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INTRODUCTION TO IBM PC ASSEMBLY LANGUAGE

INTRODUCTION TO IBM PC ASSEMBLY LANGUAGE. Assembly Language Syntax. An assembly language program consists of statements. The syntax of an assembly language program statement obeys the following rules:. RULES. Only one statement is written per line

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INTRODUCTION TO IBM PC ASSEMBLY LANGUAGE

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  1. INTRODUCTION TO IBM PC ASSEMBLY LANGUAGE CAP221

  2. Assembly Language Syntax • An assembly language program consists of statements. • The syntax of an assembly language program statement obeys the following rules: CAP221

  3. RULES • Only one statement is written per line • Each statement is either aninstructionor anassembler directive • instruction is translated into machine code • assembler directiveinstructs the assembler to perform some specific task CAP221

  4. Program Statement • The general format for an assembly language program statement is as follows: name operation operand’(s) comment Examples: START: MOV CX,5 ; initialize counter MAIN PROC CAP221

  5. Name Field • This field is used for: • instruction label: if present, a label must be followed by a colon (:) • procedure names • variable names. CAP221

  6. Name Field • Assembler translates names into memory addresses. • Names can be from 1 to 31 characters long: (letters, digits, and special characters: ?, ., _, $, @, %) • Embedded blanks are not allowed, names may not begin with a digit, period (if used) must be the first character CAP221

  7. Name Field Examples: Legal namesIllegal names COUNTER1 2ABC @CHARACTER TWO WORDS $500 A45.26 SUM_OF_DIGITS YOU&ME .TEST DONE? CAP221

  8. Operation Field For an instruction • This field consists of a symbolic operation code, known as opcode • The opcode describes the operation’s function • Symbolic opcodes are translated into machine language opcode. CAP221

  9. Operation Field For an assembler directive • This field consists of a pseudo-operation code (pseudo-op) • pseudo-ops tell assembly to do something CAP221

  10. Operand Field For an instruction • This field specifies data to be acted on. It may have one, two or no operands at all. • Examples of instructions with different operand fields NOP ; Instruction with no operand field INC AX ; Instruction with one operand field ADD AX, 2 ; Instruction with two operand field If 2 operands: the first is destination, the second is the source operand CAP221

  11. Operand Field For an assembler directive • This field contains more information about the directive CAP221

  12. Comment Field • A semicolon marks the beginning of a comment • A semicolon in the beginning of a line makes it all a comment line • Good programming practice dictates the use of a comment on almost every line. CAP221

  13. Key rules for the use of comments • Do not say something that is obvious • Put instruction in context of program CAP221

  14. Comment Field Examples of good and bad Comments MOV CX , 0 ; Move 0 to CX (This is not a good comment.) MOV CX , 0 ; CX counts terms, initially set to 0 (This is a good comment.) CAP221

  15. Numbers • Binary number is written as a bit string followed by the letter `b`. • decimal number is written as a string of decimal digits followed by the letter `d`. • Hex number is written as a string of hex digits followed by the letter `h`. • Hex number must begin with a decimal digit • Numbers may have an optional sign CAP221

  16. Numbers Examples: numbertype 1010 decimal 1010B binary -2134D decimal ABFFH illegal 0ABFFH hex 1BHH illegal 1BFFH hex 1,23 illegal CAP221

  17. Characters • Characters and character segments must be enclosed in single or double quotes; ‘A' , “hello“. • Assembler translates characters to their ASCII code CAP221

  18. Variables Declaring Integer Variables: • An integer is a whole number, such as 4 or 4444. Integers have no fractional part. Integer variables can be initialized in several ways with the data allocation directives. CAP221

  19. Variables Allocating Memory for Integer Variables: • When an integer variable is declared, the assembler allocates memory space for the variable. The variable name becomes a reference to the memory space allocated to that variable. CAP221

  20. Syntax name directive initializer initial value CAP221

  21. Variables Pseudo-optypesizerange • DB unsigned 1 byte 0 to 255. signed 1 byte -128 to +127. • DW unsigned 2 bytes 0 to 65,535 (64K). signed 2 bytes -32,768 to +32,767. • DD unsigned 4 bytes 0 to 4,294,967,295 (4 Mbytes). signed 4 bytes -2,147,483,648 to +2,147,483,647. • DQ 8-byte integer 4 consecutive words • DT 10-byte integer 10 consecutive bytes CAP221

  22. Byte variables • Syntax: Name DB initial value Examples: ALPHA DB 4 BYT DB ? CAP221

  23. Word variables • Syntax: Name DW initial value Example: WRD DW -2 • The assembler stores integers with the least significant byte in the lowest address of the memory area allocated to the integer Example: WD DW 1234H low byteWD contains 34h, high byte contains 12h CAP221

