CET 3510 – Lecture 11 Bit Manipulation in a High-Level Programming Language Dr. José M. Reyes Álamo

Bitwise Operations A logical operation that is performed between bit of the operand(s) in their corresponding position Bitwise AND Bitwise OR Bitwise NOT Bitwise XOR

Bitwise Operations Recipe: HLA Code Example: mov( eax, ecx ); Move the first operand into an appropriate register Perform the corresponding operation proving the first and second operand HLA Code Example: mov( eax, ecx ); and( ebx, ecx ); or( ebx, ecx ); xor( ebx, ecx ); mov( ebx, ecx ); not( ecx ); //BE CAREFUL

Bitwise Operations in C/C++ Result is the same, syntax is different Bitwise AND operator: & Bitwise OR operator: | Bitwise NOT operator: ~ Bitwise XOR operator: ^ Be careful: Do not confuse these with the logical comparison operators: &&, ||, !

Bitwise Operations in C/C++ Code Example: void logicalOperations(){ int x, y, z; x = 5; y = 2; z = x & y; cout << "x & y = " << z << endl; z = x | y; cout << "x | y = " << z << endl; z = x ^ y; cout << "x ^ y = " << z << endl; z = ~x; cout << "~x = " << z << endl; system("PAUSE"); }

Shifting Left in HLA Shift left move each bit n positions to the left Lower order bits becomes a 0 Higher order bit become a carry out HLA code: shl( count, dest ) | count = positions to shift; dest = register or variable Shifting n places to the left one position is equivalent to multiplying by the base (radix) n times (radix)

Shifting Right in HLA Shift right move each bit n positions to the right Higher order bits becomes a 0 Lower order bit become a carry out HLA code: shr( count, dest ) | count = positions to shift; dest = register or variable sar( count, dest ) | count = positions to shift; dest = register or variable. Used for arithmetic shifting. Shifting n places to the right one position is equivalent to dividing by the base (radix) n times

Shifting in C/C++ The bitwise shifting operators are Right shift (>>) Left shift (<<) Be careful with the compiler as shifting (especially shifting right) might return unexpected results

Shifting in C++ Code Example: void shiftingOperations(){ } int x, y, z; x = 2; x = x << 5; cout << "x shifted left = " << x << endl; x = x >> 1; cout << "x shifted right = " << x << endl; }

Bit Manipulation Example HLA HLA Code: stdout.put( “Enter the current month, day, and year: “ ); stdin.get( month, day, year ); mov( 0, ax ); mov( ax, packedDate ); //Just in case there is an error.   mov( month, al ); shl( 5, ax ); or( day, al ); shl( 7, ax ); or( year, al ); mov( ax, packedDate ); stdout.put( “Packed data = $”, packedDate, nl );

Bit Manipulation Example C++ void packedDate(){ int month, day, year, packedDate; cout << "Enter the current month, day, and year: " << endl; cin >> month >> day >> year; packedDate = 0; packedDate = packedDate | month; packedDate = packedDate << 5; packedDate = packedDate | day; packedDate = packedDate << 7; packedDate = packedDate | year; cout << "Packed date is " << hex << packedDate << endl; }