Bitwise and Logical Manipulations

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Bitwise and Logical Manipulations

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

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Boolean Data

• 0 or 1
• Requires only a single bit
• 0 FALSE
• 1 TRUE
• Boolean operators
• Unary NOT
• Binary AND, OR, XOR

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NOT

• NOT destination
• Register or memory
• Does not affect flags
• Each 0 becomes 1, 1 becomes 0
• Sometimes called the 1's complement

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AND, OR, XOR

• ANDORXOR destination, source
• reg, regmemimmed
• mem, regimmed
• SF, ZF, PF are meaningfully set, CFOF0
• x AND y 1 IFF xy1
• x OR y 0 IFF xy0
• x XOR y 0 IFF xy

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Applications of AND

• Clear a bit
• AND AH, 01111111B
• This will clear (set to 0) bit 7 of AH leaving
  all other bits unchanged
• Mask out unwanted bits
• AND AX,000Fh
• This will clear all but the low-nybble of AX,
  leaving that nybble unchanged

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Turn Off NumLock

• clear bit 5 in keyboard status byte
• mov ax,40h
• mov ds,ax set DS
• mov bx,17h byte ptr
• and byte ptr bx,0DFh

• Keyboard Status Byte is at 00400017
• bit 7 Insert Mode
• bit 6 Caps Lock
• bit 5 Num Lock
• bit 4 Scroll Lock
• bit 3 Alt Pressed
• bit 2 Ctrl Pressed
• bit 1 Left Shift
• bit 0 Right Shift

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Applications of OR

• Setting a bit
• OR BX, 0400h
• This sets bit 10 of BX, leaving all other bits
  unchanged
• Checking the value of certain bit
• OR AX,AX
• This sets flags, does not change AX
• Bit 15 sign bit (JS, JNS, JG, JGE, JL, JLE)
• ZF1 IFF AX0 (JZ, JNZ)

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Converting Data

• DL contains 0-9
• OR DL,00110000b
• DL now contains '0'-'9'
• AH contains letter ('a'-'z','A'-'Z')
• OR AH,00100000b
• AH is now lower case

• ASCII for digit x (0-9) is 3x
• Setting bits 4 and 5 will turn a digit value
  stored in a byte to the digit's ASCII code
• Upper lower case characters differ only in bit 5
  (1lowercase)

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Application of XOR

• Bit toggling
• XOR AH, 10000000B
• This will change bit 7 (only) of AH
• Clearing a byte or word
• XOR AX, AX
• This sets AX to 0
• Encryption/Decryption
• XOR AL, Key encrypts/decrypts byte in AL

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TEST

• TEST destination, source
• Performs AND, does not store result
• Flags are set as if the AND were executed
• Example
• TEST CL, 10000001b
• JZ EvenAndNonNegative
• JS Negative
• must be odd and positive

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Bit Shifting

• Slide bits in byte or word to left or right
• What happens to bit that is shifted out?
• It is copied into the CF
• What bit value is shifted in?
• SHR, SHL, SAL 0
• SAR sign bit
• ROR, ROL bit shifted out
• RCR, RCL CF

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Shifts and Rotates

• Op destination, 1
• Op destination, CL do n times (n is in CL)
• destination is any 8 or 16-bit register or memory
  location
• If CL is specified, it will be unchanged
• SF, PF, and ZF are set according to result
• CF is last bit shifted out
• OF1 if last shift changes the sign

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SxL and SxR

• SAL and SHL are identical
• Each shift doubles the numeric value (up to
  overflow)
• SHR is a logical (or unsigned) shift
• Each shift halves the unsigned value
• SAR preserves the sign bit
• Each shift halves number and rounds DOWN
• SAR AL,1 when AL-3 gives -2, not -1

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Multiplication by 5

• Assume AX contains a number N to be multiplied
  by 5
• MOV DX,AX DXN also
• SHL AX,1 AX2N
• SHL AX,1 AX4N
• ADD AX,DX AX4NN5N
• This is likely to be much faster than a multiply
  instruction

Overflow (signed or unsigned) would be checked
after each operation by examining OF or CF
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Application Binary Output

• Problem Output AX in binary format
• Each bit must be translated to '0' or '1' and
  output
• We can build up the string in memory, or output
  each character as it is determined
• Our sample solution will put the bits in memory
• Note '0' and '1' differ only in bit position 0

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Binary Output - Details

• result db 16 dup (?), ''
• mov bx,15 offset and counter
• nextbit ax contains bits to output
• mov resultbx,'0' assume 0
• ror ax, 1 CF bit 0
• jnc digit0 digit is OK if CF0
• inc resultbx '0'-gt'1'
• digit0 rotating ax preserves its value
• dec bx affects SF
• jge nextbit loop while bxgt0
• use DOS function 9 to display result

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Homework

• Page 135
• 1, 2, 4, 7 (do not use multiply or divide
  instructions)