CS 003 - Module 2

Historical Development of Computer

DEBUG

A line-oriented debugger in DOS, used as an assembler, disassembler, or hex dump tool

DEBUG allows to

• Examine memory contents interactively

• Modify memory data

• Execute assembly programs

Command: CLS | Function: ?

Clear screen

Command: CD | Function: ?

Change directory

Command: DIR | Function: ?

List directory

Command: COPY | Function: ?

Copy files

Command: DEL | Function: ?

Delete file

Command: REN | Function: ?

Rename file

Command: MD | Function: ?

Make directory

Command: RD | Function: ?

Remove directory

Advantages of DEBUG

• Free and universally available

• Simple to use

• Requires minimal memory

Command: Q | Description: ? | Example: -Q

Quit DEBUG and return to DOS

Command: H | Description: ? | Example: -H 000C 0008

Perform hex addition and subtraction (4-bit numbers)

Command: R | Description: ? | Example: -R, -R CX

Display or modify registers

Command: E | Description: ? | Example: -E 0200

Enter data or instructions into memory at a given address

Command: D | Description: ? | Example: -D 0200

Display memory contents (hex and ASCII) starting at address

Command: A | Description: ? | Example: -A 0100

Assemble program from mnemonic code into machine code

Command: T | Description: ? | Example: -T

Trace execution one instruction at a time

Command: G | Description: ? | Example: -G

Run the loaded program in memory

Command: U | Description: ? | Example: -U 0100

Unassemble machine code to mnemonic starting at address

Command: N | Description: ? | Example: -N A:SAMPLE.COM

Name the program file (8 character base + .COM extension)

Command: W | Description: ? | Example: -W

Write the program to disk

Rules of Debug Commands

• Commands are not case sensitive.

• Numbers are assumed to be hexadecimal unless specified.

• Syntax for segment:offset is <segment>:<offset>.

Command: MOV | Description: ?

Copy/transfer data

Command: ADD | Description: ?

Add values

Command: SUB | Description: ?

Subtract values

Command: MUL | Description: ?

Multiply register by AX

Command: DIV | Description: ?

Divide AX by register

Command: DIV | Description: ?

Divide AX by register

Command: INC | Description: ?

Increment register by 1

Command: DEC | Description: ?

Decrement register by 1

Command: LOOP | Description: ?

Repeat block CX times

Von Neumann Architecture

• Foundation of most modern computers

• Stores both data and instructions in the same memory

• Uses a single shared bus for data and instructions

Fixed Program Computers

Non-programmable, specific tasks (e.g., calculators)

Stored Program Computers

Can be programmed with different tasks (general-purpose)

Component: ? | Function: Stores data and instructions

Memory

Component: ? | Function: Executes instructions; includes ALU and Control Unit

CPU

Component: ? | Function: Performs arithmetic and logical operations

Arithmetic Logic Unit (ALU)

Component: ? | Function: Directs data flow, manages CPU operations

Control Unit (CU)

Component: ? | Function: Holds address of the next instruction to execute

Program Counter (PC)

Component: ? | Function: Stores current instruction being executed

Instruction Register (IR)

Component: ? | Function: Stores memory address for data/instruction fetch or store

Memory Address Register (MAR)

Component: ? | Function: Temporarily holds data moved between memory and CPU

Memory Data Register (MDR)

Component: ? | Function: Holds intermediate results in arithmetic/logical operations

Accumulator

Component: ? | Function: Temporary data storage during processing

General Purpose Registers

Component: ? | Function: Transfers data, addresses, and control signals across CPU, memory, and I/O devices

Bus

Component: ? | Function: Interface and hardware for communication with external devices

I/O Interface & Devices

Key Characteristics of Von Neumann Architecture

• Single memory for both data and instructions

• Shared bus for communication

• Sequential execution of instructions, one at a time

Von Neumann Bottleneck

• Limits CPU performance because data and instructions share the same bus and memory

• Cannot process instructions and data simultaneously

• Performance constrained by memory access speed and bus bandwidth

Advantages of Von Neumann Architecture

• Simplified design and hardware

• Cost-effective implementation

• Flexible and supports general-purpose computing

• Easier programming due to unified memory

• Widely used in modern computers

Limitations of Von Neumann Architecture

• Memory bottleneck limits performance

• Sequential processing limits speed for complex operations

• Increased power consumption due to frequent memory access

• Limited scalability for high-performance tasks

• Latency delays in fetching instructions and data

Programs & Execution

• Stored on disk (OS, data, apps)

• Loaded into memory before execution

• CPU uses registers then executes arithmetic, data movement, addressing

Unit: ? | Description: Smallest unit; 0 (off) or 1 (on)

Bit

Unit: ? | Description: 8 bits + 1 parity bit (odd parity)

Byte

Unit: ? | Description: 2 bytes (16 bits)

Word

Unit: ? | Description: 4 bytes (32 bits)

Doubleword

Unit: ? | Description: 8 bytes (64 bits)

Quadword

Unit: ? | Description: 16 bytes (128 bits)

Paragraph

Unit: ? | Description: 1,024 bytes (2¹⁰)

Kilobyte (KB)

Unit: ? | Description: 1,048,576 bytes (2²⁰)

Megabyte (MB)

Processor Registers

Provide fast storage inside the CPU to avoid slow memory access

General Purpose Register

For arithmetic, logical operations, and temporary data. Used as:

• 32-bit (EAX, EBX, ECX, EDX)

• 16-bit (AX, BX, CX, DX)

• 8-bit parts (AH, AL, etc.)

Pointer Registers

Hold addresses for instructions and stack

• Instruction Pointer (IP)

• Stack Pointer (SP)

• Base Pointer (BP).

Index Registers

Used for string and indexed operations

• Source Index (SI)

• Destination Index (DI)

Control Registers

Instruction Pointer and Flags registers controlling operation states.

Register: AX | Special Use: ?

Primary accumulator, I/O, arithmetic

Register: BX | Special Use: ?

Base register for addressing

Register: CX | Special Use: ?

Count register for loops

Register: DX | Special Use: ?

Data register, I/O, multiplication/division

Flag: Overflow (OF) | Meaning: ?

Overflow occurred in arithmetic operation

Flag: Direction (DF) | Meaning: ?

String operation direction (0=left-to-right, 1=right-to-left)

Flag: Interrupt (IF) | Meaning: ?

Enables/disables external interrupts

Flag: Trap (TF) | Meaning: ?

Enables single-step processor mode

Flag: Sign (SF) | Meaning: ?

Indicates sign of result (0=positive, 1=negative)

Flag: Zero (ZF) | Meaning: ?

Zero result flag (1 if result is zero)

Flag: Auxiliary Carry (AF) | Meaning: ?

Carry between bits 3 and 4

Flag: Parity (PF) | Meaning: ?

Parity of result (even/odd number of 1-bits)

Flag: Carry (CF) | Meaning: ?

Carry from high-order bit in arithmetic/shift

Segment Registers

Used to access different segments in memory for code, data, and stack

Register: CS | Purpose: ?

Code segment (instructions)

Register: DS | Purpose: ?

Data segment

Register: SS | Purpose: ?

Stack segment

Register: ES, FS, GS | Purpose: ?

Extra segments for data