  24. Array Declaration  • An array is a sequential collection of variables, all of the same size and type • Array elements occupy contiguous memory locations. • The program references each element relative to the start of the array. • An array is declared by giving it a name, a type, and a series of initializing values or placeholders (?). CAP221

  25. Array Examples B_ARRAY DB 10, 25, 20 If array starts at offset address 0200h, it will look like this: SymbolAddressContents B-ARRAY 0200H 10 B-ARRAY+1 0200H+1 25 B-ARRAY+2 0200H+2 20 CAP221

  26. Array Examples W_ARRAY DW 0FFFFh, 789Ah, 0BCDEh If array starts at offset address 0100h, it will look like this: SymbolAddressContents W_ARRAY 0100H FFFFH W_ARRAY+2 0102H 789AH W_ARRAY+4 0104H BCDEH CAP221

  27. Character strings • An array of characters can be initialized by a string of characters. • Inside a string, the assembler differentiates between upper and lower cases (different ASCII codes). • It is possible to combine characters and numbers in one definition CAP221

  28. Character strings Examples: 1) LETTERS DB ‘AaBCbc‘ Is equivalent to LETTERS DB 41H,61H,42H,43H,62H,63H 2) MSG DB ‘ABC‘,0AH,0DH,‘$‘ Is equivalent to MSG DB 41H,42H,43H,0AH,0DH,24H CAP221

  29. Constant Declaration  • In an assembly language program, constants are defined through the use of the EQU directive. • Syntax: Name EQU constant • The EQU directive is used to assign a name to a constant. • Use of constant names makes an assembly language easier to understand. • No memory is allocated for a constant. • The symbol on the right of EQU cab also be a string CAP221

  30. Constant Declaration Examples: 1) LF EQU 0AH ; LF can be used in place of 0Ah MOV DL LF MOV DL 0AH 2) PMT EQU ‘TYPE YOUR NAME‘ ; instead of MSG DB ‘TYPE YOUR NAME‘ We can use MSG DB PMT Have the same machine code CAP221

  31. BASIC INSTRUCTIONS MOV and XCHG CAP221

  32. MOV instruction • Is used to transfer data : • between registers, • between a register & a memory location. Or • To move a number directly into a register or memory location. CAP221

  33. Syntax MOV destination , source Example: MOV AX , WORD1 This reads “ Move WORD1 to AX “ The contents of register AX are replaced by the contents of the memory location WORD1. CAP221

  34. Mov AX , WORD1 After Before 0006 0008 AX AX 0008 0008 WORD1 WORD1 CAP221

  35. MOV AX , BX • AX gets what was previously in BX , BX is unchanged. CAP221

  36. MOV AH , ‘A’ • This is a move of the 041h ( the ASCII code of “A” ) into register AH. • The previous value of AH is overwritten ( replaced by new value ) CAP221

  37. XCHG instruction • (Exchange) operation is used to exchange the contents of • two registers, or • a register and a memory location CAP221

  38. Syntax XCHG destination , source CAP221

  39. Example XCHG AH , BL This instruction swaps the contents of AH and BL. CAP221

  40. XCHG AH , BL After Before 1A 00 05 00 AH AL AH AL 00 05 00 1A BH BL BH BL CAP221

  41. Example XCHG AX , WORD1 • This swaps the contents of AX and memory location WORD1. CAP221

  42. Restrictions on MOV & XCHG MOV Destination Operand CAP221

  43. Restrictions on MOV & XCHG XCHG Destination Operand CAP221

  44. Restrictions on MOV & XCHG Example : ILLEGAL : MOV WORD1 , WORD2 LEGAL: MOV AX , WORD2 MOV WORD1 , AX CAP221

  45. ADD & SUB • Are used to add & subtract the contents of • two registers, • a register & memory location , or • add ( subtract ) a number to ( from ) a register or a memory location. CAP221

  46. Syntax ADD destination , source SUB destination , source CAP221

  47. Example ADD WORD1 , AX This instruction , “ Add AX to WORD1 “ , causes the contents of AX & memory word WORD1 to be added, and the sum is stored in WORD1. AX is unchanged. CAP221

  48. ADD WORD1 , AX Before After 01BC 01BC AX AX 06DF 0523 WORD1 WORD1 CAP221

  49. Example SUB AX , DX This instruction , “ Subtract DX from AX “ , the value of DX is subtracted from the value of AX , with the difference being stored in AX. DX is unchanged. CAP221

  50. SUB AX , DX Before After 0000 FFFF AX AX 0001 0001 DX DX CAP221

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