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  • EE2801-L07P01

    EE2801 -- Lecture 7

    Assembly Languageand

    Computer Organization

  • EE2801-L07P02

    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

  • EE2801-L07P03

    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

  • EE2801-L07P04

    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

  • EE2801-L07P05

    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

    EE2801-L07P06

    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

    EE2801-L07P07

    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

    EE2801-L07P08

    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