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Chapter 4 Basic Instructions

Chapter 4 Basic Instructions. 4.1 Copying Data. mov Instructions. mov (“move”) instructions are really copy instructions, like simple assignment statements in a high-level language Format: mov destination , source. register or memory. register, memory or immediate.

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Chapter 4 Basic Instructions

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  1. Chapter 4 Basic Instructions

  2. 4.1 Copying Data

  3. mov Instructions • mov (“move”) instructions are really copy instructions, like simple assignment statements in a high-level language • Format: mov destination, source registerormemory register,memoryorimmediate

  4. Operand Restrictions • Operands must be same size • Can’t move from memory to memory • mov nbr1, nbr2illegal if nbr1 and nbr2 reference doublewords in memory • Instead use a registermov eax, nbr2mov nbr1, eax • Can only move one byte, word or doubleword at a time

  5. Effect on Flags • In general, an instruction may have one of three effects: • no flags are altered • specific flags are given values depending on the results of the instruction • some flags may be altered, but their settings cannot be predicted • No mov instruction changes any flag

  6. Machine Code • Depends on operand type(s), with several different opcodes used for mov instructions • Word-size and doubleword-size instructions use same opcodes, but word-size instructions have 66 prefix byte • Object and source code from listing fileB0 9B mov al, 15566| B8 009B mov ax, 155B8 0000009B mov eax, 155

  7. ModR/M Byte • Part of the object code for many instructions • Used to encode specific registers • Used to distinguish between instructions that share the same opcode • Used to specify memory modes

  8. ModR/M Fields • mod (mode), 2 bits • reg (register), 3 bits • r/m (register/memory), 3 bits • Examples of encodings • mod = 00 and r/m = 101 combined always means direct memory addressing • reg = 011 means the EBX register in a 32-bit instruction

  9. xchg Instruction • Swaps the values referenced by its two operands • Can’t have both operands in memory • Does not alter any flag

  10. 4.2 Integer Addition and Subtraction Instructions

  11. add Instruction • Format: add destination, source • The integer at source is added to the integer at destination and the sum replaces the old value at destination • SF, ZF, OF, CF, PF and AF flags are set according to the value of the result of the operation • Example: CF = 1 if there is a carry out of the sum

  12. Addition Example • BeforeEAX: 00000075ECX: 000001A2 • Instructionadd eax, ecx • AfterEAX: 00000217ECX: 000001A2SF=0 ZF=0 CF=0 OF=0

  13. sub Instruction • Format: sub destination, source • The integer at source is subtracted from the integer at destination and the difference replaces the old value at destination • SF, ZF, OF, CF, PF and AF flags are set according to the value of the result of the operation • Example: ZF = 1 if the difference is zero

  14. Subtraction Example • Beforedoubleword at Dbl: 00000100 • Instructionsub Dbl, 2 • AfterDbl: 000000FESF=0 ZF=0 CF=0 OF=0

  15. Instruction Encoding • Opcode depends on operand types • The ModR/M byte distinguishes • Between operand types • Between add, sub and other operations for certain operand types • An small immediate operand is sometimes encoded as a byte even in a 32-bit instruction

  16. Increment and Decrement Instructions • inc destination • Adds 1 to destination • dec destination • Subtracts 1 from destination • Each sets same flags as add or sub except for CF which isn’t changed

  17. neg Instruction • neg destination • Negates (takes the 2's complement of) its operand • A positive value gives a negative result • A negative value will become positive • Zero remains 0 • Affects same flags as add and sub

  18. Programming in Assembly Language • Start with a design • Plan register usage • Decide what registers will be used for what variables in the design • There are only a few available registers • Plan memory usage

  19. 4.3 Multiplication Instructions

  20. Multiplication Instruction Mnemonics • mul for unsigned multiplication • Operands treated as unsigned numbers • imul for signed multiplication • Operands treated as signed numbers and result is positive or negative depending on the signs of the operands

  21. mul Instruction Format • mul source • Single operand may be byte, word, doubleword or quadword in register or memory (not immediate) and specifies one factor • Location of other factor is implied • AL for byte-size source • AX for word source • EAX for doubleword source • RAX for quadword source

  22. mul Instruction Operation • When a byte source is multiplied by the value in AL, the product is put in AX • When a word source is multiplied by the value in AX, the product is put in DX:AX • The high-order 16 bits in DX and the low-order 16 bits in AX • When a doubleword source is multiplied by the value in EAX, the product is put in EDX:EAX • Product of two quadwords in RAX:DAX

