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Outline. Data Transfer Instructions Arithmetic Instructions Data-Related Operations and Directives Indirect Addressing JMP and LOOP Instructions. Data Transfer Instructions. MOV is for moving data between: Memory Register Immediate (constant) Almost all combinations, except:

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  1. Outline • Data Transfer Instructions • Arithmetic Instructions • Data-Related Operations and Directives • Indirect Addressing • JMP and LOOP Instructions

  2. Data Transfer Instructions • MOV is for moving data between: • Memory • Register • Immediate (constant) • Almost all combinations, except: • Memory to Memory!

  3. MOV Instruction • Move from source to destination. Syntax: • MOV destination,source • No more than one memory operand permitted • CS, EIP, and IP cannot be the destination • No immediate to segment moves .data count BYTE 100 wVal WORD 2 .code mov bl,count mov ax,wVal mov count,al mov al,wVal ; error mov ax,count ; error mov eax,count ; error

  4. Your turn . . . Explain why each of the following MOV statements are invalid: .data bVal BYTE 100 bVal2 BYTE ? wVal WORD 2 dVal DWORD 5 .code mov ds,45 ; a. mov esi,wVal ; b. mov eip,dVal ; c. mov 25,bVal ; d. mov bVal2,bVal ; e.

  5. Memory to Memory? • Must go through a register… .data Var1 WORD? Var2 WORD ? .code MOV AX, var1 MOV var2, AX

  6. MOV Instruction Format

  7. Instruction Operand Notation

  8. Zero or Sign Extension • What happens to ECX if –1 is moved from CX? • Are the higher 16 bits of ECX all 0? • What number does ECX represent now? • The solution: MOVZX and MOVSX • MOVZX always fills higher bits with 0. • MOVSX fills higher bits by “sign extension”.

  9. Zero Extension When you copy a smaller value into a larger destination, the MOVZX instruction fills (extends) the upper half of the destination with zeros. mov bl,10001111b movzx ax,bl ; zero-extension The destination must be a register.

  10. MOVZX Instruction Format

  11. Sign Extension The MOVSX instruction fills the upper half of the destination with a copy of the source operand's sign bit. mov bl,10001111b movsx ax,bl ; sign extension The destination must be a register.

  12. MOVSX Instruction Format

  13. XCHG • XCHG for exchange data between: • Register, register • Register, memory • Memory, register (again, no memory to memory)

  14. Direct-Offset Operands • Adding a displacement (or offset) to a variable name: arrayB BYTE 10h, 20h, 30, 40h, 50h … MOV AL, arrayB ; AL=10h MOV AL, [arrayB+1] ; AL=20h MOV AL, arrayB+1 ; Is it valid?

  15. Your turn. . . • Write a program that rearranges the values of three doubleword values in the following array as: 3, 1, 2. • .data • arrayD DWORD 1,2,3 • Step1: copy the first value into EAX and exchange it with the value in the second position. mov eax,arrayD xchg eax,[arrayD+4] • Step 2: Exchange EAX with the third array value and copy the value in EAX to the first array position. xchg eax,[arrayD+8] mov arrayD,eax

  16. Evaluate this .data myBytes BYTE 80h,66h,0A5h • How about the following code. Is anything missing? • movzx ax,myBytes • mov bl,[myBytes+1] • add ax,bx • mov bl,[myBytes+2] • add ax,bx ; AX = sum Yes: Move zero to BX before the MOVZX instruction.

  17. Addition and Subtraction • ADD X, Y X := X + Y • SUB X, Y X := X – Y

  18. INC, DEC, NEG • INC X X := X + 1 or X++ • DEC X X := X – 1 or X-- • NEG X X := –X

  19. Expression • Example: X=(A + B) * (D – E) MOV EAX, A ADD EAX, B MOV ECX, D SUB ECX, E IMUL EAX, ECX MOV X, EAX

  20. Flags Affected • Flags (register) tell us whether any of the following conditions occur: • Overflow, • Carry, • Zero, Sign…etc. • Used for decision in branch. • Loop (discussed next) • If…then…else

  21. Zero and Sign • Zero Flag ZF=1 if the instruction produce 0. MOV CX, 1 SUB CX, 1 ; CX=0, ZF=1 • Sign Flag SF=1 if the instruction produce a negative number. MOV CX, 0 SUB CX, 1 ; CX=-1, SF=1 ADD CX, 2 ; CX=1, SF=0

  22. Carry (Unsigned Arithmetic) • The Carry flag is set when the result of an operation generates an unsigned value that is out of range (too big or too small for the destination operand). • Example: MOV AL, 0FFh ADD AL, 1 ; CF = 1, AL=00 MOV AX, 00FFh ADD AX, 1 ; CF = 0, AX=0100h

  23. Overflow (Signed Arithmetic) • The Overflow flag is set when the signed result of an operation is invalid or out of range. • Example: MOV AL, +127 ADD AL, 1 ; OF = 1 MOV AL, -128 SUB AL, 1 ; OF = 1

  24. Detecting Carry • Detecting Carry is easy. • Adding two N-bit numbers result in an (N+1)-bit number. • Example: 0 0 0 0 0 1 0 0 + 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 • CF is ignored for signed arithmetic. For example, the above is 4 + (-1) in decimal

  25. Detecting Overflow • Carry isn’t meaningful for signed arithmetic. For example, adding any two negative numbers always produces carry. • Detecting Overflow: • Compare CF and the bit carried into MSB (Most Significant Bit).

  26. Overflow in Positive Numbers • Carry never happens. • Overflow occurs if MSB becomes 1 01111111 (127) 00000001 (1) + 01111111 (127) 00000001 (1) • Observation: • MSB=1 indicates a negative number. • But, we’re adding two positive numbers…?!

