Chapter 3

Data Representation

 Modified: 

 

3.1    Basic Elements of Assembly Language

A Complete Assembly Language Program
INCLUDE Irvine32.inc
.code
main       PROC
mov
add
sub
call

exit

 eax, 1000h
eax, 4000h
eax, 2000h
DumpRegs

main       ENDP
END        main

3.1.1    Integer Constants

[{+|-}] digits [radix]

[] means optional

{} means choice of one

| means OR

Common Radix
binary 01010b
decimal -123
hexadecimal 0ABCDh

3.1.2    Integer Expressions

Precedence as in programming languages such as Java.

5 + -7 * 3  is -16

(5 + -7) * 3 is -6

 

3.1.3    Real Number Constants

[{+|-}] integer.[integer][exponent]

+3.14
-3.8E-1

  

3.1.4    Character Constants

'X'
"X"

 

3.1.5    String Constants

'XYZ'
"XYZ"

 

3.1.6    Reserved Words

Cannot use words with special meaning.

Mnemonics, operators, etc.

BYTE
WORD
ADD
etc.

 

3.1.7    Identifiers

Name variables, labels, constants, procedures, etc.

AbC
A$$@_5?$
@endif

 

3.1.8    Directives

Provide direction to assembler, not executed.

.Data    Directive that following text is part of program data.

.Code    Directive that following text is part of program code.

See Appendix A for complete list.

 

3.1.9    Instructions

[label:] mnemonic operand(s)  [; comment]

Label is any valid identifier

: serves to mark a location in a program algorithm,

or location of a variable.

@while:

@endwhile:

Mnemonic is an instruction operation.

Add, Sub, etc.

Operands of the operation.

Add    eax, 4

 

3.2    Adding Three Integers

Adding three integers
INCLUDE Irvine32.inc
.code
main    PROC
 mov         eax, 1000h
 add         eax, 4000h
 sub         eax, 2000h
 call        DumpRegs

 exit

main    ENDP
END     main

 

Executing

Click in the left gutter next to the instruction where execution should break.

Select Debug | Start Debugging

A window should open (check the task bar) with the following.

 

3.3    Assembling, Linking and Running Programs

3.3.1    Assemble-Link-Execute Cycle

Assemble - checks syntax and generates machine code corresponding to data and code.

Link - combines machine code with external code from libraries to an executable form.

Execute - loads the executable into memory and starts execution.

Listing File

Microsoft (R) Macro Assembler Version 9.00.21022.08 12/13/09 20:57:23
.\main.asm Page 1 - 1


INCLUDE z:\d\c335\irvine\Irvine32.inc

00000000 .code
00000000                      main PROC
00000000 B8 00001000          mov eax, 1000h
00000005 05 00004000          add eax, 4000h
0000000A 2D 00002000         sub eax, 2000h
0000000F E8 00000000 E       call DumpRegs
                                             exit

0000001B                     main ENDP
END main

 

3.4    Defining Data

Definition Type Example
BYTE 8 bit unsigned X  BYTE 0A3h
SBYTE 8 bit signed Y  SBYTE -128
WORD 16 bit unsigned Z  WORD 0ABCDh
SWORD 16 bit signed A  SWORD -1234
DWORD 32 bit unsigned B  DWORD 12345678h
SDWORD 32 bit signed C  SDWORD -12345678
FWORD 48 bit integer D  FWORD  123456
REAL4 4 byte IEEE short real E  REAL4    3.1417

 

3.4.9    Little Endian Order

WORD - Stored in reverse byte order.

1234h stored as 34 12.

DWORD - Stored in reverse byte order.

12345678h stored as 78 56 34 12.

Example
 
                  .data  
   Address                                      
   00405000        X        WORD    15    
   00405002        Y        BYTE      7
   00405003        C        BYTE      'C'
   00405004        A2E    BYTE      'ABCDE'
   00405009        Z        WORD    1, 2, 3
   0040500F        A        BYTE      5 dup(9)
   00405014        N       DWORD  12345678h
   00405018
Address        Contents                       ASCII

0x00405000 0f 00 07 43 41 42 43 44 ...CABCD

0x00405008 45 01 00 02 00 03 00 09 E.......

0x00405010 09 09 09 09 78 56 34 12 ....xV4.

Arrays - occupy memory in sequential order.

A2E is an array definition of 5 bytes stored sequentially as:

0x00405000 0f 00 07 43 41 42 43 44 ...CABCD
0x00405008 45 01 00 02 00 03 00 09 E.......

A2E starts at address 0x00405004

A2E array
0 1 2 3 4
'A' 'B' 'C' 'D' 'E'
41 42 43 44 45

Z is an array definition of 3 words stored sequentially as:

0x00405008 45 01 00 02 00 03 00 09 E.......

Z array
0 1 2
1 2 3
01 00 02 00 03 00

 

3.5    Symbolic Constants

Use when a constant has special meaning (e.g. Age, PI).

Define once, use throughout program makes changing a constant simple.

Example

.data
   MYNAME    EQU   'ABCDE'
   PAYRATE   EQU   15 

   A2D          BYTE  MYNAME

.code
                   add     eax, PAYRATE