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Riyadh Philanthropic Society For Science Prince Sultan College For Woman

Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 251 Introduction to Computer Organization & Assembly Language Lecture 18 Addressing Modes. Outline.

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Riyadh Philanthropic Society For Science Prince Sultan College For Woman

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  1. Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 251 Introduction to Computer Organization & Assembly Language Lecture 18 Addressing Modes

  2. Outline • Study from Chapter 10 (10.1,10.2.1, 10.2.2, example 10.4 not included) • Introduction • Addressing Modes • Register Indirect Mode • Based and Indexed Addressing Modes Addressing Modes

  3. An operand is a register (ex. INC AX) An operand is a constant (ex. ADD A, 5) An operand is a variable(ex. ADD A, 5) Address memory operands indirectly Used with one dimensional arrays Used with two dimensional arrays Introduction • The way an operand is specified is known as its addressing mode. • Addressing modes: • Register mode • Immediate mode • Direct mode • Register Indirect • Based • Indexed • Based Indexed Addressing Modes

  4. The operand’s segment number is contained in DS The operand’s segment number is contained in SS Register Indirect Mode • The offset address of the operand is contained in a register. • I.e. The register acts as a pointer to the memory location. • Format: [register] • The register must be one of the following: • BX • SI • DI • BP Addressing Modes

  5. Register Indirect Mode – EX. 1 • Ex. suppose that SI contains 0100h, and the word at 0100h contains 1234h. • MOV AX, [SI] • The CPU: • 1. Examines SI and obtains the offset address 100h. • 2. Uses the address DS:0100h to obtain the value 1234h. • 3. Moves 1234h to AX. • MOV AX, SI • The CPU will move the value of SI, namely 100h, into AX. Addressing Modes

  6. Register Indirect Mode – EX. 2 • Ex. Write some code to sum in AX the elements of the 10-element array W defined by • W DW 10,20,30,40,50,60,70,80,90,100 • MOV AX, 0 ; AX holds sum • LEA SI, W ; SI points to array W • MOV CX, 10 ; CX has number of elements • ADDNOS: • ADD AX, [SI] ; sum = sum + element • ADD SI, 2 ; move pointer to the next element • LOOP ADDNOS ; loop until done Addressing Modes

  7. Based & Indexed Addressing Modes • The operand’s offset address is obtained by adding a number called a displacement to the contents of a register. • Displacement may be: • The offset address of a variable. (ex. A) • A constant (positive or negative). (ex. -2) • The offset address of a variable + or - a constant. (ex. A + 4) • Syntax: • [register + displacement] • [displacement + register] • [register] + displacement • displacement + [register] • displacement[register] Addressing Modes

  8. The operand’s segment number is contained in DS The operand’s segment number is contained in SS Based & Indexed Addressing Mode (Cont.) • The register must be one of the following: • BX • SI • DI • BP • The addressing mode is called based if BX (base register) or BP (base pointer) is used. • The addressing mode is called indexed if SI (source index) or DI (destination index) is used. Addressing Modes

  9. Based & Indexed Addressing Mode – EX. 1 • Ex. Suppose W is a word array, and BX contains 4 • MOV AX, W[BX] • The displacement is the offset address of variable W • The instruction moves the element at address W + 4 to AX. • (this is the third element in the array) • The instuction could also have been written in any of these forms: • MOV AX, [W+BX] • MOV AX, [BX+W] • MOV AX, W+[BX] • MOV AX, [BX]+W Addressing Modes

  10. Based & Indexed Addressing Mode – EX. 2 • Ex. suppose SI contains the address of a word array W • MOV AX, [SI+2] • The displacement is 2. • The instruction moves the contents of W + 2 to AX. • (this is the second element in the array) • The instruction could also have been written in any of these forms: • MOV AX, [2+SI] • MOV AX, 2+[SI] • MOV AX, [SI]+2 • MOV AX, 2[SI] Addressing Modes

  11. Based & Indexed Addressing Mode – EX. 3 • Ex. Write some code to sum in AX the elements of the 10-element array W defined by • W DW 10,20,30,40,50,60,70,80,90,100 • XOR AX, AX ; AX holds sum. Can also use MOV AX,0 • XOR BX, BX ; clear base register • MOV CX, 10 ; CX has number of elements • ADDNOS: • ADD AX, W[BX] ; sum = sum + element • ADD BX, 2 ; index next element • LOOP ADDNOS ; loop until done Addressing Modes

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