CSI 212 Chapter 7 Slides, Spring 2012

Assembly Language for Intel-Based Assembly Language for Intel-Based Computers, 5 Computers, 5th th Edition Edition Chapter 7: Integer Arithmetic (c) Pearson Education, 2006-2007. All rights reserved. You may modify and copy this slide show for your personal use, or for use in the classroom, as long as this copyright statement, the author's name, and the title are not changed. Slides prepared by the author Revision date: June 4, 2006 Kip R. Irvine Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 2 Web site Examples Chapter Overview Chapter Overview • Shift and Rotate Instructions • Shift and Rotate Applications • Multiplication and Division Instructions • Extended Addition and Subtraction • ASCII and Unpacked Decimal Arithmetic • Packed Decimal Arithmetic Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 3 Web site Examples Shift and Rotate Instructions Shift and Rotate Instructions • Logical vs Arithmetic Shifts • SHL Instruction • SHR Instruction • SAL and SAR Instructions • ROL Instruction • ROR Instruction • RCL and RCR Instructions • SHLD/SHRD Instructions Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 4 Web site Examples Logical vs Arithmetic Shifts Logical vs Arithmetic Shifts • A logical shift fills the newly created bit position with zero: • An arithmetic shift fills the newly created bit position with a copy of the number’s sign bit: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 5 Web site Examples SHL Instruction SHL Instruction • The SHL (shift left) instruction performs a logical left shift on the destination operand, filling the lowest bit with 0. • Operand types for SHL: SHL reg,imm8 SHL mem,imm8 SHL reg,CL SHL mem,CL (Same for all shift and rotate instructions) Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 6 Web site Examples Fast Multiplication Fast Multiplication mov dl,5 shl dl,1 Shifting left 1 bit multiplies a number by 2 mov dl,5 shl dl,2 ; DL = 20 Shifting left n bits multiplies the operand by 2n For example, 5 * 22 = 20 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 7 Web site Examples SHR Instruction SHR Instruction • The SHR (shift right) instruction performs a logical right shift on the destination operand. The highest bit position is filled with a zero. mov dl,80 shr dl,1 ; DL = 40 shr dl,2 ; DL = 10 Shifting right n bits divides the operand by 2n Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 8 Web site Examples SAL and SAR Instructions SAL and SAR Instructions • SAL (shift arithmetic left) is identical to SHL. • SAR (shift arithmetic right) performs a right arithmetic shift on the destination operand. An arithmetic shift preserves the number's sign. mov dl,-80 sar dl,1 ; DL = -40 sar dl,2 ; DL = -10 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 9 Web site Examples Your turn . . . Your turn . . . mov al,6Bh shr al,1 a. shl al,3 b. mov al,8Ch sar al,1 c. sar al,3 d. Indicate the hexadecimal value of AL after each shift: 35h A8h C6h F8h Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 10 Web site Examples ROL Instruction ROL Instruction • ROL (rotate) shifts each bit to the left • The highest bit is copied into both the Carry flag and into the lowest bit • No bits are lost mov al,11110000b rol al,1 ; AL = 11100001b mov dl,3Fh rol dl,4 ; DL = F3h Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 11 Web site Examples ROR Instruction ROR Instruction • ROR (rotate right) shifts each bit to the right • The lowest bit is copied into both the Carry flag and into the highest bit • No bits are lost mov al,11110000b ror al,1 ; AL = 01111000b mov dl,3Fh ror dl,4 ; DL = F3h Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 12 Web site Examples Your turn . . . Your turn . . . mov al,6Bh ror al,1 a. rol al,3 b. Indicate the hexadecimal value of AL after each rotation: B5h ADh Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 13 Web site Examples RCL Instruction RCL Instruction • RCL (rotate carry left) shifts each bit to the left • Copies the Carry flag to the least significant bit • Copies the most significant bit to the Carry flag CF clc ; CF = 0 mov bl,88h ; CF,BL = 0 10001000b rcl bl,1 ; CF,BL = 1 00010000b rcl bl,1 ; CF,BL = 0 00100001b Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 