introduction to ibm pc assembly language n.
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  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 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

  19. 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

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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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


  28. 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

  29. 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

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

  31. 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

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

  33. Syntax XCHG destination , source CAP221

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

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

  36. Restrictions on MOV & XCHG MOV Destination Operand CAP221

  37. Restrictions on MOV & XCHG XCHG Destination Operand CAP221

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

  39. 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

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

  41. 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

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

  43. 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

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

  45. Legal combinations of operands forADD & SUB Destination operand CAP221

  46. ILLEGAL ADD BYTE1 , BYTE2 Solution : move BYTE2 to a register before adding MOV AL , BYTE2 ; AL gets BYTE2 ADD BYTE1 , AL; add it to BYTE1 CAP221

  47. INC ( increment ) Is used to add 1to the contents of a • Register or • Memory location CAP221

  48. DEC ( decrement ) Is used to subtract 1 from the contents of a • Register or • Memory location CAP221

  49. Syntax INC destination DEC destination CAP221

  50. Example INC WORD1 adds 1 to the contents of WORD1 CAP221