  23. Double-Length Product • The “double-length” product ensures that the result will always fit in the destination location • If significant bits of the product actually “spill over” into the high-order half (AH, DX or EDX), then CF and OF are both set to 1 • If the high-order half is not significant, then CF and OF are both cleared to 0 • For unsigned multiplication, this is when the high-order half is all 0’s

  24. mul Instruction Example • BeforeEAX: 00000005 EBX: 00000002EDX: ???????? • Instructionmul ebx • AfterEAX: 0000000AEBX: 00000002EDX: 00000000CF=OF=0

  25. imul Instruction Formats • imul source • imul register, source • imul register, source, immediate

  26. imulsource • “Single-operand format” • Similar to mul source except for signed operands • CF=OF=0 if each bit in the high-order half is the same as the sign bit in the low-order half • CF=OF=1 otherwise (the bits in the high-order half are significant)

  27. Single-Operand Example • BeforeAX: ??05 byte at Factor: FF • Instructionimul Factor • AfterAX: FFFBCF=OF=0

  28. imulregister,source • “Two-operand format” • Source operand can be in a register, in memory, or immediate • Register contains other factor, and also specifies the destination • Both operands must be word-size or doubleword-size, not byte-size • Product must “fit” in destination register • CF and OF are cleared to 0 if result fits • CF and OF are set to 1 if it doesn’t fit

  29. Two-operand Example • BeforeEBX: 0000000A • Instructionimul ebx, 10 • AfterEBX: 00000064CF=OF=0

  30. imul register,source,immediate • “Three-operand format” • The two factors are given by source (register or memory) and the immediate value • The first operand, a register, specifies the destination for the product • Operands register and source are the same size, both 16-bit or both 32-bit (not 8-bit) • If the product will fit in the destination register, then CF and OF are cleared to 0; if not, they are set to 1

  31. Three-Operand Example • Beforeword at Value: 08F2 BX: ???? • Instructionimul bx, Value, 1000 • AfterBX: F150CF=OF=1

  32. 4.4 Division Instructions

  33. Division Instruction Formats • idiv sourcefor signed operands • div sourcefor unsigned operands • source identifies the divisor • Byte, word, doubleword or quadword • In memory or register, but not immediate

  34. Implicit Dividend for div and idiv • Byte source divided into word in AX • Word source divided into doubleword in DX:AX • Doubleword source divided into quadword in EDX:EAX • Quadword source divided into RDX:RAX

  35. Results of div and idiv • Byte-size divisor:quotient in AL and remainder in AH • Word-size divisor:quotient in AX and remainder in DX • Doubleword-size divisor: quotient in EAX and remainder in EDX • Quadword-size divisor: quotient in RAX and remainder in RDX

  36. Results of div and idiv • All division operations satisfy the relation dividend = quotient*divisor + remainder • For signed division, the remainder will have same sign as dividend

  37. Flag Settings • Division instructions do not set flags to any meaningful values • They may change previously set values of AF, CF, OF, PF, SF or ZF

  38. Unsigned Division Example • BeforeEDX: 00 00 00 00EAX: 00 00 00 64EBX: 00 00 00 0D • Instructiondiv ebx ; 100/13 • AfterEDX: 00000009EAX: 00000007 100 = 7 * 13 + 9

  39. Signed Division Example • BeforeEDX: FF FF FF FFEAX: FF FF FF 9CECX: 00 00 00 0D • Instructionidiv ecx ; -100/13 • AfterEDX: FFFFFFF7EAX: FFFFFFF9 –100 = (–7) * 13 + (–9)

  40. Errors in Division • Caused by • Dividing by 0, or • Quotient too large to fit in destination • Triggers an exception • The interrupt handler routine that services this exception may vary from system to system • When a division error occurs for a program running under Visual Studio, an error window pops up

  41. Preparing for Division • Dividend must be extended to double length • Example • Copy a doubleword dividend to EAX • Extend dividend to EDX:EAX • For unsigned division, use mov edx, 0 • For signed division, use cdq instruction • Finally use div or idiv instruction

  42. Convert Instructions • No operand • cbwsign extends the byte in AL to the word in AX • cwdsign extends the word in AX to the doubleword in DX:AX • cdqsign extends the doubleword in EAX to the quadword in EDX:EAX • cqosign extends the quadword in RAX to RDX:RAX

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