Computer Organization & Assembly Language Logic Instructions
- 1. Computer Organization & Assembly Language Logic Instructions
- 2. Logic Instructions To manipulate individual bits Binary Value 0 treated as false Binary Value 1 treated as true In Assembly Language: AND OR XOR NOT TEST 2
- 3. Truth Tables 3 a b a AND b a OR b a XOR b 0 0 0 0 0 0 1 0 1 1 1 0 0 1 1 1 1 1 1 0 a NOT a 0 1 1 0
- 4. Examples 1. 1010 1010 AND 1111 0000 = 1010 0000 2. 1010 1010 OR 1111 0000 = 1111 1010 3. 1010 1010 XOR 1111 0000 = 0101 1010 4. NOT 1010 1010 = 0101 0101 4
- 5. Syntax AND destination, source OR destination, source XOR destination, source Destination: Stores result Can be Register or Memory Location Source: May be a Constant, Register or Memory Location 5
- 6. Effects on Flags SF, ZF, PF reflects the result AF is undefined CF, OF = 0 6
- 7. MASK To modify only selective bits in destination, we construct a source bit pattern known as MASK. To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 7
- 8. MASK To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 8
- 9. MASK To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 9
- 10. MASK To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 10
- 11. MASK To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 11
- 12. MASK To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 12
- 13. MASK To choose mask, use following properties: b AND 1 = b b AND 0 = 0 b OR 1 = 1 b OR 0 = b b XOR 0 = b b XOR 1 = ~b (complement of b) Where b represents a bit (0 or 1) 13
- 14. Contd.. 1. The AND instruction: - May be used to clear specific destination bits while preventing the others. - A 0 mask bit clears the corresponding destination bit. - A 1 mask bit preserves the corresponding destination bit. 14
- 15. Example 1 Clear the sign bit of AL while leaving the other bits unchanged. Solution: AND AL, 7Fh Where 7Fh (0111 1111) is the mask. Suppose: AL = 0D4h Binary: 1101 0100 → AND 1101 0100 0111 1111 = 0101 0100 54h → 15
- 16. Contd.. 2. The OR instruction: - May be used to set specific destination bits while preventing the others. - A 1 mask bit sets the corresponding destination bit. - A 0 mask bit preserves the corresponding destination bit. 16
- 17. Example 2 Set the MSB and LSB of AL while preserving the other bits. Solution: OR AL, 81h Where 81h (1000 0001) is the mask. Let AL = 42h Binary: 0100 0010 → OR AL, 81h OR 0100 0010 → 1000 0001 = 1100 0011 C3h → 17
- 18. Contd.. 3. The XOR instruction: - May be used to complement specific destination bits while preventing the others. - A 1 mask bit complements the corresponding destination bit. - A 0 mask bit preserves the corresponding destination bit. 18
- 19. Example 3 Change the sign bit of DX. Solution: XOR DX, 8000h Where 80h (1000 0000) is the mask. DX = 7230h 0111 0010 0011 0000 → XOR 0111 0010 0011 0000 1000 0000 0000 0000 = 1111 0010 0011 0000 F230h → 19
- 20. Nibbles A nibble is a unit of data in computing that consists of four bits. A bit is the smallest unit of data in a computer and can have a value of either 0 or 1. A nibble, being four bits, can represent 16 different values (2^4 = 16), ranging from 0000 to 1111 in binary notation. Four bits that are at the higher-order positions within a byte (or word). In a byte, which is typically composed of eight bits, the high nibble consists of the four bits at positions 4-7 (counting from right to left, where the rightmost bit is bit 0). 20
- 21. Nibbles contd… For example, consider the byte 1010 1101 in binary notation. In this byte: The high nibble is 1010, corresponding to bits 4-7. The low nibble is 1101, corresponding to bits 0-3. 21
- 22. Converting an ASCII digit to a Number ASCII code for digit “0-9” is “30h-39h” AND AL, 0Fh ;Clears the high nibble. How to convert decimal digit to ASCII code? Add 48 to every decimal digit. E.g. to convert 213 into ASCII code, just add 48 to 2 then 48 to 1 and lastly 48 to three So we get 50 49 51 22
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- 24. Lowercase to Uppercase letter Lower case: 61h to 7Ah ( 97 to 122) Uppercase: 41h to 5Ah (65 to 90) 24
- 25. Clearing a Register MOV AX, 0 ;machine code 3 bytes OR SUB AX, AX ;machine code 2 bytes OR XOR AX, AX ;machine code 2 bytes 25
- 26. Testing a Register for zero CMP CX, 0 Is same like: OR CX, CX ;sets ZF = 1 if CX is 0 26
- 27. NOT Instruction Performs the one’s complement operation on the destination. Syntax: NOT destination No effect on flags Example: Complement the bit in AX: NOT AX 27
- 28. TEST Instruction Performs an AND operation without changing destination i.e. only status flags updated. Syntax: TEST destination, source MOV AL, F0h ; AL = 1111 0000 TEST AL, 0Fh ; = 0000 1111 28
- 29. TEST Instruction Performs an AND operation without changing destination i.e. only status flags updated. Syntax: TEST destination, source Effects on flags: SF, ZF and PF reflects the results AF is undefined CF, OF = 0 29