10010100 00000100 00001000 It means: z = x + y;

Incomprehensible? • Though possible, it is extremely difficult, tedious (and error-prone) for humans to read and write “raw” machine-language • When unavoidable, a special notation can help (called hexadecimal representation): A1 BC 93 04 08 03 05 C0 93 04 08 A3 C0 94 04 08 • But still this looks rather meaningless!

Hence assembly language • There are two key ideas: -- mnemonic opcodes: we use abbreviations of English language words to denote operations -- symbolic addresses: we invent “meaningful” names for memory storage locations we need • These make machine-language understandable to humans – if they know their machine’s design • Let’s see our example-program, rewritten using actual “assembly language” for Intel’s Pentium

Simplified Block Diagram Central Processing Unit Main Memory system bus I/O device I/O device I/O device I/O device

Pentium’s visible “registers” • Four general-purpose registers: eax, ebx, ecx, edx • Four memory-addressing registers: esp, ebp, esi, edi • Six memory-segment registers: cs, ds, es, fs, gs, ss • An instruction-pointer and a flags register: eip, eflags

The sixteen x86 registers EAX ESP EBX EBP ECX ESI EDX EDI CS DS ES FS GS SS EIP EFLAGS Intel Pentium processor

The “Fetch-Execute” Cycle main memory central processor Temporary Storage (STACK) ESP Program Variables (DATA) EAX EAX EAX EAX Program Instructions (TEXT) EIP the system bus

Define symbolic constants .equ device_id, 1 .equ sys_write, 4 .equ sys_exit, 1

our program’s ‘data’ section .section .data x: .int 4 y: .int 5 fmt: .asciz “%d + %d = %d \n”

Our program’s ‘bss’ section .section .bss z: .int 0 n: .int 0 buf: .space 80

our program’s ‘text’ section .section .text _start: # comment: assign z = x + y movl x, %eax addl y, %eax movl %eax, z

‘text’ section (continued) # comment: prepare program’s output pushl z # arg 5 pushl y # arg 4 pushl x # arg 3 pushl $fmt # arg 2 pushl $buf # arg 1 call sprintf # function-call addl $20, %esp # discard the args movl %eax, n # save return-value

‘text’ section (continued) # comment: request kernel assistance movl $sys_write, %eax movl $device_id, %ebx movl $buf, %ecx movl n, %edx int $0x80

‘text’ section (concluded) # comment: request kernel assistance movl $sys_exit, %eax movl $0, %ebx int $0x80 # comment: make label visible to linker .global _start .end

program translation steps demo.s demo.o program source module demo program object module assembly linking the executable program object module library object module library other object modules

The GNU Assembler and Linker • With Linux you get free software tools for compiling your own computer programs • An assembler (named ‘as’): it translates assembly language (called the ‘source code’) into machine language (called the ‘object code’) $ as demo.s -o demo.o • A linker (named ‘ld’): it combines ‘object’ files with function libraries (if you know which ones)

How a program is loaded 0xFFFFFFFF Kernel’s code and data stack runtime libraries .bss uninitialized variables .data initialized variables .text program instructions 0x00000000 Main memory

What must programmer know? • Needed to use CPU register-names (eax) • Needed to know space requirements (int) • Needed to know how stack works (pushl) • Needed to make symbol global (for linker) • Needed to understand how to quit (exit) • And of course how to use system tools: (e.g., text-editor, assembler, and linker)

Summary • High-level programming (offers easy and speedy real-world problem-solving) • Low-level programming (offers knowledge and power in utilizing machine capabilities) • High-level language hides lots of details • Low-level language reveals the workings • High-level programs: readily ‘portable’ • Low-level programs: tied to specific CPU

In-class exercise #1 • Download the source-file for ‘demo1’, and compile it using the GNU C compiler ‘gcc’: $ gcc demo1.c -o demo1 Website: http://cs.usfca.edu/~cruse/cs210/ • Execute this compiled applocation using: $ ./demo1

In-class exercise #2 • Download the two source-files needed for our ‘demo2’ application (i.e., ‘demo2.s’ and ‘sprintf.s’), and assemble them using: $ as demo2.s -o demo2.o $ as sprintf.s -o sprintf.o • Link them using: $ ld demo2.o sprintf.o -o demo2 • And execute this application using: $ ./demo2

In-class exercise #3 • Use your favorite text-editor (e.g., ‘vi’) to modify the ‘demo2.s’ source-file, by using different initialization-values for x and y • Reassemble your modified ‘demo2.s’ file, and re-link it with the ‘sprintf.o’ object-file • Run the modified ‘demo2’ application, and see if it prints out a result that is correct

In-class exercise #4 • Download the ‘ljpages.cpp’ system-utility from our class website and compile it: $ g++ ljpages.cpp –o ljpages • Execute this utility-program to print your modified assembly language source-file: $ ./ljpages demo2.s • Write your name on the printed hardcopy and turn it in to your course instructor

Summary of the exercises Download and Compile a high-level program Assemble and Link a low-level program Edit and recompile an assembly program Print out and turn in your hardcopy

What is Assembly Language?