CET 3510 – Lecture 11

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

2. 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

3. Bitwise Operations • Recipe: • 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 ); • mov( eax, ecx ); • or( ebx, ecx ); • mov( eax, ecx ); • xor( ebx, ecx ); • mov( ebx, ecx ); • not( ecx ); //BE CAREFUL

4. 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: &&, ||, !

5. 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"); }

6. 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)

7. 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

8. 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

9. 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; }

10. 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 );

11. 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; }