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 register or memory register, memory or immediate

4. Operand Restrictions Operands must be same size Can’t move from memory to memory –mov nbr1, nbr2 illegal if nbr1 and nbr2 reference doublewords in memory –Instead use a register mov eax, nbr2 mov 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 file B0 9B mov al, 155 66| B8 009B mov ax, 155 B8 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 Before EAX: 00000075 ECX: 000001A2 Instruction add eax, ecx After EAX: 00000217 ECX: 000001A2 SF=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 Before doubleword at Dbl: 00000100 Instruction sub Dbl, 2 After Dbl: 000000FE SF=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 Before EAX: 00000005 EBX: 00000002 EDX: ???????? Instruction mul ebx After EAX: 0000000A EBX: 00000002 EDX: 00000000 CF=OF=0

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

26. imul source “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 Before AX: ??05 byte at Factor: FF Instruction imul Factor After AX: FFFB CF=OF=0

28. imul register,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 Before EBX: 0000000A Instruction imul ebx, 10 After EBX: 00000064 CF=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 Before word at Value: 08F2 BX: ???? Instruction imul bx, Value, 1000 After BX: F150 CF=OF=1

32. 4.4 Division Instructions

33. Division Instruction Formats idiv source for signed operands div source for 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 Before EDX: 00 00 00 00 EAX: 00 00 00 64 EBX: 00 00 00 0D Instruction div ebx ; 100/13 After EDX: 00000009 EAX: 00000007 100 = 7 * 13 + 9

39. Signed Division Example Before EDX: FF FF FF FF EAX: FF FF FF 9C ECX: 00 00 00 0D Instruction idiv ecx ; -100/13 After EDX: FFFFFFF7 EAX: 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 cbw sign extends the byte in AL to the word in AX cwd sign extends the word in AX to the doubleword in DX:AX cdq sign extends the doubleword in EAX to the quadword in EDX:EAX cqo sign extends the quadword in RAX to RDX:RAX