registers   and   memory   locations.   How   machine instructions within an instruction set are classified differ by computer type and manufacturer. Instructions Classified by Function Instructions can be classified in general terms by the type of operation they peform—data movement, transfer of control or program sequencing, arithmetic, and  logical. MOVEMENT  INSTRUCTIONS.—  Movement instructions literally move the data in some way. They include   internal,   external,   and   data   assignment instructions. Internal  Instructions.—Internal  instructions move  the  data  within  the  confines  of  the  computer. They include the following examples: l l l Load —Load the A register with the contents specified by the operand Move (transfer) —Move the register to another register Store —Store the contents of specified memory address contents of one a register into a External  Instructions.—Instructions dedicated to I/O are external instructions. They include inputting data from a peripheral device, such as a magnetic tape unit or outputting data to a peripheral, such as a printer. Data   Assignment   (Special-Purpose) Instructions.— Data  assignment  instructions  include those that set or clear status indicating bits that are normally held in an active status or flag register. Some examples  of  active  status  registers  include  state indicators, upper/lower control indicator of half-word instructions,  interrupt  lockouts,  memory  lockout inhibit, bootstrap mode, fixed point overflow, and compare designators. Some examples of flag bits are equal to zero, sign (+ or –), carry, and parity (odd or even). TRANSFER OF CONTROL OR PROGRAM SEQUENCING CONTROL INSTRUCTIONS.— Transfer of control or program sequencing control instructions enable the programmer to change the sequence  in  which  instructions  are  executed  by branching to another area of a program. They also include instructions for a subroutine to perform a function. Branching  Instructions.—  Branching   instructions make it possible to change the sequence in which the computer peforms instructions. An unconditional branching instruction always causes a jump to a new area  of  memory. An  example  is  a  jump  (JMP) instruction.  Branching  instructions  often  use  a  modifier in the instruction to establish a condition to be met. These  types  of  branching  instructions  are  called conditional branching instructions. A conditional branching instruction causes a jump to a new area of memory only when a specific condition is met, such as IF  A  =  0  JUMP  TO....    A  conditional  branching instruction may also rely on the setting of a switch on the computer’s controlling device to be included with the instruction, such as IF JUMP 1 SWITCH IS SET JMP TO . . . . Subroutine.— A subroutine may include a function that is routinely repeated, such as incrementing or decrementing an index register or a short multiply routine when no multiply instruction exists. Some functions performed in a subroutine may include stack pointer management  and  data  buffering  algorithms such as last-in, first-out (LIFO) and first-in, first-out (FIFO) methods. ARITHMETIC INSTRUCTIONS.— Arithmetic instructions  include  add,  subtract,  multiply,  divide, shift,  increment,  decrement,  clear,  and  negation instructions. Depending on the design of the computer, absolute   numbers   are   involved   in   arithmetic calculations. Also depending on the design, math pac and numeric data coprocessor  are used in some computers  in  addition  to  the  normal  arithmetic instructions available. They execute the arithmetic instructions  the  CPU’s  ALU  cannot  and  are  still controlled by the CPU’s program control. LOGICAL  INSTRUCTIONS.—  Logical in- structions include and, or, not, exclusive or/nor, com- pares,  and  shift  instructions.  They  are  often  used  in computers with multiply or divide instructions, in calculations to isolate bits. They also include compare type instructions. Compare type instructions are greater than (>), less than (<), equal to (=), not equal to (<>), check for positive, and check for negative. Instructions Classified by Their Action on Operands Instructions may also be classified by their action on an operand. They may read, store, or replace an operand. For example, ADD LOGICAL PRODUCT is classified as a read instruction; STORE LOGICAL PRODUCT is classified as a store instruction; and REPLACE SELECTIVE CLEAR is classified as a 8-4