Arrays and addressing modes

Arrays and Addressing Modes Muhammad Bilal Amjad www.facebook.com/bilal.amjad bilalamjad78633@yahoo.com

Presentation Outline Operand Types Memory operands Addressing Modes Copying a String Summing an Array of Integers

Three Basic Types of Operands Immediate Constant integer (8, 16, or 32 bits) Constant value is stored within the instruction Register Name of a register is specified Register number is encoded within the instruction Memory Reference to a location in memory Memory address is encoded within the instruction, or Register holds the address of a memory location

Instruction Operand Notation Operand Description r8 8-bit general-purpose register: AH, AL, BH, BL, CH, CL, DH, DL r16 16-bit general-purpose register: AX, BX, CX, DX, SI, DI, SP, BP r32 32-bit general-purpose register: EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP reg Any general-purpose register sreg 16-bit segment register: CS, DS, SS, ES, FS, GS imm 8-, 16-, or 32-bit immediate value imm8 8-bit immediate byte value imm16 16-bit immediate word value imm32 32-bit immediate doubleword value r/m8 8-bit operand which can be an 8-bit general-purpose register or memory byte r/m16 16-bit operand which can be a 16-bit general-purpose register or memory word r/m32 32-bit operand which can be a 32-bit general register or memory doubleword mem 8-, 16-, or 32-bit memory operand

Next . . . Operand Types Memory operands Addition and Subtraction Addressing Modes Jump and Loop Instructions Copying a String Summing an Array of Integers

Direct Memory Operands Variable names are references to locations in memory Direct Memory Operand: Named reference to a memory location Assembler computes address (offset) of named variable .DATA var1 BYTE 10h .CODE Direct Memory Operand mov al, var1 ; AL = var1 = 10h mov al,[var1] ; AL = var1 = 10h Alternate Format

Direct-Offset Operands  Direct-Offset Operand: Constant offset is added to a named memory location to produce an effective address  Assembler computes the effective address  Lets you access memory locations that have no name .DATA arrayB BYTE 10h,20h,30h,40h .CODE mov al, arrayB+1 ; AL = 20h mov al,[arrayB+1] ; alternative notation mov al, arrayB[1] ; yet another notation Q: Why doesn't arrayB+1 produce 11h?

Direct-Offset Operands - Examples .DATA arrayW WORD 1020h, 3040h, 5060h arrayD DWORD 1, 2, 3, 4 .CODE mov ax, arrayW+2 ; AX = 3040h mov ax, arrayW[4] ; AX = 5060h mov eax,[arrayD+4] ; EAX = 00000002h mov eax,[arrayD-3] ; EAX = 01506030h mov ax, [arrayW+9] ; AX = 0200h mov ax, [arrayD+3] ; Error: Operands are not same size mov ax, [arrayW-2] ; AX = ? Out-of-range address mov eax,[arrayD+16] ; EAX = ? MASM does not detect error 1020 3040 5060 1 2 3 4 20 10 40 30 60 50 01 00 00 00 02 00 00 00 03 00 00 00 04 00 00 00 +1 +2 +3 +4 +5 +1 +2 +3 +4 +5 +6 +7 +8 +9 +10 +11 +12 +13 +14 +15 arrayW arrayD

Next . . . Operand Types Memory operands Addressing Modes Copying a String Summing an Array of Integers

Addressing Modes  Two Basic Questions  Where are the operands?  How memory addresses are computed?  Intel IA-32 supports 3 fundamental addressing modes  Register addressing: operand is in a register  Immediate addressing: operand is stored in the instruction itself  Memory addressing: operand is in memory  Memory Addressing  Variety of addressing modes  Direct and indirect addressing  Support high-level language constructs and data structures

Register and Immediate Addressing  Register Addressing Most efficient way of specifying an operand: no memory access Shorter Instructions: fewer bits are needed to specify register Compilers use registers to optimize code  Immediate Addressing Used to specify a constant Immediate constant is part of the instruction Efficient: no separate operand fetch is needed  Examples mov eax, ebx ; register-to-register move add eax, 5 ; 5 is an immediate constant

Direct Memory Addressing Used to address simple variables in memory Variables are defined in the data section of the program We use the variable name (label) to address memory directly Assembler computes the offset of a variable The variable offset is specified directly as part of the instruction Example .data var1 DWORD 100 var2 DWORD 200 sum DWORD ? .code mov eax, var1 add eax, var2 var1, var2, and sum are direct mov sum, eax memory operands

