Computer Systems: High-Level vs Low-Level Languages

• The lecture is on Computer Systems and is taught at Montclair State University by Dr. Prince Waqas Khan.

High-Level vs Low-Level Languages

• High-level languages feature human-readable syntax, platform independence, abstraction from hardware, and are productivity-focused.
• Low-level languages are hardware-specific, allow direct memory manipulation, provide minimal abstraction, and focus on performance.

Programming Hierarchy

• High-level languages include Python, Java, and C++, which are human-readable, portable, and abstract.
• Mid-level languages like C offer a balance between abstraction and hardware control.
• Assembly language is architecture-specific and has a direct hardware correspondence.
• Machine code consists of binary instructions executed by the CPU.

The Four Stages of Compilation

• Preprocessing removes comments, expands included macros, performs code maintenance, and prepares code for the compiler, and uses C preprocessor directives like #include and #define.
• Compiling translates preprocessed code into assembly language, performs syntax and semantic analysis, generates an intermediate representation, and optimizes code; assembly language code is the final output as .s or .asm files.
• Assembling converts assembly language to machine code, translates mnemonics to binary opcodes, resolves symbolic names and addresses, generates object code (.o or .obj files), and is an architecture-specific process.
• Linking combines multiple object files, resolves external references, incorporates library code, assigns final memory addresses, and produces an executable program.

Example High Level Language conversion to Assembly Language

• High-level language may be converted to assembly language.
• The high-level will initialize a variable with “var a=1 while(true) { a=a+1; }”.
• The assembly converts it to: “mov r0, #1 @ Initialize a = 1 in register ro”.
• Then adds a loop “loop: @ Label for loop start add r0, r0, #1 @ a = a + 1. b loop @ Unconditional branch back to loop”

CPU Instruction Execution

• Fetch involves the CPU retrieving an instruction from memory.
• Decode refers to the instruction being decoded to determine the operation.
• Execute means the CPU performs the operation.
• Store means results are stored in memory or registers.
• Repeat is the process continuing with the next instruction.

Instruction Set Architecture (ISA)

• ISA serves as the abstract interface between hardware and software that defines supported instructions, data types and registers, memory management, and binary encoding.
• Supported instructions are basic operations like ADD and LOAD that a processor can execute.
• Data Types & Registers define how data is stored/accessed, for example, as 32-bit integers or floating-point numbers.
• Memory Management includes addressing modes, virtual memory, and I/O operations.
• Binary Encoding defines the format of the machine code, including opcodes and operands.

CISC vs RISC

• CISC (Complex Instruction Set Computer) is an ISA where single instructions execute multiple low-level operations, it contains examples like Intel x86 and Motorola 68000.
• RISC (Reduced Instruction Set Computer) is an ISA with simple, fixed-length instructions optimized for single-cycle execution and includes examples like ARM, RISC-V, and MIPS.

Examples of ISA

• x86 (CISC: Complex instructions, desktop CPUs).
• ARM, MIPS (RISC: Simple instructions, mobile devices).
• RISC-V (Open-source, modular ISA).

Load/Store Architecture

• Load transfers data from memory (RAM) to CPU registers.
• Store transfers data from CPU registers to memory (RAM).
• MIPS uses a strict load/store architecture.
• ARM mainly primarily uses load/store, but includes more features than MIPS.

Assembly Language

• Assembly language is a low-level programming language with a strong correspondence to machine code (1 assembly instruction ≈ 1 machine instruction), is architecture-specific, and uses mnemonics instead of binary.
• Key features include minimal abstraction, direct hardware control, and compiler transparency.
• Learning assembly helps understand computer architecture, optimize critical code, and debug/reverse engineer compiled programs.
• ISAs are hardware standards like x86-64, ARM, and MIPS.
• Assembly languages are text-based representations of those ISAs.
• Assemblers convert assembly language to ISA machine code.