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Experiment 0

COE 205Computer Organization & Assembly

Language ProgrammingTerm 043

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Outline

• Definition of a program

• Instruction atomicity

• Assembly language

• Tools

• Setting up the environment

• Policy

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Definition of a Program• A sequence of instructions and data that

causes a machine to perform a given task.• An Instruction has two fields: Operation &

Operands▪ Operation (Opcode)▪ Operands: Data

ADDITIONA

BY

Operation Addition

Data A & B

Instruction Y ← ADD A, B

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• An instruction is atomic.▪ Indivisible▪ Non-interruptible operation

• Hardware ensures the non-interruptible execution of an instruction.

• A complex operation is made of simple instructions.

• Example:▪ Multiplication

- SHIFT- ADD

Instruction Atomicity

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A User’s View of Computer Systems

• Depends on the degree of abstraction provided by the underlying software

• We consider a hierarchy of six levels– Moving to the top of hierarchy shields the

user from the lower-level details

– The top two levels are system independent

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A User’s View of Computer Systems (cont.)

– The other lower four levels are system dependent

• Assembly and machine languages are specific to a particular processor

• One-to-one correspondence between assembly language and machine language

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A User’s View of Computer Systems (cont.)

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General Properties of a Personal Computer

• I/O devices – keyboard, scanner, monitor, printer, projector, light pen, etc.

• Memory unit – where program is kept (debug, d100 lets look at memory) (u 100 unassemble memory, see what is there)

• CPU – registers, ALU and stack pointer, control unit and IP (instruction pointer)

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RAM-Random Access Memory

• This is where our program stays when we run it. 1600:0100 is an example of a RAM address.

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ROM–Read Only Memory• ROM is read only memory and cannot be

changed. ROM has a program burned in the chip.

• PROM

• EPROM is erasable programmable read only memory and is expensive and is done with very special equipment.

• EEPROM

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BOOTUP- FIRMWARE• Firmware lies between hardware and

software. Firmware is the software that is burned into the chip.

• When you turn on the PC. An address is loaded into the CPU’s IP. The address is of the ROM’s firmware. This firmware program is called the BOOTSTRAP program. Then the program runs and loads the operating system. The computer is ready to be used !

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ROM BIOS• Address 0 – 1024 interrupt vectors---------------------------------------------------------------• 1024 – (2028 – 4096) BIOS Data---------------------------------------------------------------• (about 2028-4096) – 640 KB (about 640,000)

RAM for your program---------------------------------------------------------------• Above 640KB < 1 MB Video Memory---------------------------------------------------------------• 1MB – 1.5 MB ROM BIOS – this is Basic Input

Output System.

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Real Mode / Protected Mode• All PCs even the real old ones start in REAL

MODE where the first 1 MB is usable. All modern operating systems such as LINUX, Widows 95/98/2000/Millennium / XP immediately switch to PROTECTED MODE letting us use all the memory (except ROM BIOS and where the OS is located and memory where other’s programs run)

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CPU• Contained in one IC – integrated circuit• 8080, 80x86 = 80286, 80386, 80486, etc.• Pentium, Pentium 2, Celeron (similar),

Pentium 3, Pentium 4, etc.• ALU – arithmetic logical unit • CU – control unit• Fetch, decode, execute

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A Small Program in Debug• C:\windows> debug• - a 100 • 1600:0100 MOV AX,5• 1600:0103 INC AX• Hit Enter• -r• -t • Watch IP and current instruction

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Machine Cycles• Business Executives, e.g., Discuss Machine

Cycles when they say their PC is 900 MHZ or 3 GHz.

• Each command such as MOV AX, 5 (move 5 to AX register) takes a few computer cycles to execute.

• The faster your computer, the more cycles per second it performs. Cycles per second.

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Advantages of High-Level Languages• Program development is faster

– Higher level instructions = less coding– Some simple high-level language instructions can be

expressed by a single assembly instruction– But most high-level language instructions need more

than one assembly instruction• Programs maintenance is easier

– For the same reasons as above• Programs are portable

– Contain few machine-dependent details – Can be used with little or no modifications on different

types of machines– Compiler translates to the target machine language

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Why Program in Assembly Language?• Two main reasons:

1. Efficiency• Space-efficiency• Time-efficiency

2. Accessibility to system hardware

• Assembly language can be used as a tool to learn computer organization

– You will know more about the organization and internal workings of a computer system

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Space-efficiency• Assembly code tends to be more compact

than when written in HLL.

• Not a big plus point for most applications

• Code compactness is important in some cases– Portable and hand-held device software

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Time-efficiency• Assembly language programs tend to run

faster– Only a well-written assembly language program

runs faster

• Time Convenience – (Graphics, hardware drivers)

• Time Critical – (Aircraft navigational systems, Process control systems, Robot control software, Missile control software)

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Accessibility to system hardware

• System software typically requires direct control of the system hardware devices– Assemblers, linkers, compilers– Network interfaces and other device drivers– Video games

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Bits and Bytes

• 1 or 0 is a bit

• 4 bits = a nibble (1/4 of a register)

• 8 bits = 1 byte

• 2 bytes = 1 word = register size

• 1024 bytes = 1 KB = 1kilobyte

• 1,048,576 bytes = 1 MB = 1 megabyte

• 1,073,741,824 bytes = 1 GB = 1 gigabyte

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BITS, decimal, hexadecimal• 0 off, 1 on 1001 = 9• 01 = 1 1010 = 10 or A• 10 = 2 1011 = 11 or B• 011 =3 1100 = 12 or C• 100 = 4 1101 = 13 or D• 101 =5 1110 = 14 or E• 110 =6 1111 = 15 or F• 111 = 7• 1000 = 8

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Bits, Bytes, OpCode

• Viewing machine code equivalent of instruction set

• C:\WINDOWS>debug

• -a 100

• 106C:0100 MOV AX,5

• 106C:0103

• -d 100

• B8 05 in memory means MOV AX, 05

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16 Bit Addressing

• The IBM uses 16 bit addressing which lets us use only 65,536 bytes. How do we address up to 1 MB ?

• 1600:0100 (Segment Number:Offset)• 1600 is really 16000, a zero is added. • Then 0100 is the other part added on to give the exact

location !

16000 (add 0 to 16-bit segment base value)

+0100 (offset value)

16100 (Physical address)

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Memory location

• 106C:0100 This is a memory location.

• It is actually 106C0

+0100

107C0

• B8 05 is what is there. The OpCode is B8

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MNEMONIC

• MOV, INC, are examples of mnemonics– MOV – move– INC – increment

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CISC architecture

• Complex Instruction Set Computing

• This is used by the INTEL 80X86 chips. It is pronounced SISK. CISC architecture uses very few registers so the instructions take many cycles and run slower.

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RISC Architecture

• Reduced Instruction Set Computing

• This is used by APPLE 680X0 family chips and one instruction takes 1 machine cycle. Very fast, very efficient, but many registers and more difficult to program since there are more registers and mnemonics to use.

• SPARC stations use RISC

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PROGRAMS

• Programs are basically a set of instructions to do some task. Here is a simple assembly program.

MOV AH,02 // get display ready

MOV DL,41 // Capital A

INT 21h // Interrupt 21

INT 20h //interrupt 20, quit

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NEW Tools

• ASCII Chart

• What is an A (41 hex)

• MOV AH,02h - monitor

• MOV DL,41 – 41 is an A

• INT 21h - interrupt 21 to execute I/O

• (cout << “A”)

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Exiting a program

• INT 21h

• This is like EXIT_SUCCESS