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Lecture 4 Chapter 6 –Symbolic Instruction and Addressing

Lecture 4 Chapter 6 –Symbolic Instruction and Addressing. Chapter Outline Data Transfer Instruction Basic Arithmetic Instruction The INT Instruction. Registers. Information inside the microprocessor is stored in registers.

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Lecture 4 Chapter 6 –Symbolic Instruction and Addressing

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  1. Lecture 4 Chapter 6 –Symbolic Instruction and Addressing

  2. Chapter Outline • Data Transfer Instruction • Basic Arithmetic Instruction • The INT Instruction

  3. Registers • Information inside the microprocessor is stored in registers. • The registers are classified according to the functions they perform • In general there are fourteen 16-bit registers: • Data registers: • There are four general data registers. • They hold data for an operation. • Address registers: • They are divided into segment, pointer, and index registers. • They hold the address of an instruction or data. • Status register: • It is called the FLAGS register. • It keeps the current status of the processor.

  4. AH BH CH DH AL BL CL DL Registers Data Registers AX BX CX DX Segment Registers CS DS SS ES Pointer and Index Registers SI DI SP BP IP FLAGS Register

  5. Data Registers: AX, BX, CX, DX • These four registers, in addition to being general-purpose registers, • also perform special functions. • The high and low bytes of these registers can be accessed separately. • Ex. The high byte of AX is called AH, and the low byte is called AL. • This arrangement gives us more registers to use when dealing • with byte-size data.

  6. Data Registers: AX, BX, CX, DX • AX (Accumulator Register) is the preferred register to use in arithmetic, logic, and data • transfer instructions • BX (Base Register) also serves as an address register. • CX (Count Register) Program loop constructions are facilitated by the use of CX, which • serves as a loop counter. • DX (Data Register) is used in multiplication and division.

  7. Address Registers - Segment Registers CS, DS, SS, ES • Address registers store addresses of instructions and data in memory. • These values are used by the processor to access memory locations. • In the 8086 processor (16-bit processor): • Memory is a collection of bytes, each memory byte has an • address, starting with 0. • The processor assigns a 20-bit physical address to its memory • locations thus it is possible to address 2 = 1,048,576 bytes • (one megabyte) of memory. • The bytes in memory have addresses 00000h to FFFFFh. 20

  8. Address Registers - Segment Registers CS, DS, SS, ES • To keep track of the various program segments, the 8086 is • equipped with four segments registers to hold segment numbers: • CS (Code Segment): contains the code segment number. • DS (Data segment): contains the data segment number. • SS (Stack Segment): contains the stack segment number. • ES (Extra Segment): is used if a program needs to access a • second data segment.

  9. Address Registers - Pointer and Index Registers: SP, BP, SI, DI • SP (Stack Pointer) register is used in conjunction with SS for • accessing the stack segment. • BP (Base Pointer) register is used primarily to access data on the • stack. However, unlike SP, BP can be used to access data in the • other segments. • SI (Source Index) register is used to point to memory locations in • the data segment addressed by DS. By incrementing the contents • of SI, we can easily access consecutive memory locations. • DI (Destination Index) register performs the same functions as SI. • There is a class of instructions, called string operations, that use DI • to access memory locations addressed by ES.

  10. Address Registers - Instruction Pointer (IP) • IP is updated each time an instruction is executed so that it will point • to the next instruction. • Unlike other registers, the IP cannot be directly manipulated by an • instruction (i.e. The instruction cannot contain IP as its operand).

  11. Data Transfer Instruction • MOV • MOVSX-MOVZX • XCHG • LEA

  12. Note: any register can be used except CS & IP Before After 0006 0008 AX AX 0008 0008 WORD1 WORD1 MOV Instruction • The MOV (move) instruction is used to: • Transfer data between Registers. • Transfer data between registers and memory locations. • Move a number directly into a register or memory location. • Syntax: • MOV destination, source • Example: • MOV AX, WORD1 • MOV AX, BX • MOV AX, 'A'

  13. Legal Combinations of operands for MOV Destination Operand General Segment Memory Source operand register register location Constant General register yes yes yes no Segment register yes no yes no Memory location yes yes no no Constant yes no yes no • Illegal: MOV WORD1, WORD2 • Legal: MOV AX, WORD2 • MOV WORD1, AX • Illegal: MOV DS, CS • Legal: MOV AX, CS • MOV DS, AX

  14. Type Agreement of Operands • The operands of any two-operand instruction must be of the same • type (i.e. Both bytes or words). • Illegal: MOV AX, BYTE1 • However, the assembler will accept both of the following: • MOV AH, 'A' moves 41H into AH • MOV AX, 'A' moves 0041H into AX

