An Introduction to

1
Chapter 3

• An Introduction to
• 8086 Family Assembly Language
• Programming

2
Review of Registers

• In EU
• General purpose registers
• AX, BX, CX, DX
• Others
• SP - Stack Pointer (top of the stack)
• ST - Status flag
• BP - Base Pointer, SI - Source Index, DI -
  Destination Index
• these usually hold the 16 bit offset of a data
  word in one of the segments

3

• In BIU
• Segments registers
• CS, DS, SS, ES
• Others
• IP - Instruction Pointer (next instruction in the
  CS)
• Queue - a set of registers

4
Review of Addressing Modes

• Immediate - operand is a constant value contained
  in the instruction
• MOV AX, 0100H
• Register - operand is an 8-bit or a 16-bit
  register
• MOV SP, BX
• Direct - operand is a fixed location in memory
• MOV CL, 48H
• MOV 48H, CL

5
Another Addressing Mode

• Indirect - operand is a memory location
  determined at run-time by using the address
  stored in one or two registers
• MOV AX, BXSI

6
Program Development Steps

• Define the problem
• a statement of purpose
• Design
• an algorithm
• Develop/Implement
• source code
• Test
• test plan

7
Design Approaches

• Top-Down
• Break the problem into sub-problems repeat!
• Bottom-Up
• Write low-level modules independently, and then
  hope the pieces fit together!
• Which do you think is better?
• Which do you think is more common?

8
Representing Your Program

• Flowcharts
• use symbols to represent processes, subroutines,
  input output, and decisions
• Pseudocode
• any desired program operation can be represented
  by three basic types of operations
• sequential
• selection
• iteration

9
Flowchart Symbols
10
Standard Programming Structures

• Sequence
• IF-THEN, IF-THEN-ELSE, CASE
• WHILE-DO, REPEAT-UNTIL, FOR-DO

11
Types of Assembly Instructions

• Data Transfer
• MOV, LEA, PUSH, POP, IN, OUT
• Arithmetic
• ADD, SUB, MUL, DIV
• Bit Manipulation
• NOT, AND, OR, ROL, SHR
• String Instructions
• LODS, STOS, INS

12

• Program Execution
• LOOP, JNS, CALL, RET, JMP
• Processor Control
• HLT, NOP, STI
• These are listed in chapter 3 with some detail,
  and in chapter 6 in with great detail
• Appendix B contains the mnemonics with their
  english translation

13
Coding Template for 8086 Instructions
14
Instruction Coding Examples

• MOV SP, BX
• MOV CL, BX
• MOV CSBX, DL
• segment override prefix

15
Machine Code Translations

• This is what an assembler does for us!
• Very cumbersome
• Sources for information
• Chapter 3 has some useful tables
• Appendix A which has the data sheet for the 8086
• Appendix B which has templates

16
Assemblers and Assembly Language

• As weve observed, creating machine code programs
  is tough!
• Hard to remember numbers for instructions
• And what about calculating addresses for jumps!
• Assembly language is the answer (somewhat!)
• symboliccan use words to represent operations
  and operands
• low-level
• each assembly instruction corresponds to one
  machine code instruction
• translated by an assembler into machine code

17
Two-Pass Assembly

• Label may be used before it is defined
• Address which label refers to is unknownforward
  reference
• CMP DX, BX
• JLE FISHPOND
• FISHPOND PUSH AX
• Two-Pass Assemblers read input file twice to
  overcome this problem

18
Two-Pass Assembly

• Opcode table
• contains an entry for each valid combination of
  mnemonic and operand(s)
• used to translate mnemonic into assembly

19

• Symbol table
• holds values of symbols as they are defined
• two ways to define symbols
• a label specifying a jump address
• an explicit definition using a pseudo-op
  (assembler directives)
• TEMP EQU 2FF0H

20
Two Passes

• First Pass
• Build the symbol table
• scan input file
• identify labels and pseudo-ops
• store ltlabel, valuegt pairs in the symbol table
• Second Pass
• Generate the machine code output
• scan input file
• look-up mnemonics in the opcode table
• look-up symbols in the symbol table
• generate machine code output
• generate reports

