Introduction

Overview Administrivia More Languages! Setup Student Voice

Reference

Visual Studio Code VSC Cloning replit.com Git Gitlab Setup Using GitLab VSC GitLab Terminal;GitLab

1. Basic Operations

Basic Operations Binary Recap Basic Operations Floating Point Numbers Basic Operations - Summary Basic Operations - Task Sheet 1 Basic Operations - Task Sheet 2

2. Memory

Memory The Stack Methods & Frames Stack Step By Step Recursion Primitives Vs Objects I The Heap Primitives Vs Objects II Frame Detail Summary Java Memory Exercise

3. C Programming

The C Programming Language C Basics C Variables C Branching & Loops C Operators C Functions C Strings C Input/Output C Code Organisation C Programming - Task Sheet

4. C Memory

C Memory C Pointers C Arrays C Lab Session 2

5. C Struct

C Data Structures C Struct Pointer Guide

C Linked Lists Exercises 1 External Pages Pointers To Pointers Doubly Linked Lists Doubly Linked Lists Node Based Stacks Stacks - Node Based Exercises 2 Trees Binary Trees Binary Search Trees Exercises 2

7. Assembly Language

Assembly PC Hardware Assembly Environment Setup Assembly Basics Registers System Calls Variables & Constants Arithmetic Logic Conditions Loops Exercises Numbers Procedures & The Stack Debugging Assembly Generating Assembly From C

Assembly Basics

Structure / Sections

An assembly program can be divided into three sections −

The data section
The bss section
The text section

The data Section

The data section is used for declaring initialized data or constants. This data does not change at runtime. You can declare various constant values, file names, or buffer size, etc., in this section.

The syntax for declaring data section is −

section.data

The bss Section

The bss section is used for declaring variables. The syntax for declaring bss section is −

section.bss

The text section

The text section is used for keeping the actual code. This section must begin with the declaration global _start, which tells the kernel where the program execution begins.

The syntax for declaring text section is −

section.text
   global _start
_start:

Comments

Assembly language comment begins with a semicolon (;). It may contain any printable character including blank. It can appear on a line by itself, like −

; This program displays a message on screen

or, on the same line along with an instruction, like −

add eax, ebx     ; adds ebx to eax

Assembly Language Statements

Assembly language programs consist of three types of statements −

Executable instructions or instructions,
Assembler directives or pseudo-ops, and
Macros.

The executable instructions or simply instructions tell the processor what to do. Each instruction consists of an operation code (opcode). Each executable instruction generates one machine language instruction.

The assembler directives or pseudo-ops tell the assembler about the various aspects of the assembly process. These are non-executable and do not generate machine language instructions.

Macros are basically a text substitution mechanism.

Syntax of Assembly Language Statements

Assembly language statements are entered one statement per line. Each statement follows the following format −

[ label ]   mnemonic   [ operands ]   [ ;comment ]

The fields in the square brackets are optional. A basic instruction has two parts, the first one is the name of the instruction (or the mnemonic), which is to be executed, and the second are the operands or the parameters of the command.

Following are some examples of typical assembly language statements −

INC COUNT        ; Increment the memory variable COUNT

MOV TOTAL, 48    ; Transfer the value 48 in the 
                 ; memory variable TOTAL
					  
ADD AH, BH       ; Add the content of the 
                 ; BH register into the AH register
					  
AND MASK1, 128   ; Perform AND operation on the 
                 ; variable MASK1 and 128
					  
ADD MARKS, 10    ; Add 10 to the variable MARKS
MOV AL, 10       ; Transfer the value 10 to the AL register

The Hello World Program in Assembly

The following assembly language code displays the string ‘Hello World’ on the screen −

section	.text
   global _start     ;must be declared for linker (ld)
	
_start:	            ;tells linker entry point
   mov	edx,len     ;message length
   mov	ecx,msg     ;message to write
   mov	ebx,1       ;file descriptor (stdout)
   mov	eax,4       ;system call number (sys_write)
   int	0x80        ;call kernel
	
   mov	eax,1       ;system call number (sys_exit)
   int	0x80        ;call kernel

section	.data
msg db 'Hello, world!', 0xa  ;string to be printed
len equ $ - msg     ;length of the string

When the above code is compiled and executed, it produces the following result −

Hello, world!

The code is in replit.com at https://replit.com/@andyguest/assemblyHelloWorld

Memory Segments

We can replace the keyword section with the keyword segment and the code will still run.

Live Demo
segment .text	   ;code segment
   global _start    ;must be declared for linker 
	
_start:	           ;tell linker entry point
   mov edx,len	   ;message length
   mov ecx,msg     ;message to write
   mov ebx,1	   ;file descriptor (stdout)
   mov eax,4	   ;system call number (sys_write)
   int 0x80	   ;call kernel

   mov eax,1       ;system call number (sys_exit)
   int 0x80	   ;call kernel

segment .data      ;data segment
msg	db 'Hello, world!',0xa   ;our dear string
len	equ	$ - msg          ;length of our dear string

A segmented memory model divides the system memory into groups of independent segments referenced by pointers located in the segment registers. Each segment is used to contain a specific type of data. One segment is used to contain instruction codes, another segment stores the data elements, and a third segment keeps the program stack.

In the light of the above discussion, we can specify various memory segments as −

Data segment − It is represented by .data section and the .bss. The .data section is used to declare the memory region, where data elements are stored for the program. This section cannot be expanded after the data elements are declared, and it remains static throughout the program.

The .bss section is also a static memory section that contains buffers for data to be declared later in the program. This buffer memory is zero-filled.
Code segment − It is represented by .text section. This defines an area in memory that stores the instruction codes. This is also a fixed area.
Stack − This segment contains data values passed to functions and procedures within the program. This is both a stack and The Stack.

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