  15. MOVE-and-Fill Instruction (MOVSX-MOVZX) • The MOVSX (move) instruction is used to: • Move a byte or word source to a word or doubleword destination. • Use with signed arithmetic values • Syntax: • MOVSX destination, source • Example: • MOVSX CX,10110000B . CX= 11111111 10110000

  16. MOVE-and-Fill Instruction (MOVSX-MOVZX) • The MOVZX (move) instruction is used to: • Move a byte or word source to a word or doubleword destination. • Use with unsigned arithmetic values • Syntax: • MOVZX destination, source • Example: • MOVZX CX,10110000B . CX= 00000000 10110000

  17. Before After 1A 00 05 00 AH AL AH AL 00 05 00 1A BH BL BH BL XCHG Instruction • The XCHG (exchange) operation is used to exchange the contents • of: • Two registers. • A register and a memory location. • Syntax: • XCHG destination, source • Example: • XCHG AH, BL • XCHG AX,WORD1

  18. Legal Combinations of operands for XCHG Destination Operand General Memory Source Operand register location General register yes yes Memory location yes no

  19. LEA Instruction • LEA (Load Effective Address) puts a copy of the source offset • address into the destination. • Syntax: • LEA destination, source • Where destination is a general register and source is a memory • location • Example: • MSG DB 41H, 42H, 43H • LEA DX, MSG • puts the offset address of the variable MSG into DX. Data Definition + Basic Instructions 17

  20. Basic Arithmetic Instruction • ADD • SUB • INC • DEC

  21. Before After Before After 0000 FFFF 01BC 01BC AX AX AX AX 0001 0001 0523 06DF DX DX WORD1 WORD1 ADD and SUB Instructions • The ADD (add) and SUB (subtract) instructions are used to: • Add/subtract the contents of: • Two registers. • A register and a memory location. • Add/subtract a number to/from a register or memory location. • Syntax: • ADD destination, source SUB destination, source • Examples: • ADD WORD1, AX SUB AX, DX

  22. Legal Combinations of operands for ADD & SUB Destination Operand General Memory Source Operand register location General register yes yes Memory location yes no Constant yes yes • Illegal: ADD BYTE1, BYTE2 • Legal: MOV AL, BYTE2 • ADD BYTE1, AL

  23. Before After Before After 0002 0003 FFFE FFFD WORD1 WORD1 BYTE1 BYTE1 INC and DEC Instructions • INC (increment) is used to add 1 to the contents of a register or • memory location. • DEC (decrement) is used to subtract 1 from a register or memory • location. • Syntax: • INC destination DEC destination • Examples: • INC WORD1 DEC BYTE1

  24. INT Instruction • To invoke a DOS or BIOS routine, the INT (interrupt) instruction is • used. • Format: • INT interrupt_number • where interrupt_number is a number that specifies a routine.

  25. INT 21h • INT 21h may be used to invoke a large number of DOS functions. • A particular function is requested by placing a function number in • the AH register and invoking INT 21h. • Some of the functions are: • INT21h functions expect input values to be in certain registers and • return output values in other registers. Function number Routine 1 single-key input 2 single-character output 9 character string output

  26. INT 21h Function 1: Single-Key Input Input: AH = 1 Output: AL = ASCII code if character key is pressed = 0 if non-character key is pressed • To invoke the routine, the following instructions should be executed: • MOV AH,1 ; input key function • INT 21H ; ASCII code in AL

  27. INT 21h Function 2: Display a character or execute a control function Input: AH = 2 DL = ASCII code of the character Output AL = ASCII code of the character • To invoke the routine, the following instructions should be executed: • MOV AH, 2 ; display character function • MOV DL, '?' ; character is '?' (or any other character) • INT 21H ; display character

  28. INT 21h • Function 2 may be used to perform control functions. • If DL contains the ASCII code of a control character, INT 21h • causes the control function to be performed. • The principal control characters are : ASCII code (Hex) Symbol Function 07H BEL beep (sounds a tone) 08H BS backspace 09H HT tab 0AH LF line feed (new line) 0DH CR carriage return (start of current line)

  29. INT 21h Function 9: Display a string Input: AH = 9 DX = offset address of string. The string must end with a '$' character • To invoke the routine, the following instructions should be executed: • MOV AX, @DATA • MOV DS, AX • MOV AH, 9 ; display string function • LEA DX, MSG ; get message (Load Effective Address) • INT 21H ; display string A program containing a data segment should begins with these two instructions

  30. INT 21h Function 4CH: Returning control to DOS Input: AH = 4CH • To invoke the routine, the following instructions should be executed: • MOV AH, 4CH ; DOS exit function • INT 21H ; exit to DOS

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