  27. Overflow in Negative Numbers • Carry always happens. • Overflow occurs if MSB becomes 0 10000000 (-128) 11111111 (-1) + 11111111 (-1) 11111111 (-1) • Observation: • MSB=0 indicates a positive number. • But, we’re adding two negative numbers…?!

  28. Detecting Overflow • Overflow: CF  MSB ? • Doesn’t work if adding a positive number to a negative number (or vice versa)! • Overflow: (CF  MSB) and not the case of (positive+negavive) • positive+negavive: • Overflow never happens. • Carry happens when carry-in to MSB • Overflow: CF  (carry-in to MSB)

  29. Flags Affect in ADD, SUB • Carry: unsigned arithmetic out of range • Overflow: signed arithmetic out of range • Sign: result is negative • Zero: result is zero • Auxiliary Carry: carry from bit 3 to bit 4 • Parity: sum of 1 bits is an even number

  30. LAHF/SAHF • LAHF: load the low byte of EFLAGS register into AH. • SAHF: store the low byte of EFLAGS register into AH.

  31. Data Related Operators • Who are they? • OFFSET, PTR, TYPE, LENGTHOF, SIZEOF • They are only understood by the assembler. • They are not instructions!

  32. Operand Sizes • Operands may have the size of 1 byte, 2 bytes, or 4 bytes. • Most of time, we can tell the size from the register names or the variable definition. For examples: Var1 BYTE “Hello” MOV ECX, 13 MOV AL, Var1

  33. PTR • But sometimes we want to override the default. • myDouble DWORD 12345678h MOV AL, myDouble ; error MOV AL, BYTE PTR myDouble MOV AX, WORD PTR myDouble MOV AX, WORD PTR [myDouble+2] MOV EAX, myDouble

  34. OFFSET • OFFSET returns the distance in bytes, of a label from the beginning of its enclosing segment • Assume that the data segment begins at 00404000h: .data bVal BYTE ? wVal WORD ? dVal DWORD ? dVal2 DWORD ? .code mov esi,OFFSET bVal ; ESI = 00404000 mov esi,OFFSET wVal ; ESI = 00404001 mov esi,OFFSET dVal ; ESI = 00404003 mov esi,OFFSET dVal2 ; ESI = 00404007

  35. TYPE • TYPE returns the size (in bytes) of each element. .data var1 BYTE ? var2 WORD ? var3 DWORD ? var4 QWORD ? .code mov eax,TYPE var1 ; 1 mov eax,TYPE var2 ; 2 mov eax,TYPE var3 ; 4 mov eax,TYPE var4 ; 8

  36. LENGTHOF • LENGTHOF returns the number of elements. .data byte1 BYTE 10,20,30 ; 3 array1 WORD 30 DUP(?),0,0 ; 32 array2 WORD 5 DUP(3 DUP(?)) ; 15 array3 DWORD 1,2,3,4 ; 4 digitStr BYTE "12345678",0 ; 9 .code mov ecx,LENGTHOF array1 ; 32

  37. SIZEOF • SIZEOF returns the size of the variable (the whole array).  SIZEOF = LENGTHOF * TYPE .data SIZEOF byte1 BYTE 10,20,30 ; 3 array1 WORD 30 DUP(?),0,0 ; 64 array2 WORD 5 DUP(3 DUP(?)) ; 30 array3 DWORD 1,2,3,4 ; 16 digitStr BYTE "12345678",0 ; 9 .code mov ecx,SIZEOF array1 ; 64

  38. Indirect Operands • An indirect operand holds the address of a variable, usually an array or string. It can be dereferenced (just like a pointer). .data val1 BYTE 10h,20h,30h .code mov esi,OFFSET val1 mov al,[esi] ; dereference ESI (AL = 10h) inc esi mov al,[esi] ; AL = 20h inc esi mov al,[esi] ; AL = 30h

  39. Array Sum Example .data arrayW WORD 1000h,2000h,3000h .code mov esi,OFFSET arrayW mov ax,[esi] add esi,2 ; or: add esi,TYPE arrayW add ax,[esi] add esi,2 ; increment ESI by 2 add ax,[esi] ; AX = sum of the array

  40. Indexed Operands arrayW WORD 1000h,2000h,3000h .code mov esi,0 mov ax,[arrayW + esi] ; AX = 1000h mov ax,arrayW[esi] ; alternate format add esi,2 add ax,[arrayW + esi]

  41. Pointers .data arrayW WORD 1000h,2000h,3000h ptrW DWORD arrayW .code mov esi,ptrW mov ax,[esi] ; AX = 1000h

  42. Implementation of Loops • JMP instruction: Unconditional Branch. • LOOP instruction: • Step 1: Set ECX to n for a loop of n iterations. • Step 2: Use LOOP instruction at the end of loop. • Hidden action: DEC ECX

  43. Example: Summation • For I := 10 downto 1 {Sum := Sum+I} MOV ECX, 10 MOV EAX, 0 L1: ADD EAX, ECX LOOP L1

  44. Your turn What will be the final value of AX? mov ax,6 mov ecx,4 L1: inc ax loop L1 10 How many times will the loop execute? mov ecx,0 X2: inc ax loop X2 4,294,967,296 (=232)

  45. Copying a String .data source BYTE "This is the source string",0 target BYTE SIZEOF source DUP(0),0 .code 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 ; move to next character loop L1 ; repeat for entire string

  46. Nested Loop • .data • count DWORD ? • .code • mov ecx,100 ; set outer loop count • L1: • mov count,ecx ; save outer loop count • mov ecx,20 ; set inner loop count • L2: . • . • loop L2 ; repeat the inner loop • mov ecx,count ; restore outer loop count • loop L1 ; repeat the outer loop

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