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Microprocessor & Assembly Language

Microprocessor & Assembly Language. Logical, shift, rotate, and bit Instructions. Basic Logic Instructions. Include AND, OR, Exclusive-OR, and NOT. – also TEST, a special form of the AND instruction – NEG, similar to the NOT instruction

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Microprocessor & Assembly Language

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  1. Microprocessor & Assembly Language Logical, shift, rotate, and bit Instructions

  2. Basic Logic Instructions • Include AND, OR, Exclusive-OR, and NOT. – also TEST, a special form of the AND instruction – NEG, similar to the NOT instruction • Logic operations provide binary bit control in low-level software. –Can manipulate bits, do logical operations, allow bits to be set, cleared, or complemented and do arithmetic operations • Low-level software appears in machine language or assembly language form and often controls the I/O devices in a system. • Logic operations always clear the carry and overflow flags, while the other flags change to reflect the condition of the result • Except not instruction it does not affect any flags

  3. AND Instruction • The AND operation performs logical multiplication • Figures: (a) The truth table for the AND operation and (b) the logic symbol of an AND gate • The AND operation clears bits of a binary number • The task of clearing a bit in a binary number is called masking

  4. AND Examples • AND Form: • AND dest, source ;dest := dest and source • AND instruction uses any addressing mode except memory-to-memory and segment register addressing • The two operand must be the same size

  5. OR Instruction • The OR operation performs logical addition and is often called the Inclusive-OR function • Figure: (a) The truth table for the OR operation and (b) the logic symbol of an OR gate • The OR operation clears bits of a binary number

  6. OR Example • OR uses the same formats as AND

  7. XOR Instruction • Exclusive-OR: • If the inputs are both 0 or both 1, the output is 0 • If the inputs are different, the output is 1 • (a) truth table of XOR (b) Symbol of XOR gate • Exclusive-OR is sometimes called a comparator • XOR instruction allows part of a number to be inverted • A common use for the Exclusive-OR instruction is to clear a register to zero. For example, the XOR CH,CH instruction clears register CH to 00H

  8. XOR Example • XOR uses the same formats as AND

  9. TEST Instruction • The TEST instruction performs the AND operation BUT • AND instruction changes the destination operand • TEST instruction does not changes the destination operand • TEST only affects the condition of the flag register, which indicates the result of the test • The zero flag (Z) is a logic 1 if the bit under test is a zero, and (indicating a nonzero result) if the bit under test is not zero. • The TEST instruction functions in the same manner as a CMP instruction, The difference is • TEST instruction normally tests a single bit (or occasionally multiple bits) • CMP instruction tests the entire byte, word, or doubleword. • The TEST instruction uses the same addressing modes as the AND instruction • The destination operand is normally tested against immediate data • The value of immediate data is 1 to test the rightmost bit position, 2 to test the next bit, 4 for the next,and so on. • Usually the TEST instruction is followed by either the JZ (jump if zero) or JNZ (jump if not zero) instruction

  10. Test Example • Example 1: TEST AL,1 ;test right bit JNZ RIGHT ;if set TEST AL,128 ;test left bit JNZ LEFT ;if set • Examples:

  11. Additional Test Instructions • The 80386 through the Pentium 4 processors contain additional test instructions that test single bit positions • Example:

  12. NOT and NEG Instructions • Logical inversion, or the one’s complement (NOT), and arithmetic sign inversion, or the two’s complement (NEG) • NOT and NEG can use any addressing mode except segment register addressing • Examples:

  13. ShiftInstructions • Shift instructions position or move numbers to the left or right within a register or memory location. • They also perform :simple arithmetic such as • multiplication by powers of 2+n (left shift) • division by powers of 2-n (right shift) • The microprocessor’s instruction set contains four different shift instructions as shown in figure

  14. ShiftInstructions • Logical shift function with unsigned numbers • Arithmetic shift function with signed numbers • Logical shifts multiply or divide unsigned data; • arithmetic shifts multiply or divide signed data. – a shift left always multiplies by 2 for each bit position shifted – a shift right always divides by 2 for each position – shifting a two places, multiplies or divides by 4 • Shift form: • shl dest, count • shl dest, CL ; cl aontain count number • Destination can be memory location or register; Segment shift not allowed.

  15. Shift Examples • Two different forms of shifts:

  16. Rotate • Rotate instructions position binary data by rotating the information in a register or memory location, either from one end to another or through the carry flag • Rotation dressing modes are the same shifts • The four available rotate instructions appear:

  17. Rotation Examples

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