Assembly Language Lecture 9 Array and Addressing Modes

Assembly Language Lecture 9 (Array and Addressing Modes)

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

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 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 Addressing Modes 2

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 The operand’s segment number is contained in DS • SI • DI The operand’s segment number is contained in SS • BP Addressing Modes 3

Register Indirect Mode • Ex. suppose that SI contains 0100 h, and the word at 0100 h contains 1234 h. • MOV AX, [SI] The CPU: 1. Examines SI and obtains the offset address 100 h. 2. Uses the address DS: 0100 h to obtain the value 1234 h. 3. Moves 1234 h to AX. • MOV AX, SI The CPU will move the value of SI, namely 100 h, into AX. Addressing Modes 4

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 LEA SI, W MOV CX, 10 ADDNOS: ADD AX, [SI] ADD SI, 2 LOOP ADDNOS Addressing Modes ; AX holds sum ; SI points to array W ; CX has number of elements ; sum = sum + element ; move pointer to the next element ; loop until done 6

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 + [register] displacement [register] Addressing Modes 7

Based and Indexed Addressing Modes • The register must be one of the following: • BX The operand’s segment number is contained in DS • SI • DI The operand’s segment number is contained in SS • 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

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

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

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 XOR BX, BX MOV CX, 10 ADDNOS: ADD AX, W[BX] ADD BX, 2 LOOP ADDNOS Addressing Modes ; AX holds sum. Can also use MOV AX, 0 ; clear base register ; CX has number of elements ; sum = sum + element ; index next element ; loop until done 11

Based and Indexed Addressing Modes • Ex. Suppose that ALPHA is declared as ALPHA DW 0123 H, 0456 H, 0789 H, 0 ABCDH In segment address by DS. Suppose also that BX contains 2 offset 0002 contains 1084 h SI contains 4 offset 0004 contains 2 BACh DI contains 1 What will be the result of following instructions: instruction NUMBER MOVED MOV AX, [ALPHA+BX] 0456 H MOV BX, [BX+2] 2 BACH MOV CX, ALPHA[SI] 0789 H MOV AX, -2[SI] 1084 H MOV BX, [ALPHA+3+DI] 0789 H MOV AX, [BX]2 Addressing Modes illegal 11

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], 0 DFH ; convert to upper case NEXT: INC SI ; Index next byte LOOP TOP ; loop until done Addressing Modes 11