Assembly Language and Compilation Flow

Assembly Language

• Low-level programming language for microprocessors and other devices
• Basic programming language for processors
• Processors understand machine language instructions

Compilation Flow

• C/C++ Compilation Process
  • Preprocessing: Processes include files, conditional compilation instructions, and macros
  • Compilation: Takes preprocessor output and source code, generates assembler source code
  • Assembly: Produces assembly listing with offsets from assembly source code
  • Linking: Combines object files or libraries to produce a single file

Machine Language

• Example: 10100001 10111100 10010011 00000100
  • z=x+y example, a machine language instruction

Machine and Assembly Language

• Assembly Language Overview
  • Uses symbolic names for operations, registers, memory locations
  • Higher-level language with better instruction readability
  • Provides one-to-one correspondence with machine language instructions
• Assemblers translate assembly to machine code
• Compilers translate high-level programs to machine code, either directly or indirectly, via a compiler

Assembler

• A program that converts source code programs written in assembly language into object files in machine language

MNEMONICS

• Specify opcode for complete machine language instruction
• Assembler translates to generate object code
• MOV used for data copy/movement between registers/memory locations

Instruction

• Program commands instructions computer actions
• Computers handle binary data; hence instructions must be in binary format (0s, 1s)
• All binary instructions form the computer's machine language, also known as the instruction set

Instruction Fields

• Opcode field specifies specific operation
• Each operation has a unique opcode
• Operands fields specify source and destination operands
• Source/destination can be constant, memory, or general-purpose registers

Assembly Instruction

• Built from two pieces: Opcode, Operands

Types of Opcodes

• Arithmetic & Logical (ADD, SUB, MULT, AND, OR, CMP)
• Memory Load/Store (LOAD, LD, LOD, ST)
• Control Transfer (JMP, BNE)

Complex

• ASCII
  • Scheme for assigning numeric values to punctuation marks, spaces, numbers, etc.
  • Uses 7 bits for character representation
  • Uses values 0000000-1111111 or 00-7F for 128 characters
  • Extended version assigns characters from 80-FF

Parts of Assembly Language Program

• Header Files
• Directives
• Labels
• Comments

Assembly Language: Header Files

• No direct concept of header files
• Used to define functions, variables, macros
• Can be included in multiple source files

Macros

• Defined using .macro and .endm directives
• Useful for defining reusable code blocks
• Can take parameters and be expanded inline during assembly

Include

• Supports .include for file inclusion
• Facilitates sharing of common definitions or code snippets

External Reference

• Declareable using .extern directive
• Symbols can be in assembly files or C/C++ code
• Linker resolves external references during linking process

Org 100h

• Specific to MASM and TASM
• Sets code segment origin
• Instructs assembler to place instructions at memory address 100h

Directives

• Commands for controlling assembly process
• Guide assembler in interpreting and translating assembly code

Data Definition Directives

• DB/db : This data definition directive defines bytes of data
• DW/dw : This data definition directive defines words of data
• DD/dd : This data definition directive defines doublewords of data
• DQ/dq : This data definition directive defines quadwords of data

Segment Definition Directives

• .data : This segment definition directive defines the data segment
• .code : This segment definition directive defines the code segment

.bss and .align

• .bss : Defines initialized data segment (not supported by EMU8086)
• .align : Aligns the next data or instruction to a specified boundary

Memory Model

• Organize and manage memory segments for code, data, and stack
• Crucial in older 16-bit x86 architectures
• Ensured limited and segmented memory

Memory Model Types

• Small
• Medium
• Compact
• Large
• Huge

Labels

• Symbolic names for memory locations
• Mark jump instructions target
• Define entry procedures
• Reference data locations

Comments

• Supplementary text added to the code to enhance comprehension, but typically ignored by the assembler

Best Practice for Labels & Comments

• Use meaningful labels
• Consistent formatting
• Comment on complex or tricky parts of the code
• Explain the overall purpose of code sections
• Avoid over-commenting
• Update comments as code changes

Control Flow Instructions

• Unconditional Jump
• Conditional Jump
• Looping
• Procedures & Subroutines

Unconditional Jump

• Fundamental control flow
• Allows abrupt program execution transfer

Conditional Jump

• Instruction altering program's control flow
• Condition evaluated through value comparison or processor flag status check

Looping

• Executes code block repeatedly until condition is met
• Combines conditional jumps and instruction counters
  • Loop instruction decrements CX register, jumps back to label if CX isn't zero
  • Conditional jump checks the condition, jumps back to loop start.

Procedures and Subroutines

• Facilitates modularity
• Enhances code reusability
• Improves program organization
  • Procedures: sequence of instructions for specific tasks
  • Subroutines: smaller, specialized tasks for multiple locations

Test your knowledge on assembly language, its integration with machine language, and the compilation process in C/C++. This quiz covers fundamental concepts such as preprocessing, linking, and the significance of symbolic representation in programming. Perfect for students in computer science!