CS401 Midterm Short Notes by Almas Afzal

CS401 – Preface

Q1. What is Assembly Language?

•  A programming language that talks directly to the computer.
•  In high-level languages (like C, C++, Java), there is a “gap” between the

programmer and the machine.

•  This gap is filled by compilers or interpreters, but in assembly, you bypass most of them.
•  It works close to hardware, giving you more control.

Q2. Is it difficult?

•  People think it’s hard, but it’s actually simple.
•  It looks hard because we are used to thinking computers are complicated.
•  Assembly is just direct and clear — what you write is exactly what the computer does.

3. Course focus

•  Examples will use IBM PC with Intel processors.
•  Concepts will be platform-independent (can work on any system).

Basic knowledge needed:

1. Digital logic (AND, OR, NOT)
2. Binary numbers and arithmetic

Q4. Why learn Assembly?

Optimization: Makes code faster by removing unnecessary instructions. Real-time systems: Needed when operations must finish in a fixed time. Fine control: You control each instruction — no hidden steps like in “for” loops in high-level languages.

Q5. When to use it?

•  In performance-critical parts of a program (e.g., image processing).
•  To make programs run faster by saving CPU cycles.
•  To meet exact timing requirements in hardware control systems.

Q6. How to learn it?

•  You can’t just read; you must practice.
•  Try every example, run code, test, and experiment.
•  Only by doing this will you see how powerful and simple it is.

Chapter 1 · Introduction to Assembly Language · 1.1. BASIC COMPUTER ARCHITECTURE · 1. Computer main parts:

1. Processor (CPU) – Does the operations.
2. Memory (RAM) – Stores data.
3. I/O devices – Connects computer to outside world (keyboard, display, etc.).

For now, we focus only on Processor + Memory.

2. Communication between CPU & Memory

To work, CPU must:

•  Know which data to read/write.
•  Tell memory whether to read or write.
•  Actually transfer the data.

This is done via 3 types of buses: · Bus · Purpose · Direction

Sends memory address (location) to access. Unidirectional (CPU →Memory)

Data Bus

Transfers actual data between CPU & Memory.

Bus · Purpose · Direction

Control Bus Sends control signals (read/write, status info).

3. How each bus works

•  Tells memory exactly which cell to use (like giving an address to a postman).
•  Always one-way from CPU to Memory.
•  Example: “Fifth book from top” = specific memory location.
•  Memory cell size is called cell width (often 8-bit = 1 byte).

Data Bus

•  Transfers actual data.
•  Can go both directions: 1. CPU →Memory (write) 2. Memory →CPU (read)

Control Bus

Sends read/write commands and special status signals. Also lets peripherals send “status” back to CPU (e.g., “door locked” message). Ensures synchronization — CPU reads when memory is ready, writes when memory can accept.

4. Why synchronization is important

•  If CPU wants to read but memory is trying to write →No communication happens.
•  Control bus ensures CPU and memory act in correct order (one speaks, one listens).

1.2 Registers

What are Registers?

•  Tiny, super-fast storage areas inside the CPU.
•  Used to hold data (operands) temporarily while processing.
•  Faster than main memory.
•  Very few in number →precious resource.

Why do we need Registers?

•  CPU can get only one data item at a time from memory via the data bus.