Data Transfer and Manipulation

Data Transfer Instructions

Data transfer instructions move data from one place in the computer to another without changing the data content. The most common transfers are between memory and processor registers, between processor registers and input or output, and between the processor registers themselves.

The load instruction is used to transfer for memory to a processor register, usually an accumulator. The store instruction is used to transfer data to memory. The move instruction is used to transfer data from one register to other. It has also been used for data transfers between CPU registers and memory or between two memory words. The exchange instruction swaps information between two registers or a register and a memory word. The input and output instructions transfer data among processor registers and input or output terminals. The push and pop instructions transfer data between processor registers and a memory stack.

Dta Manipulation Instructions:

Data manipulation instructions perform operations on data and provide the computational capabilities for the computer. The data manipulation instructions in a typical computer are usually divided into three basic types: 

1.  Arithmetic instructions 
2.  Logical and bit manipulation instructions 
3.  Shift instructions 

Arithmetic Instructions

The four basic arithmetic operations are addition, subtraction, multiplication, and division. Most computers provide instructions for all four operations. Some small computers have only addition and possibly subtraction instructions. The multiplication and division must then be generated by means of software subroutines. The increment instruction adds 1 to the value stored in a register or memory word. The decrement instruction subtracts 1 from a value stored in a register or memory word. The instruction "add with carry" performs the addition on two operands plus the value of the carry from the previous computation. Similarly, the "subtract with borrow" instruction subtracts two words and a borrow which may have resulted from a previous subtract operation.The negate instruction forms the 2' s complement of a number, effectively reversing the sign of an integer when represented in the signed-2's complement form.

Logical and Bit Manipulation Instructions

Logical instructions perform binary operations on strings of bits stored in registers. They are useful for manipulating individual bits or a group of bits that represent binary-coded information. The AND instruction is used to clear a bit or a selected group of bits of an operand. The OR instruction is used to set a bit or a selected group of bits of an operand. Similarly, the XOR instruction is used to selectively complement bits of an operand. Individual bits such as a carry can be cleared, set, or complemented with appropriate instructions.

Shift Instructions

Instructions to shift the content of an operand are quite useful and are often provided in several variations. Shifts are operations in which the bits of a word are moved to the left or right. The bit shifted in at the end of the word determines the type of shift used. Shift instructions may specify either logical shifts, arithmetic shifts, or rotate-type operations. In either case the shift may be to the right or to the left.

The logical shift inserts 0 to the end bit position. The end position is the leftmost bit for shift right and the rightmost bit position for the shift left.

The arithmetic shift-right instruction must preserve the sign bit in the leftmost position. The sign bit is shifted to the right together with the rest of the number, but the sign bit itself remains unchanged. This is a shift-right operation with the end bit remaining the same. The arithmetic shift-left instruction inserts 0 to the end position and is identical to the logical shift-left instruction.

The rotate instructions produce a circular shift. Bits shifted out at one end of the word are not lost as in a logical shift but are circulated back into the other end.