![Example
• Show how character string “RG 2z” is stored in memory
starting at address 0.
• Solution:
12
Address Character ASCII Code (HEX) ASCII Code (Binary)
[Memory Contents]
0 R 52 0101 0010
1 G 47 0100 0111
2 Space 20 0010 0000
3 2 32 0011 0010
4 z 7A 0111 1010](https://image.slidesharecdn.com/labsheet03-240327150928-4057261d/85/Introduction-to-Assembly-Language-various-basic-things-12-320.jpg)
Introduction to Assembly Language & various basic things
- 1. Introduction to IBM PC Assembly Language
- 2. Outline • Assembly language syntax • Program data • Variables • Named constants • A few basic instructions • Translation of high-level language to assembly language • Program structure • Input and output instructions 2
- 3. Assembly Language Syntax • Assembler • Assembly language programs are translated into machine language instructions by assembler • Assembly language code is not case sensitive • Statements • Program consists of statements one per line • Two types: instruction and assembler directive • Statements have four fields • name operation operand(s) comment • Instruction- START: MOV CX,5 ;initialize counter • Assembler directive- MAIN PROC 3
- 4. Name Field • Used for instruction labels, procedure names and variable names • Assembler translates name into memory address • Names can be 1 to 31 characters long and may consist of letters, digits or special characters. • If period is used, it must be first character. • Embedded blanks are not allowed. • May not begin with a digit. • Not case sensitive 4
- 5. Name Field Legal names Illegal Names COUNTER_1 TWOWORDS @character 2abc .TEST A45.28 DONE? YOU&ME 5
- 6. Operation Field • Contains symbolic (Operation code) • Assembler translates op code translated into machine language op code • Examples: ADD, MOV, SUB • In an assembler directive, the operation field represents pseudo-op code • Pseudo-op code is not translated into machine code, it only tells assembler to do something. • Example: PROC psuedo-op is used to create a procedure 6
- 7. Operand Field • Specifies the data that are to be acted on by the operation • An instruction may have zero, one or more operands. • In two-operand instruction, first operand is destination, second operand is source. • For an assembler directive, operand field represents more information about the directive • Examples NOP ;no operand, does nothing INCAX ;one operand, adds 1 to the contents of AX ADDWORD1, 2; two operands, adds value 2 to the contents of memory locationWORD1 7
- 8. Comment Field • Say something about what the statement does • Marked by semicolon in the beginning • Assembler ignores anything after semicolon • Optional • Good practice 8
- 9. Program Data • Processor operates only on binary data. • In assembly language, you can express data in: • Binary • Decimal • Hexadecimal • Characters • Numbers • For Hexadecimal, the number must begin with a decimal digit. E.g.: write 0ABCh not only ABCH. • Cannot contain any non-digit character. E.g.: 1,234 not allowed • Characters enclosed in single or double quotes. • ASCII codes can be used • No difference in “A” and 41h 9
- 10. Variables • Each variable has a data type and is assigned a memory address by the program. • PossibleValues: • 8 Bit Number Range: Signed (-128 to 127), Unsigned (0-255) • 16 Bit Number Range: Signed (-32,678 to 32767), Unsigned (0-65,535) • ?To leave variable uninitialized • Mainly three types • Byte variables • Word variables • Arrays 10
- 11. Data Defining Pseudo-Ops 11 Examples Bytes Description Pseudo-ops var1 DB ‘A’ Var2 DB ? array1 DB 10, 20,30,40 1 Define Byte DB var2 DW ‘AB’ array2 DW 1000, 2000 2 DefineWord DW Var3 DD -214743648 4 Define Double Word DD Var DQ ? 8 Define Quad Word DQ Var DT ? 10 DefineTen Bytes DT
- 12. Example • Show how character string “RG 2z” is stored in memory starting at address 0. • Solution: 12 Address Character ASCII Code (HEX) ASCII Code (Binary) [Memory Contents] 0 R 52 0101 0010 1 G 47 0100 0111 2 Space 20 0010 0000 3 2 32 0011 0010 4 z 7A 0111 1010
- 13. Named Constants • Use symbolic name for a constant quantity • Syntax: • name EQU constant • Example: • LF EQU 0Ah • No memory allocated 13
- 14. A Few Basic Instructions • MOV • XCHG • ADD • SUB • INC • DEC • NEG 14
- 15. MOV • Transfer data • Between registers • Between register and a memory location • Move a number directly to a register or a memory location • Syntax • MOV destination, source • Example • MOVAX,WORD1 15 0006 0008 0008 0008 AX WORD1 Before After
- 16. Legal Combinations of Operands for MOV 16 Destination Operand Source Operand Legal General Register General Register YES General Register Memory Location YES General Register Segment Register YES General Register Constant YES Memory Location General Register YES Memory Location Memory Location NO Memory Location Segment Register YES Memory Location Constant YES
