Req. For Assembly Language

• Assembly Language Features


  • Program comments
  • Reserved words
  • Identifiers
  • Statements
  • Directives
• Program comments

  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. Good programming practice dictates a comment on almost every line.
• It is almost impossible to understand an assembly language program without comments
• Comments are used to put the instruction into the context of the program
• 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
• Identifiers

  • Name refers to the address of a data item
  • Label refers to the address of an instruction, procedure, or segment
• Identifiers


  • Can be from 1 to 31 characters long (not case sensitive)
  • May consist of letters, digits, and the special characters ? . @ _ $ %
  • Names may not begin with a digit
  • If a dot is used, it must be the first character
• Embedded blanks are not allowed in identifiers
• YOU&ME is an example of an identifier containing an illegal character
• Statements


  • Both instructions and directives have up to four fields: [identifier] operation [operand(s)] [comment]
  • 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
• Directives


  • SEGMENT Directive: Data Segment, Stack segment, Code Segment
  • END Directive: ENDP directive ends a procedure, END directive ends the entire program and appears as the last statement
• 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
  • 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
• The appropriate model is SMALL, unless there is a lot of code or data
• The .MODEL directive should come before segment definitions
• 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
• Program Structure - Stack Segment

  • .STACK 100H ; sets aside 100H bytes for the stack area
• A segment is 2^16 = 64k
• Program Structure - Data Segment


  • The data segment contains all the variable definitions
  • Constant definitions are often made here as well
  • To declare a data segment, we use the directive .DATA, followed by variable and constant declarations
• Program Structure - Code Segment


  • The code segment contains a program's instructions
  • Syntax: .CODE name ; where name is an optional name of segment
  • Inside a code segment, instructions are organized as procedures
• Program Structure - Code Segment (Procedures)

  The simplest procedure definition is: name PROC ; name: is the name of the procedure. ; body of the procedure ; PROC & ENDP: are pseudo-ops that delineate the procedure name ENDP
• There is no need for a name in a SMALL program, However, the assembler will generate an error
• 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