Lecture 1 Chapter 4 –Requirements for coding in Assembly Language

1. Lecture 1 Chapter 4 –Requirements for coding in Assembly Language

2. Chapter Outline • Assembly Language Features • Simplified segment Directive • Defining Types of data • Equate Directive 1

3. Assembly Language Features • Program comments • Reserved words • Identifiers • Statements • Directives 4

4. Program Comment • The comment field of a statement is used by the programmer to say • something about what the statement does. • A semicolon marks the beginning of this field, and the assembler • ignores anything typed after the semicolon. • It is almost impossible to understand an assembly language program • without comments. • Good programming practice dictates a comment on almost every • line. 13

5. Program Comment • Examples: • MOV CX, 0 ; CX counts terms, initially 0 • Thus, comments is used to put the instruction into the context of the • program. • It is permissible to make an entire line a comment, and to use them • to create space in a program. 14

6. Reserved words • Instructions, such as MOV and ADD • Directives, such as END that used to provide information to the assembler • Operators • Predefined symbols, such as @Data, which return information to your program during the assembly

7. Identifiers Two types of Identifiers : name and label 1. Name refers to the address of a data items ex: COUNTER ,SUM,ID 2. Label refers to the address of an instruction,procedure,or segment ex: MAIN

8. Identifiers • Can be from 1 to 31 characters long (not case sensitive). • May consist of letters, digits, and the special characters • ? . @ _ $ % (Thus, embedded blanks are not allowed). • Names may not begin with a digit. • If a dot is used, it must be the first character.

9. Identifiers • Examples: • COUNTER1 • 2abc • Begins with a digit • @CHARACTER • A45. 28 • . Not first character • TWO WORDS • Contains a blank • STD_NUM • .TEST • YOU&ME • Contains an illegal character 8

10. Statements • Both instructions and directives have up to four fields: • [identifier ]operation [operand(s)][comment] • [Name Fields are optional] • At least one blank or tab character must separate the fields. • The fields do not have to be aligned in a particular column, but they • must appear in the above order. • An example of an instruction: • START:MOVCX,5; initialize counter • An example of an assembler directive: • MAINPROC 6

11. Directives • SEGMENT Directive • Data Segment • Stack segment • Code Segment • END Directive • ex: ENDP directive ends a procedure • ex: END directive ends the entire program and appears as the last statement 9

12. SIMPLIFIED SEGMENT Directives • SEGMENT Directive • Data Segment • Stack segment • Code Segment • END Directive • ex: ENDP directive ends a procedure • ex: END directive ends the entire program and appears as the last statment 9

13. Program Structure - Memory Models • The size of code and data a program can have is determined by • specifying a memory model using the .MODEL directive. • Syntax: • .MODEL memory_model Model Description SMALL code in 1 segment data in 1 segment MEDIUM code > 1 segment data in 1 segment COMPACT code in 1 segment data > 1 segment LARGE code > 1 segment data > 1 segment no array larger than 64k bytes HUGE code > 1 segment data > 1 segment arrays may be larger than 64k bytes

14. Program Structure - Memory Models • The appropriate model is SMALL, unless there is a lot of code or • data. • .MODEL directive should come before segment definitions. • A segment is 216 (64 k)

15. Program Structure - Stack Segment • The purpose of the stack segment declaration is to set aside a block • of memory (the stack area) to store the stack. • The stack area should be big enough to contain the stack at its • maximum size. • Syntax: • .STACK size ; where size is an optional number that specifies • ; the stack area size in bytes. • Example: • .STACK 100H ; sets aside 100H bytes for the stack area. • ; (reasonable size for most applications). • If size is omitted, 1KB is set aside for the stack area.

16. Program Structure - Data Segment • A program’s data segment contains all the variable definitions. • Constant definitions are often made here as well. (they may be • placed elsewhere in the program since no memory allocation is • involved). • To declare a data segment, we use the directive .DATA, followed by • variable and constant declarations. • Example: • .DATA • WORD1 DW 2 • MSG DB ‘this is a message’

17. Program Structure - Code Segment • The code segment contains a program’s instructions. • Syntax: • .CODE name ; where name is an optional name of segment. • There is no need for a name in a SMALL program, However, the • assembler will generate an error. • Inside a code segment, instructions are organized as procedures.

18. Program Structure - Code Segment • The simplest procedure definition is: • name PROC ; name: is the name of the procedure. • ; body of the procedure ; PROC & ENDP: are pseudo-ops that • name ENDP ; delineate the procedure • Example of a code segment definition: • .CODE • MAIN PROC • ; main procedure instructions • MAIN ENDP • ; other procedures go here

19. Program Structure - A General Form of a .SMALL model program .MODEL SMALL .STACK 100H .DATA ; data definitions go here .CODE MAIN PROC ; instructions go here MAIN ENDP ; other procedures go here END MAIN