Register Indirect Addressing  Problem with Direct Memory Addressing Causes problems in addressing arrays and data structures  Does not facilitate using a loop to traverse an array Indirect memory addressing solves this problem  Register Indirect Addressing The memory address is stored in a register Brackets [ ] used to surround the register holding the address For 32-bit addressing, any 32-bit register can be used  Example mov ebx, OFFSET array ; ebx contains the address mov eax, [ebx] ; [ebx] used to access memory EBX contains the address of the operand, not the operand itself

Array Sum ideal for traversing an array  Indirect addressing is Example .data array DWORD 10000h,20000h,30000h .code mov esi, OFFSET array ; esi = array address mov eax,[esi] ; eax = [array] = 10000h add esi,4 ; why 4? add eax,[esi] ; eax = eax + [array+4] add esi,4 ; why 4? add eax,[esi] ; eax = eax + [array+8] Note that ESI register is used as a pointer to array ESI must be incremented by 4 to access the next array element Because each array element is 4 bytes (DWORD) in memory

Ambiguous Indirect Operands  Consider the following instructions: mov [EBX], 100 add [ESI], 20 inc [EDI]  Where EBX, ESI, and EDI contain memory addresses  The size of the memory operand is not clear to the assembler  EBX, ESI, and EDI can be pointers to BYTE, WORD, or DWORD  Solution: use PTR operator to clarify the operand size mov BYTE PTR [EBX], 100 ; BYTE operand in memory add WORD PTR [ESI], 20 ; WORD operand in memory inc DWORD PTR [EDI] ; DWORD operand in memory

Indexed Addressing  Combines a variable's name with an index register  Assembler converts variable's name into a constant offset  Constant offset is added to register to form an effective address  Syntax: [name + index] or name [index] .data array DWORD 10000h,20000h,30000h .code mov esi, 0 ; esi = array index mov eax,array[esi] ; eax = array[0] = 10000h add esi,4 add eax,array[esi] ; eax = eax + array[4] add esi,4 add eax,[array+esi] ; eax = eax + array[8]

Index Scaling  Useful to index array elements of size 2, 4, and 8 bytes  Syntax: [name + index * scale] or name [index * scale]  Effective address is computed as follows:  Name's offset + Index register * Scale factor .DATA arrayB BYTE 10h,20h,30h,40h arrayW WORD 100h,200h,300h,400h arrayD DWORD 10000h,20000h,30000h,40000h .CODE mov esi, 2 mov al, arrayB[esi] ; AL = 30h mov ax, arrayW[esi*2] ; AX = 300h mov eax, arrayD[esi*4] ; EAX = 30000h

Based Addressing Syntax: [Base + Offset] Effective Address = Base register + Constant Offset Useful to access fields of a structure or an object Base Register → points to the base address of the structure Constant Offset → relative offset within the structure .DATA mystruct is a structure mystruct WORD 12 consisting of 3 fields: DWORD 1985 a word, a double BYTE 'M' word, and a byte .CODE mov ebx, OFFSET mystruct mov eax, [ebx+2] ; EAX = 1985 mov al, [ebx+6] ; AL = 'M'

Based-Indexed Addressing  Syntax: [Base + (Index * Scale) + Offset] Scale factor is optional and can be 1, 2, 4, or 8  Useful in accessing two-dimensional arrays Offset: array address => we can refer to the array by name Base register: holds row address => relative to start of array Index register: selects an element of the row => column index Scaling factor: when array element size is 2, 4, or 8 bytes  Useful in accessing arrays of structures (or objects) Base register: holds the address of the array Index register: holds the element address relative to the base Offset: represents the offset of a field within a structure

Based-Indexed Examples .data matrix DWORD 0, 1, 2, 3, 4 ; 4 rows, 5 cols DWORD 10,11,12,13,14 DWORD 20,21,22,23,24 DWORD 30,31,32,33,34 ROWSIZE EQU SIZEOF matrix ; 20 bytes per row .code mov ebx, 2*ROWSIZE ; row index = 2 mov esi, 3 ; col index = 3 mov eax, matrix[ebx+esi*4] ; EAX = matrix[2][3] mov ebx, 3*ROWSIZE ; row index = 3 mov esi, 1 ; col index = 1 mov eax, matrix[ebx+esi*4] ; EAX = matrix[3][1]

LEA Instruction LEA = Load Effective Address LEA r32, mem (Flat-Memory) LEA r16, mem (Real-Address Mode) Calculate and load the effective address of a memory operand Flat memory uses 32-bit effective addresses Real-address mode uses 16-bit effective addresses LEA is similar to MOV … OFFSET, except that: OFFSET operator is executed by the assembler  Used with named variables: address is known to the assembler LEA instruction computes effective address at runtime  Used with indirect operands: effective address is known at runtime

