ICS312 Set 4 Program Structure

Outline for a SMALL Model Program Note the quiz at the next lecture will be to reproduce this slide.MODEL SMALL.586 ; allows Pentium instructions to be used.STACK 100H.DATA ; put data definitions here.CODE MAIN PROC ; put instructions for MAIN procedure here MAIN ENDP ; include additional procedures after MAIN procedure SUB1 PROC... SUB1 ENDP END MAIN ; END directive ends program ; Label used with END directive matches ; label of first instruction to be executed in the ; code segment. (Any legal label can be used.)

Program Structure (1) Memory Model Use SMALL memory model for assignments --- creates 1 code segment, 1 data segment and 1 stack segment..MODEL SMALL Processor Directive To use the Pentium instruction set for PCs, code the 586 processor directive immediately after the.MODEL directive Example.MODEL SMALL.586

Program Structure (2) Stack Segment Set the stack size to 100h bytes, using.STACK 100h If no stack size is set, the default size is 1000h bytes. A smaller stack size (100h bytes) is more than enough for programs in this course.

Program Structure (3) Data Segment Define all variables in the data segment, so the memory allocated for them will be in the program's data area. Different ways data can be coded Numbers (all converted into binary) binary numbers with suffix “b”, eg: b hexadecimal numbers with suffix “h”, eg: 3Fh decimal number Characters Use single or double quotes eg: ‘HI THERE’ or “HI THERE” Variables Variables require that memory space be allocated for storing values of a specified data type. In assembler programming, a variable's data type determines the amount of space needed: one byte, two bytes, 4 bytes, etc. Byte Variables declared using DB pseudo-opcode Word Variables (two bytes) declared using DW pseudo-opcode Double Word Variables (four bytes) declared using DD pseudo-opcode

Program Structure (4) Example of a Data Segment. DATA CRLF EQU 0DH, 0AH PROMPT DB 'Enter a digit between 0 and 9$’ VAR1 DB ? ARRAY DW 1234h, 23h, 0FF54h

Program Structure (5) Code Segment The code segment contains all the source code for the program. Starts with the directive:.CODE Use procedures to organize the program code (example below) Assembly Language Syntax General form of assembly language Instructions: [name] [operation code] [operand(s)] [;comment] Types of Assembly Language Statements: Instructions - part of the machine language instruction set. Instructions will be converted into machine code by the assembler translator. Directives/Pseudo-Operations - directives are NOT part of the machine language instruction set and will be processed by the assembler translator at assembly time.

Input and Output Instructions Some basic INT 21H Input/Output Functions: Functio n DescriptionRegister Setup Result 1 Input one character from keyboard with echo to screen AH = 1 AL = ASCII code of the character, if char key pressed, or AL = 0, if non-char key pressed eg:the up arrow 2 Output one character to screen AH = 2 DL = ASCII code or control character code Contents of DL are written on screen Examples shown above: Read a character from the keyboard Display a character on the monitor

ASCII codes for control characters ASCII codeSymbolFunction 07hBELbeep 08hBSbackspace 09hHThorizontal tab 0AhLFline feed 0DhCRcarriage return

Displaying a String Function:Description:Input: 9 Display a string AH = 9 DX = offset address of string. String must end with '$'

Example Write a program that displays a message on the screen. Define the message in the data segment of the program. Use LEA Instruction to put the address of the message in the DX register. Display the Message. TITLE SAMPLE PROGRAM.MODEL SMALL.586.STACK 100H.DATA MES DB ‘This is a sample program$’.CODE MAIN PROC MOV MOV DS, AX MOV AH, 9 LEA DX, MES INT 21H MOV AH, 4CH INT 21H MAIN ENDP END MAIN The portions in the blue boxes will occur in every main program exactly as shown, except that the label “main” (in its three ocurrences) can be replaced by any other label.

HMW: Your First Program Write a program to read a character from the keyboard and display it at the beginning of the next line. Program outline: Display a question mark to prompt for input. Read a character Display the character on the next line save the character in another register move the cursor to the next line (execute carriage return and line feed) display the character

Creating and running your program Create the source file in text format using any word processor. Give it a name with “asm” as suffix, e.g: first.asm To use the word processor available in dos, employ: edit first.asm Refer to the instructions on downloading Masm 6.15 on the web page. Assemble and run first.asm as illustrated there, i.e: ml first.asm first

Notes: 1. For the purpose of readibility, employ function 2 to output a single character, and function 9 to output a string of more than one character 2. You can’t assume that the int 21h subroutines will preserve the values of al, ax or eax. So your code for a new line might wipe out any character that you have just read into al.

Textbook Reading (Jones): Chapter 2 Assembler Overview & first part of Chapter 3