21
Language Components of MASM

• Reserved words
• Identifiers
• Predefined symbols
• Constants
• Expressions
• Operators
• Data types
• Registers
• Statements

22
Reserved Words

• instructions
• operations the processor can execute
• directives
• give commands to the assembler
• operators
• used in expressions such as product2
• predefined symbols
• return info to your program such as _at_data
• reserved words are not case sensitive, except for
  the predefined symbols

23
Identifiers

• a name that you invent and attach to a definition
• can represent variables, constants, procedure
  names, segment names, and user-defined data types
  such as structures
• cannot exceed 247 characters
• the first character can be alphabetic (A-Z), or
  _at_, _ ?
• the other characters can by any of the above
  characters or a decimal digit (0 - 9)
• its best to avoid starting an identifier with _at_,
  because many predefined symbols begin with _at_

24
Some Predefined Symbols

• _at_code
• returns the name of the code segment
• _at_data
• returns the name of the data segment
• _at_Model
• returns the selected memory model
• remember these are caps sensitive!

25
Integer Constants

• an integer constant is a series of one or more
  numerals followed by an optional radix specifier
• mov ax, 25
• mov ax, 0b3h
• 25 and 0b3h are integer constants..h is a radix
  specifier
• the default radix is decimal
• b is for binary
• d is for decimal
• h is hex
• hex constants must start with a decimal digitif
  necessary, add a leading zero

26
Symbolic Constant Expressions

• You can create symbolic integer constants using
  the EQU directive
• column EQU 80 Constant - 80
• row EQU 25 Constant - 25
• screen EQU column row Constant - 2000
• you cannot change these!

27
Operators

• operators are used in expressions
• expressions are evaluated at assembly time and do
  not change during execution
• do not confuse with instructions such as ADD,
  MUL, SUB
• examples
• screen EQU column row
• mov amount2, 01dah

28
Data Types

• DB - type byte (8 bits)
• myvar DB 20h an 8 bit word named myvar
• mov ah, myvar move myvar to ah
• DW - type word (16 bits)
• mywordvar DW 20afh a 16 bit word named
  mywordvar
• mov ax, mywordvar move mywordvar to ax
• DD - type double (32 bits)

29
SEGMENT and ENDS Directives

• a logical segment is a group of instruction
  statements or data statements contained between
  SEGMENT and ENDS directives
• similar to the block concept in C
• DATA_HERE SEGMENT
• DATA_HERE ENDS

30
Other Directives

• ASSUME
• tells the assembler which logical segment to use
  for each physical segment
• remember that just because I named my data
  segment DATA_HERE, doesnt tell the assembler
  that thats my data segment!
• ASSUME DSDATA_HERE, CSCODE_HERE
• segment registers other than the code segment
  must be initialized
• mov ax, DATA_HERE
• mov ds, ax

31
MASM Directives

• .TITLE
• give the title of the program
• .DOSSEG
• use the MSDOS segment order
• .MODEL small
• use a small memory model
• .8086
• 8086/88 instructions only
• .STACK 0100h
• start stack segment and specify size
• .DATA
• start data segment
• .CODE
• start code segment
• END
• tells the assembler to STOP readinganything
  after END will be ignored

32
DOS INT 21H instruction

• int 21h interrupt program execution
• 21h specifies that this is a DOS interrupt
• value contained in ax can determine behavior of
  interrupt
• mov ah, 9 specify print to stdout
• mov dx, offset greeting specify what to print
  in dx
• int 21h
• mov ax, 4c00h return to DOS
• int 21h
• see webpage for complete listing of codes

33
Simple Example I

• .model small
• .dosseg
• .data
• greeting db 'hello there',13,10,''
• .code
• mov ax,_at_data
• mov ds,ax
• mov ah,9
• mov dx,offset greeting
• int 21h
• mov ah,4ch
• int 21h
• end

34
Simple Example II
.TITLE TEST .DOSSEG .MODEL SMALL .8086 .DATA
multiplicand DW 204AH multiplier DW
3B2AH product DW 2 DUP(0) .CODE
START mov ax, _at_DATA mov ds,
ax mov ax, multiplicand mul
multiplier mov product, ax mov
product2, dx mov ax, 4C00H int
21H END START