LEA Examples .data array WORD 1000 DUP(?) .code ; Equivalent to . . . lea eax, array ; mov eax, OFFSET array lea eax, array[esi] ; mov eax, esi ; add eax, OFFSET array lea eax, array[esi*2] ; mov eax, esi ; add eax, eax ; add eax, OFFSET array lea eax, [ebx+esi*2] ; mov eax, esi ; add eax, eax ; add eax, ebx

Summary of Addressing Modes Assembler converts a variable name into a constant offset (called also a displacement) For indirect addressing, a base/index register contains an address/index CPU computes the effective address of a memory operand

Registers Used in 32-Bit Addressing  32-bit addressing modes use the following 32-bit registers Base + ( Index * Scale ) + displacement EAX EAX 1 no displacement EBX EBX 2 8-bit displacement ECX ECX 4 32-bit displacement EDX EDX 8 ESI ESI Only the index register can have a scale factor EDI EDI EBP EBP ESP can be used as a base register, but not as an index ESP

16-bit Memory Addressing Old 16-bit Used with real-address mode addressing mode Only 16-bit registers are used No Scale Factor Only BX or BP can be the base register Only SI or DI can be the index register Displacement can be 0, 8, or 16 bits

Default Segments  When 32-bit register indirect addressing is used … Address in EAX, EBX, ECX, EDX, ESI, and EDI is relative to DS Address in EBP and ESP is relative to SS In flat-memory model, DS and SS are the same segment  Therefore, no need to worry about the default segment  When 16-bit register indirect addressing is used … Address in BX, SI, or DI is relative to the data segment DS Address in BP is relative to the stack segment SS In real-address mode, DS and SS can be different segments  We can override the default segment using segment prefix mov ax, ss:[bx] ; address in bx is relative to stack segment mov ax, ds:[bp] ; address in bp is relative to data segment

Copying a String The following code copies a string from source to target .DATA source BYTE "This is the source string",0 target BYTE SIZEOF source DUP(0) .CODE main PROC Good use of SIZEOF mov esi,0 ; index register mov ecx, SIZEOF source ; loop counter L1: mov al,source[esi] ; get char from source mov target[esi],al ; store it in the target inc esi ; increment index loop L1 ESI is used to index ; loop for entire string exit source & target main ENDP strings END main

Summing an Integer Array This program calculates the sum of an array of 16-bit integers .DATA intarray WORD 100h,200h,300h,400h,500h,600h .CODE main PROC mov esi, OFFSET intarray ; address of intarray mov ecx, LENGTHOF intarray ; loop counter mov ax, 0 ; zero the accumulator L1: add ax, [esi] ; accumulate sum in ax add esi, 2 ; point to next integer loop L1 ; repeat until ecx = 0 exit esi is used as a pointer main ENDP contains the address of an array element END main

Summing an Integer Array – cont'd This program calculates the sum of an array of 32-bit integers .DATA intarray DWORD 10000h,20000h,30000h,40000h,50000h,60000h .CODE main PROC mov esi, 0 ; index of intarray mov ecx, LENGTHOF intarray ; loop counter mov eax, 0 ; zero the accumulator L1: add eax, intarray[esi*4] ; accumulate sum in eax inc esi ; increment index loop L1 ; repeat until ecx = 0 exit main ENDP esi is used as a scaled index END main

PC-Relative Addressing The following loop calculates the sum: 1 to 1000 Offset Machine Code Source Code 00000000 B8 00000000 mov eax, 0 00000005 B9 000003E8 mov ecx, 1000 0000000A L1: 0000000A 03 C1 add eax, ecx 0000000C E2 FC loop L1 0000000E . . . . . . When LOOP is assembled, the label L1 in LOOP is translated as FC which is equal to –4 (decimal). This causes the loop instruction to jump 4 bytes backwards from the offset of the next instruction. Since the offset of the next instruction = 0000000E, adding –4 (FCh) causes a jump to location 0000000A. This jump is called PC-relative.

PC-Relative Addressing – cont'd Assembler: Calculates the difference (in bytes), called PC-relative offset, between the offset of the target label and the offset of the following instruction Processor: Adds the PC-relative offset to EIP when executing LOOP instruction If the PC-relative offset is encoded in a single signed byte, (a) what is the largest possible backward jump? (b) what is the largest possible forward jump? Answers: (a) –128 bytes and (b) +127 bytes

Summary Data Transfer MOV, MOVZX, and XCHG instructions Addressing Modes Register, immediate, direct, indirect, indexed, based-indexed Load Effective Address (LEA) instruction 32-bit and 16-bit addressing JMP and LOOP Instructions Traversing and summing arrays, copying strings PC-relative addressing

Arrays and addressing modes

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Arrays and addressing modes