14 Web site Examples RCR Instruction RCR Instruction • RCR (rotate carry right) shifts each bit to the right • Copies the Carry flag to the most significant bit • Copies the least significant bit to the Carry flag stc ; CF = 1 mov ah,10h ; CF,AH = 1 00010000b rcr ah,1 ; CF,AH = 0 10001000b Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 15 Web site Examples Your turn . . . Your turn . . . stc mov al,6Bh rcr al,1 a. rcl al,3 b. Indicate the hexadecimal value of AL after each rotation: B5h AEh Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 16 Web site Examples SHLD Instruction SHLD Instruction • Shifts a destination operand a given number of bits to the left • The bit positions opened up by the shift are filled by the most significant bits of the source operand • The source operand is not affected • Syntax: SHLD destination, source, count • Operand types: SHLD reg16/32, reg16/32, imm8/CL SHLD mem16/32, reg16/32, imm8/CL Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 17 Web site Examples SHLD Example SHLD Example .data wval WORD 9BA6h .code mov ax,0AC36h shld wval,ax,4 Shift wval 4 bits to the left and replace its lowest 4 bits with the high 4 bits of AX: Before: After: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 18 Web site Examples SHRD Instruction SHRD Instruction • Shifts a destination operand a given number of bits to the right • The bit positions opened up by the shift are filled by the least significant bits of the source operand • The source operand is not affected • Syntax: SHRD destination, source, count • Operand types: SHRD reg16/32, reg16/32, imm8/CL SHRD mem16/32, reg16/32, imm8/CL Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 19 Web site Examples SHRD Example SHRD Example mov ax,234Bh mov dx,7654h shrd ax,dx,4 Shift AX 4 bits to the right and replace its highest 4 bits with the low 4 bits of DX: Before: After: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 20 Web site Examples Your turn . . . Your turn . . . mov ax,7C36h mov dx,9FA6h shld dx,ax,4 ; DX = shrd dx,ax,8 ; DX = Indicate the hexadecimal values of each destination operand: FA67h 36FAh Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 21 Web site Examples What's Next What's Next • Shift and Rotate Instructions • Shift and Rotate Applications • Multiplication and Division Instructions • Extended Addition and Subtraction • ASCII and Unpacked Decimal Arithmetic • Packed Decimal Arithmetic Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 22 Web site Examples Shift and Rotate Applications Shift and Rotate Applications • Shifting Multiple Doublewords • Binary Multiplication • Displaying Binary Bits • Isolating a Bit String Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 23 Web site Examples Shifting Multiple Doublewords Shifting Multiple Doublewords • Programs sometimes need to shift all bits within an array, as one might when moving a bitmapped graphic image from one screen location to another. • The following shifts an array of 3 doublewords 1 bit to the right (view complete source code): .data ArraySize = 3 array DWORD ArraySize DUP(99999999h) ; 1001 1001... .code mov esi,0 shr array[esi + 8],1 ; high dword rcr array[esi + 4],1 ; middle dword, include Carry rcr array[esi],1 ; low dword, include Carry Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 24 Web site Examples Binary Multiplication Binary Multiplication • We already know that SHL performs unsigned multiplication efficiently when the multiplier is a power of 2. • You can factor any binary number into powers of 2. • For example, to multiply EAX * 36, factor 36 into 32 + 4 and use the distributive property of multiplication to carry out the operation: EAX * 36 = EAX * (32 + 4) = (EAX * 32)+(EAX * 4) mov eax,123 mov ebx,eax shl eax,5 ; mult by 25 shl ebx,2 ; mult by 22 add eax,ebx Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 25 Web site Examples Your turn . . . Your turn . . . mov ax,2 ; test value mov dx,ax shl dx,4 ; AX * 16 push dx ; save for later mov dx,ax shl dx,3 ; AX * 8 shl ax,1 ; AX * 2 add ax,dx ; AX * 10 pop dx ; recall AX * 16 add ax,dx ; AX * 26 Multiply AX by 26, using shifting and addition instructions. Hint: 26 = 16 + 8 + 2. Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 26 Web site Examples Displaying Binary Bits Displaying Binary Bits Algorithm: Shift MSB into the Carry flag; If CF = 1, append a "1" character to a string; otherwise, append a "0" character. Repeat in a loop, 32 times. .data buffer BYTE 32 DUP(0),0 .code mov ecx,32 mov esi,OFFSET buffer L1: shl eax,1 mov BYTE PTR [esi],'0' jnc L2 mov BYTE PTR [esi],'1' L2: inc esi loop L1 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 27 Web site Examples Isolating a Bit String Isolating a Bit String • The MS-DOS file date field packs the year, month, and day into 16 bits: mov ax,dx ; make a copy of DX shr ax,5 ; shift right 5 bits and al,00001111b ; clear bits 4-7 mov month,al ; save in month variable Isolate the Month field: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 28 Web site Examples What's Next What's Next • Shift and Rotate Instructions • Shift and Rotate Applications • Multiplication and Division Instructions • Extended Addition and Subtraction • ASCII and Unpacked Decimal Arithmetic • Packed Decimal Arithmetic Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 29 Web site Examples Multiplication and Division Instructions Multiplication and Division Instructions • MUL Instruction • IMUL Instruction • DIV Instruction • Signed Integer Division • CBW, CWD, CDQ Instructions • IDIV Instruction • Implementing Arithmetic Expressions Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 30 Web site Examples MUL Instruction MUL Instruction • The MUL (unsigned multiply) instruction multiplies an 8-, 16-, or 32-bit operand by either AL, AX, or EAX. • The instruction formats are: MUL r/m8 MUL r/m16 MUL r/m32 Implied operands: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 31 Web site Examples MUL Examples MUL Examples 100h * 2000h, using 16-bit operands: .data val1 WORD 2000h val2 WORD 100h .code mov ax,val1 mul val2 ; DX:AX = 00200000h, CF=1 The Carry flag indicates whether or not the upper half of the product contains significant digits. mov eax,12345h mov ebx,1000h mul ebx ; EDX:EAX = 0000000012345000h, CF=0 12345h * 1000h, using 32-bit operands: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 32 Web site Examples Your turn . . . Your turn . . . mov ax,1234h mov bx,100h mul bx What will be the hexadecimal values of DX, AX, and the Carry flag after the following instructions execute? DX = 0012h, AX = 3400h, CF = 1 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 33 Web site Examples Your turn . . . Your turn . . . mov eax,00128765h mov ecx,10000h mul ecx What will be the hexadecimal values of EDX, EAX, and the Carry flag after the following instructions execute? EDX = 00000012h, EAX = 87650000h, CF = 1 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 34 Web site Examples IMUL Instruction IMUL Instruction • IMUL (signed integer multiply ) multiplies an 8-, 16-, or 32-bit signed operand by either AL, AX, or EAX • Preserves the sign of the product by sign-extending it into the upper half of the destination register Example: multiply 48 * 4, using 8-bit operands: mov al,48 mov bl,4 imul bl ; AX = 00C0h, OF=1 OF=1 because AH is not a sign extension of AL. Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 35 Web site Examples IMUL Examples IMUL Examples Multiply 4,823,424 * 423: mov eax,4823424 mov ebx,-423 imul ebx ; EDX:EAX = FFFFFFFF86635D80h, OF=0 OF=0 because EDX is a sign extension of EAX. Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 36 Web site Examples Your turn . . . Your turn . . . mov ax,8760h mov bx,100h imul bx What will be the hexadecimal values of DX, AX, and the Carry flag after the following instructions execute? DX = FF87h, AX = 6000h, OF = 1 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 37 Web site Examples DIV Instruction DIV Instruction • The DIV (unsigned divide) instruction performs 8-bit, 16-bit, and 32-bit division on unsigned integers • A single operand is supplied (register or memory operand), which is assumed to be the divisor • Instruction formats: DIV r/m8 DIV r/m16 DIV r/m32 Default Operands: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 38 Web site Examples DIV Examples DIV Examples Divide 8003h by 100h, using 16-bit operands: mov dx,0 ; clear dividend, high mov ax,8003h ; dividend, low mov cx,100h ; divisor div cx ; AX = 0080h, DX = 3 Same division, using 32-bit operands: mov edx,0 ; clear dividend, high mov eax,8003h ; dividend, low mov ecx,100h ; divisor div ecx ; EAX = 00000080h, DX = 3 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 39 Web site Examples Your turn . . . Your turn . . . mov dx,0087h mov ax,6000h mov bx,100h div bx What will be the hexadecimal values of DX and AX after the following instructions execute? Or, if divide overflow occurs, you can indicate that as your answer: DX = 0000h, AX = 8760h Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 40 Web site Examples Your turn . . . Your turn . . . mov dx,0087h mov ax,6002h mov bx,10h div bx What will be the hexadecimal values of DX and AX after the following instructions execute? Or, if divide overflow occurs, you can indicate that as your answer: Divide Overflow Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 41 Web site Examples Signed Integer Division Signed Integer Division • Signed integers must be sign-extended before division takes place • fill high byte/word/doubleword with a copy of the low byte/word/doubleword's sign bit • For example, the high byte contains a copy of the sign bit from the low byte: Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 42 Web site Examples CBW, CWD, CDQ Instructions CBW, CWD, CDQ Instructions • The CBW, CWD, and CDQ instructions provide important sign-extension operations: • CBW (convert byte to word) extends AL into AH • CWD (convert word to doubleword) extends AX into DX • CDQ (convert doubleword to quadword) extends EAX into EDX • Example: mov eax,0FFFFFF9Bh ; (-101) cdq ; EDX:EAX = FFFFFFFFFFFFFF9Bh Your copy of the book may have an error on page 243: 9Bh equals –101 rather than –65. Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 43 Web site Examples IDIV Instruction IDIV Instruction • IDIV (signed divide) performs signed integer division • Same syntax and operands as DIV instruction Example: 8-bit division of –48 by 5 mov al,-48 cbw ; extend AL into AH mov bl,5 idiv bl ; AL = -9, AH = -3 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 44 Web site Examples IDIV Examples IDIV Examples Example: 32-bit division of –48 by 5 mov eax,-48 cdq ; extend EAX into EDX mov ebx,5 idiv ebx ; EAX = -9, EDX = -3 Example: 16-bit division of –48 by 5 mov ax,-48 cwd ; extend AX into DX mov bx,5 idiv bx ; AX = -9, DX = -3 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 45 Web site Examples Your turn . . . Your turn . . . mov ax,0FDFFh ; -513 cwd mov bx,100h idiv bx What will be the hexadecimal values of DX and AX after the following instructions execute? Or, if divide overflow occurs, you can indicate that as your answer: DX = FFFFh (1), AX = FFFEh (2) Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 46 Web site Examples Unsigned Arithmetic Expressions Unsigned Arithmetic Expressions • Some good reasons to learn how to implement integer expressions: • Learn how do compilers do it • Test your understanding of MUL, IMUL, DIV, IDIV • Check for overflow (Carry and Overflow flags) Example: var4 = (var1 + var2) * var3 ; Assume unsigned operands mov eax,var1 add eax,var2 ; EAX = var1 + var2 mul var3 ; EAX = EAX * var3 jc TooBig ; check for carry mov var4,eax ; save product Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 47 Web site Examples Signed Arithmetic Expressions Signed Arithmetic Expressions (1 of 2) (1 of 2) Example: eax = (-var1 * var2) + var3 mov eax,var1 neg eax imul var2 jo TooBig ; check for overflow add eax,var3 jo TooBig ; check for overflow Example: var4 = (var1 * 5) / (var2 – 3) mov eax,var1 ; left side mov ebx,5 imul ebx ; EDX:EAX = product mov ebx,var2 ; right side sub ebx,3 idiv ebx ; EAX = quotient mov var4,eax Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 48 Web site Examples Signed Arithmetic Expressions Signed Arithmetic Expressions (2 of 2) (2 of 2) Example: var4 = (var1 * -5) / (-var2 % var3); mov eax,var2 ; begin right side neg eax cdq ; sign-extend dividend idiv var3 ; EDX = remainder mov ebx,edx ; EBX = right side mov eax,-5 ; begin left side imul var1 ; EDX:EAX = left side idiv ebx ; final division mov var4,eax ; quotient Sometimes it's easiest to calculate the right-hand term of an expression first. Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 49 Web site Examples Your turn . . . Your turn . . . mov eax,20 imul ebx idiv ecx Implement the following expression using signed 32-bit integers: eax = (ebx * 20) / ecx Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 50 Web site Examples Your turn . . . Your turn . . . push edx push eax ; EAX needed later mov eax,ecx imul edx ; left side: EDX:EAX pop ebx ; saved value of EAX idiv ebx ; EAX = quotient pop edx ; restore EDX, ECX Implement the following expression using signed 32-bit integers. Save and restore ECX and EDX: eax = (ecx * edx) / eax Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 51 Web site Examples Your turn . . . Your turn . . . mov eax,var1 mov edx,var2 neg edx imul edx ; left side: EDX:EAX mov ecx,var3 sub ecx,ebx idiv ecx ; EAX = quotient mov var3,eax Implement the following expression using signed 32-bit integers. Do not modify any variables other than var3: var3 = (var1 * -var2) / (var3 – ebx) Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 52 Web site Examples What's Next What's Next • Shift and Rotate Instructions • Shift and Rotate Applications • Multiplication and Division Instructions • Extended Addition and Subtraction • ASCII and UnPacked Decimal Arithmetic • Packed Decimal Arithmetic Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 53 Web site Examples Extended Addition and Subtraction Extended Addition and Subtraction • ADC Instruction • Extended Precision Addition • SBB Instruction • Extended Precision Subtraction Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 54 Web site Examples Extended Precision Addition Extended Precision Addition • Adding two operands that are longer than the computer's word size (32 bits). • Virtually no limit to the size of the operands • The arithmetic must be performed in steps • The Carry value from each step is passed on to the next step. Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 55 Web site Examples ADC Instruction ADC Instruction • ADC (add with carry) instruction adds both a source operand and the contents of the Carry flag to a destination operand. • Operands are binary values • Same syntax as ADD, SUB, etc. • Example • Add two 32-bit integers (FFFFFFFFh + FFFFFFFFh), producing a 64-bit sum in EDX:EAX: mov edx,0 mov eax,0FFFFFFFFh add eax,0FFFFFFFFh adc edx,0 ;EDX:EAX = 00000001FFFFFFFEh Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 56 Web site Examples Extended Addition Example Extended Addition Example • Task: Add 1 to EDX:EAX • Starting value of EDX:EAX: 00000000FFFFFFFFh • Add the lower 32 bits first, setting the Carry flag. • Add the upper 32 bits, and include the Carry flag. mov edx,0 ; set upper half mov eax,0FFFFFFFFh ; set lower half add eax,1 ; add lower half adc edx,0 ; add upper half EDX:EAX = 00000001 00000000 Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 57 Web site Examples SBB Instruction SBB Instruction • The SBB (subtract with borrow) instruction subtracts both a source operand and the value of the Carry flag from a destination operand. • Operand syntax: • Same as for the ADC instruction Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 58 Web site Examples Extended Subtraction Example Extended Subtraction Example • Task: Subtract 1 from EDX:EAX • Starting value of EDX:EAX: 0000000100000000h • Subtract the lower 32 bits first, setting the Carry flag. • Subtract the upper 32 bits, and include the Carry flag. mov edx,1 ; set upper half mov eax,0 ; set lower half sub eax,1 ; subtract lower half sbb edx,0 ; subtract upper half EDX:EAX = 00000000 FFFFFFFF Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 59 Web site Examples What's Next What's Next • Shift and Rotate Instructions • Shift and Rotate Applications • Multiplication and Division Instructions • Extended Addition and Subtraction • ASCII and UnPacked Decimal Arithmetic • Packed Decimal Arithmetic Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 60 Web site Examples ASCII and Packed Decimal Arithmetic ASCII and Packed Decimal Arithmetic • Binary Coded Decimal • ASCII Decimal • AAA Instruction • AAS Instruction • AAM Instruction • AAD Instruction • Packed Decimal Integers • DAA Instruction • DAS Instruction