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Understanding Assembly Language Addressing Modes

Learn about Register Indirect, Based, and Indexed Addressing Modes with examples and explanations in assembly language. Understand how operands are addressed using different modes.

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Understanding Assembly Language Addressing Modes

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  1. Assembly Language Lecture 7 (Array and Addressing Modes)

  2. Lecture Outline • Introduction • Addressing Modes • Register Indirect Mode • Based and Indexed Addressing Modes Addressing Modes 1

  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 two dimensional arrays Used with one 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 2

  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 3

  5. Register Indirect Mode • 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 4

  6. Register Indirect Mode • 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 6

  7. Based and 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 7

  8. The operand’s segment number is contained in DS The operand’s segment number is contained in SS Based and Indexed Addressing Modes • 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 8

  9. Based and Indexed Addressing Modes • 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 9

  10. Based and Indexed Addressing Modes • 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 10

  11. Based and Indexed Addressing Modes • 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 11

  12. Based and Indexed Addressing Modes • Ex. Suppose that ALPHA is declared as • ALPHA DW 0123H,0456H,0789H,0ABCDH • In segment address by DS. Suppose also that • BX contains 2 offset 0002 contains 1084 h • SI contains 4 offset 0004contains 2BACh • DI contains 1 • What will be the result of following instructions: Addressing Modes 11

  13. Based and Indexed Addressing Modes • Ex. Replace each lower case letter in the following string by it’s upper case equivalence. Use index addressing mode • MSG DB ‘this is a message’ • Solution: • MOV CX,17 ; NO of chars in string • XOR SI, SI ; clear SI indexes a char • TOP: • CMP MSG[SI],’ ‘ ;BLANK ? • JE NEXT ; yes skip over • AND MSG[SI],0DFH ; convert to upper case • NEXT: • INC SI ;Index next byte • LOOP TOP ; loop until done Addressing Modes 11

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