- 17. XCHG • Exchange the contents of • Two registers • Register and a memory location • Syntax • XCHG destination, source • Example • XCHGAH, BL 17 1A 00 00 05 AH BH Before After AL BL 00 1A 05 00 AH AL BH BL
- 18. Destination Operand Source Operand Legal General Register General Register YES General Register Memory Location YES Memory Location General Register YES Memory Location Memory Location NO Legal Combinations of Operands for XCHG
- 19. ADD • To add contents of • Two registers • A register and a memory location • A number to a register • A number to a memory location • Syntax • ADD destination, source • Example • ADDWORD1,AX 19 01BC 0523 01BC 06DF AX WORD1 Before After
- 20. SUB • To subtract the contents of: • Two registers • A register and a memory location • A number from a register • A number from a memory location • Syntax • SUB destination, source • Example • SUB AX, DX 20 0000 0001 FFFF 0001 AX DX Before After
- 21. Destination Operand Source Operand Legal General Register General Register YES General Register Memory Location YES General Register Constant YES Memory Location General Register YES Memory Location Memory Location NO Memory Location Constant YES 21 Legal Combinations of Operands for ADD and SUB
- 22. Memory to Memory Instruction • ADD BYTE1, BYTE2 Illegal instruction • Solution? • MOVAL, BYTE2 ADD BYTE1, AL • What can be other possible solutions? • How can you add two word variables? 22
- 23. INC • INC (increment) instruction is used to add 1 to the contents of • a register • memory location. • Syntax: • INC destination • Example: • INCWORD1 23 0002 0003 WORD1 Before After
- 24. DEC • DEC (decrement) instruction is used to subtract 1 from the contents of • a register • memory location. • Syntax: • DEC destination • Example: • DEC BYTE1 24 FFFE FFFD BYTE1 Before After
- 25. NEG • Used to negate the contents of destination. • Replace the contents by its 2’s complement. • Syntax • NEG destination • Example • NEG BX 25 0002 FFFE BX Before After
- 26. Translation of High-level Language to Assembly Language • Consider instructions: MOV, ADD, SUB, INC, DEC, NEG • A and B are two word variables • Translate statements into assembly language: 26 Statement Translation B = A MOV AX, A MOV B, AX A = 5 - A MOV AX, 5 SUB AX, A MOV A, AX OR NEG A ADD A, 5 A = B – 2 x A MOV AX, B SUB AX, A SUB AX, A MOV A, AX
- 27. Program Structure • Machine Programs consists of • Code • Data • Stack • Each part occupies a memory segment. • Same organization is reflected in an assembly language program as Program Segments. • Each program segment is translated into a memory segment by the assembler. 27
- 28. Memory Models • Determines the size of data and code a program can have. • Syntax: • .MODEL memory_model 28 Model Description SMALL code in one segment, data in one segment MEDIUM code in more than one segment, data in one segment COMPACT code in one segment, data in more than one segment LARGE Both code and data in more than one segments. No array larger than 64KB HUGE Both code and data in more than one segments.Array may be larger than 64KB
- 29. Data Segment • All variable definitions • Constant definitions are often made here • Use .DATA directive • For Example: • .DATA WORD1 DW 2 BYTE1 DB 10h 29
- 30. Stack Segment • A block of memory to store stack • Syntax • .STACK size • Where size is optional and specifies the stack area size in bytes • If size is omitted, 1 KB set aside for stack area • For example: • .STACK 100h 30
- 31. Code Segment • Contains a program’s instructions • Syntax • .CODE name • Where name is optional • Do not write name when using SMALL as a memory model • Inside a code segment instructions are organized as procedures 31
- 32. The Format of a Code .MODEL SMALL .STACK 100h .DATA ;data definition go here .CODE MAIN PROC ;instructions go here MAIN ENDP ;other procedures go here END MAIN 32
- 33. Input and Output Instructions Function Number Routine Function Code 1 Single key input Input:AH=1 Output:AL=ASCIICode if character is pressed =0 if non-character key is pressed MOV AH,1 ;input key function INT 21H ;ASCII code in AL 2 Single character output Input:AH=2 DL=ASCII code of the display character or control character Output:AL=ASCII code of the display character or control character MOV AH,2 ;display character function MOV DL,’? ’;character is ? INT 21H; display character 9 Character string output Input: DX=offset address of string.The string must end with a $ character MSG DB ‘HELLO$’ 33
- 34. LEA Instruction • It means Load Effective Address • To get the offset address of the character string in DX we use LEA instruction • Syntax • LEA destination, source • Destination is a general register and source is a memory location • Example • LEA DX, MSG 34
- 35. Assignment 04 • Write a program to input an uppercase letter and display its corresponding lowercase letter. • Write a program to read two decimal digits whose sum is less than 10 and display them and their sum in the next line with appropriate message. 35