Boolean and comparison_instructions
Download as PPT, PDF1 like934 views
1. The document describes various bitwise operations including AND, OR, XOR, NOT, TEST, and CMP and how they affect flags and values. 2. It provides examples of using each operation to clear/set bits, convert between binary and ASCII, check register parity, and compare values. 3. In-class problems are presented and answered involving manipulating register bits through single instructions.
Boolean and comparison_instructions
- 1. Boolean and Comparison Instructions Operation Description AND AND Destination, Source OR OR Destination, Source XOR XOR Destination, Source NOT NOT Destination TEST Implied AND Destination, Source, Only FLAGS changed BT,BTC,BTR, BTS 01/20/16 SHOUA IQBAL 1
- 2. Bitwise AND (result placed in destination) Mov al, 00001111b And al, 00111011b al = ? • al = 00001011b (0Bh) Mov al, 6Dh And al, 4Ah al = ? • al = 48h • AND with a 0 to clear bits • AND with a 1 to preserve bits (bit extraction) • Always clears the Overflow and Carry flags. • May modify the Sign, Zero, and Parity flag AND Instruction 01/20/16 SHOUA IQBAL 2
- 3. Lowercase A (‘a’) is 61 0 1 1 0 0 0 0 1 Uppercase A (‘A’) is 41 0 1 0 0 0 0 0 1 Need to clear bit 5 Mov al, 61h AND al, 11011111b ;(DFh) ;al = 41h Application:Converting Characters to Uppercase 01/20/16 SHOUA IQBAL 3
- 4. Bitwise OR (result placed in destination) Mov al, 0Fh Or al, 61h al = ? • al = 6Fh Mov al, 3Dh Or al, 74h al = ? • al = 7Dh OR with 1 to set selected bits OR with 0 to preserve bits Always clears the Overflow and Carry Flags May modify the Sign, Zero, and Parity Flags OR Instruction 01/20/16 SHOUA IQBAL 4
- 5. Binary representation for 9 0 0 0 0 1 0 0 1 Ascii 9 is 39h 0 0 1 1 1 0 0 1 Need to set bits 4 and 5 Mov al, 9 OR al, 30h ;al = 00111001b (39h) Application: Converting a Decimal Digit (byte) to ASCII 01/20/16 SHOUA IQBAL 5
- 6. Application:Determining the sign of a value by ORing register with itself OR al, al Zero Flag Sign Flag Value in AL is … clear clear greater than 0 set clear equal to 0 clear set less than 0 01/20/16 SHOUA IQBAL 6
- 7. Bitwise XOR (result stored in destination) Mov al, 94h XOR al, 37h al = ? al = 10100011b Mov al, 72h Xor al, 0DCh al = ? al = 10101110b • XOR reverses itself when applied twice to the same operand (data encryption) • Clears the Overflow and Carry flags. • May modify the sign, zero, and parity flags XOR Instruction 01/20/16 SHOUA IQBAL 7
- 8. Indicates if the LOWEST BYTE of the result of a bitwise or arithmetic operation has an even or odd number of 1 bits. Mov al, 10110101b XOR al, 0 ; al unchanged parity flag clear (PO) Mov al, 11001100b XOR al, 0 ; al unchanged parity flag set (PE) Parity Flag 01/20/16 SHOUA IQBAL 8
- 9. Perform an XOR between upper-and lower bytes Mov ax, 64C1h ;0110 0100 1100 0001 XOR ah, al ;ah = 10100101 Parity bit set (PE) Application:How to check the parity of 16-bit registers 01/20/16 SHOUA IQBAL 9
- 10. Perform an XOR between bytes Mov eax, 56D764C1h ;0101 0110 1101 0111 0110 0100 1100 0001 XOR ah, al ;ah = 1010 0101 Shr eax, 8 XOR ah, al ;ah = 0111 0010 Shr eax, 8 XOR ah, al ;ah = 0010 0100 Parity bit set (PE) Application: How to check the parity of 32-bit registers 01/20/16 SHOUA IQBAL 10
- 11. Toggles all bits in an operand No flags are affected NOT Instruction 01/20/16 SHOUA IQBAL 11
- 12. Implied AND (no registers are changed) (flags may be modified) • Valuable for determining if individual bits are set. • Always clears the Overflow and Carry flags • May modify the sign, zero, and parity flags TEST Instruction 01/20/16 SHOUA IQBAL 12
- 13. Implied SUB (no registers are changed) (flags may be modified) • May modify Overflow, Sign, Zero, Aux. Carry, and Parity flags CMP Instruction 01/20/16 SHOUA IQBAL 13
- 14. CMP Instruction Results Zero Flag Carry Flag CMP Results 0 1 destination < source CMP 5, 10 0 0 destination > source CMP 105, 10 1 0 destination = source CMP 10, 10 01/20/16 SHOUA IQBAL 14
- 15. CMP of Signed Integers Flags CMP Results SF ≠ OF destination < source SF = OF destination > source ZF = 1 destination = source 01/20/16 SHOUA IQBAL 15
- 16. 1. Write a single instruction that clears the high 8 bits of AX and does not change the low 8 bits. AND AX, 00FFh In Class Problems 01/20/16 SHOUA IQBAL 16
- 17. 2. Write a single instruction that sets the high 8 bits of AX and does not change the low 8 bits. OR AX, FF00h In Class Problems 01/20/16 SHOUA IQBAL 17
- 18. 3. Write a single instruction that complements all the bits in EAX (do not use the NOT instruction) XOR EAX, FFFFFFFFh In Class Problems 01/20/16 SHOUA IQBAL 18
- 19. 4. Write instructions that set the Zero flag if the 32-bit value in EAX is even, and clear the Zero flag if EAX is odd. TEST EAX, 0000 0000 0000 0000 0000 0000 0000 0001b In Class Problems 01/20/16 SHOUA IQBAL 19