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Chapter 5

Chapter 5. Arithmetic and Logic Instructions Barry B. Brey bbrey@ee.net. Addition. The ADD instruction is used for binary addition. The addition causes the flag bits to change. Addition can be 8-, 16-, and 32-bits. All of the addressing modes presented in Chapter 2 are used by addition.

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Chapter 5

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  1. Chapter 5 Arithmetic and Logic Instructions Barry B. Brey bbrey@ee.net

  2. Addition • The ADD instruction is used for binary addition. • The addition causes the flag bits to change. • Addition can be 8-, 16-, and 32-bits. • All of the addressing modes presented in Chapter 2 are used by addition. ADD EAX,EBX ;EAX = EAX + EBX

  3. Increment • The INC instruction adds a one to a register or the contents of a memory location. INC BX ;BX = BX + 1 INC BYTE PTR [EBX]

  4. Add with Carry • The ADC instruction adds the carry bit into the sum. Used for wide additions (wider than 32-bits) and other reasons. ADC AX,DX ;AX = AX + DX + C ADX ESI,[EBX]

  5. Subtraction • The SUB instruction performs subtraction and the flags change to reflect condition of the result. • As with other arithmetic and logic instructions, subtraction exists for 8-, 16-, and 32-bit data. SUB AL,3 ;AL = AL - 3 SUB ECX,ESI

  6. Decrement • The DEC instruction subtracts one from a register or the contents of a memory location. DEC EBX ;EBX = EBX - 1 DEC DWORD PTR [EAX]

  7. Subtract with Borrow • The SBB instruction subtracts with borrow (where the borrow is held in the carry flag). SBB EAX,EBX ;EAX = EAX – EBX – C

  8. Compare • The CMP instruction is a special form of the SUB instruction. A compare does not result in a difference that is saved, on the flag bits change to reflect the difference. CMP AL,3 ;if AL = 3 the result to zero (flag)

  9. Multiplication • The MUL (unsigned) and IMUL (signed) instruction exist to perform 8-, 16-, or 32-bit multiplication. • The result is always a double wide result. • The carry and overflow bits indicate conditions about the multiplication. • A special IMUL exists with 3 operands.

  10. Division • The DIV (unsigned) and IDIV (signed) instruction exist to perform division on 8-, 16-, or 32-bit numbers. • Division is always performed o a double wide dividend. • The result is always in the form of an integer quotient and an integer remainder.

  11. AND • The AND instruction performs logical multiplication (the AND operation).

  12. OR • The OR instruction generates the logical sum (OR operation).

  13. Exclusive OR • The XOR instruction performs the Exclusive OR operation.

  14. NEG and NOT • The NEG (negate) instruction 2’s complements a number, • The NOT instruction 1’s complements a number. NOT EAX NEG DWORD PTR [EBX]

  15. Shifts • There are 4 shift instructions. Two are logical shifts and two are arithmetic shifts. • The logical shifts reposition the bits in a number. The arithmetic shifts multiply or divide signed numbers by powers of two. • SHR and SHL are logical and SAR and SAL are arithmetic shifts. SHL AL,3 or SHL AL,CL

  16. Rotates • Rotates are shifts that re-circulate the bit that moves out of an end of the register or memory location. • Four rotates exist where two just rotate the target and two rotate the target through the carry flag. ROL AL,3 or RCL AL,3

  17. TEST • The TEST instruction is a special form of the AND instruction that produces no result except for a change of the flags. • This instruction is often used to test multiple bits of a number. TEST AL,3 ;test the right two bits (if both are zero the result is zero)

  18. Bit Test Instructions • There are four bit test instructions BT (bit test), BTR (bit test and reset), BTS (bit test and set), and BTC (bit test and complement). • Each tests the prescribed bit by moving it into carry. Then the bit is modified (except for BT) BT AL,3 ;bit 3 is moved to carry BTS AL,3 ;bit 3 is moved to carry then set BTR AL,3 ;bit 3 is moved to carry then reset BTC AL,3 ;bit 3 is moved to carry then inverted.

  19. String Compares • The SCAS and CMPS instruction perform comparisons on blocks of data. • SCAS compares a memory block to the accumulator and CMPS compares two blocks of memory. • SCASB, SCASW, and SCASD are available for 8-, 16-, and 32-bit comparisons as are CMPSB, CMPSW, and CMPSD.

  20. SCAS is often used to search for a value and CMPS is often used to compare two blocks. • Both instruction change the flags to indicate the outcome of the comparison. • The Direction flag determines whether the pointer increments or decrements. • REPE and REPNE are often used to repeat the SCAS or CMPS instructions.

  21. AAM • About the only ASCII adjust instruction commonly used is the AAM instruction. • This instruction divides by 10, but leaves the quotient in AH instead of AL. The remainder is found in AL instead of AH. • By dividing by 10, a number up to 100 decimal can be converted to BCD.

  22. DAA and DAS • These instructions are used to adjust the result after a BCD addition or subtraction. • DAS and DAA does find some application in systems.

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