EE2801-L07P01

EE2801 -- Lecture 7

Assembly Languageand

Computer Organization

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Assembly Programming Is A 3 Step Process

Everything in a computer system is located at some well-defined location in the memory or

the IO space of the computer.

Doing something useful with a computer involves reading data from one location, operating

on it, and storing it in another location. It really doesnt matter what the information is, or

what the desired action is, the process remains the same.

For assembly language programs, creating a program is a 3 step process:

ASCII

Source

(.asm)

Object

Files

(.obj)

Executable

(.exe)

TASM

TLINK

EDIT

TD

Assembler

Debugger

Linker

Text

Editor

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Addresses - A Place For Everything

Both memory and IO are accessed using addresses. Data is sent to a memory or IO

location by to an address. Data is received from memory or IO locations by

from an address.

Portability is achieved (in part) by having rules, or standards, for how addresses are

assigned and for the behavior that is supposed to occur when addresses are accessed.

For example, the PC has a that looks something like:

writing reading

memory map

64Kb Boot

ROM

64Kb Option

ROM

128Kb Reserved

for device ROMs

128Kb Video

Memory

640Kb

Conventional

Memory

FFFFFh

F0000h

EFFFFh

E0000h

DFFFFh

C0000h

BFFFFh

A0000h

9FFFFh

00000h

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Program Loading And The Operating System

Executable

(.exe)

In order to execute, somehow the executable program image must be mapped to the

memory space of the computer. In general, parts of the executable will be stored in ROM

and some in RAM. Under an operating system like Linux or Windows, the program will

load into RAM, but theres no way to tell exactly where!

It is the job of a loader to assign the image to the proper memory space(s).

640Kb

Conventional

Memory

9FFFFh

00000h

Loader

Stack

Code

Data

Note: On a PC, when a program

starts, its stack, data, and extra

segments are initialized!not

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Segment-Based Addressing In The 80x86

In order to calculate a real, physical memory address the 80x86 uses a 16 bit segment

register and combines it with a 16 bit offset.

For example, consider the instruction pointer (IP). To determine the actual physical

memory location of the code, we shift the CS register left 4 bits, and add it to the IP.

Thus, if CS = A000h and IP = 27A5H then CS:IP = A27A5h as follows:

All of the segment registers in the 80x86 work this way, however the offset does not usually

come from the IP register when the DS, SS, and ES registers are used.

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 0

0

0

0

0

0

0 0

0

0

0

0

0

0 0

0

0

0

0

0

0 0

0

0

0

0

0

CS = A000h

IP = 27A5h

CS:IP = A27A5h

Shift CS Left

What Segmenting Is Really Doing

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Any segment register can hold any 16 bit value. Likewise, any 16 bit offset can be added

to any segment register. Thus, there are 65,536 possible segment locations, and each can

be thought of as a pointer to an array of 65536 memory locations

00000h

FFFFFh

64K Code

Segment

64K Extra

Segment

64K Stack

Segment

64K Data

Segment

CS

DS

SS

ES

Working With Segment Registers

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Although segments and offset might sound a little confusing, there some typical ways that

segments and offsets are associated with each other, and some typical applications of the

various segments.

This register is used to find addresses for program instructions. The physical address of an

instruction is normally calculated using the IP register (CS:IP).

CS - Code Segment Register

DS - Data Segment Register

This register is used to find addresses for data items in memory. There are a number of

ways that an offset may be obtained such as direct input of a number or using the BX, DI,

or SI registers (or a combination).

The extra segment is not used very often in simple programs. When it is, it is usually

associated with string operations and the offset usually comes from the DI register.

The SP register is the most common source of an offset into the stack segment. BP is also

commonly used.

ES - Extra Segment Register

SS - Stack Segment Register

Keeping Track Of Segment Usage

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Lets review the assembly language program we saw in the last lecture and think about

what the segments are doing:

Notice:

not all instructions access data from memory.

some data values are compiled in

.Data ;Create a symbolic name for the beginning of the data segment.

A db 7d ;Create a symbolic name for DS:0000 put a value there.

B db 12d ;

C db ? ;

.Code ;

start: nop ;Instruction at CS:0000

mov ax,@data ;

mov ds,ax ;

mov al,byte ptr[A] ;

mov bl,byte ptr[B] ;

add al, bl ;

mov byte ptr[C],al ;

stop: jmp stop ;

and

Create a symbolic name for DS:0001 put a value there.

Create a symbolic name for DS:0002

Create a symbolic name for the beginning of the code segment.

Instruction at CS:0001

Instruction at CS:0004

Instruction at CS:0006 gets data from location DS:0000

Instruction at CS:0009 gets data from location DS:0001

Instruction at CS:000D

Instruction at CS:000F stores data in location DS:0002

Instruction at CS:0012

and