Computer Organization | Basic Computer Instructions



The basic computer has 16 bit instruction register (IR) which can denote either memory reference or register reference or input-output instruction.

  1. Memory Reference – These instructions refer to memory address as an operand. The other operand is always accumulator. Specifies 12 bit address, 3 bit opcode (other than 111) and 1 bit addressing mode for direct and indirect addressing.

    memory reference instruction

    Example –
    IR register contains = 0001XXXXXXXXXXXX, i.e. ADD after fetching and decoding of instruction we find out that it is a memory reference instruction for ADD operation.

    Hence, DR <- M[AR]
    AC <- AC+ DR, SC <- 0
  2. Register Reference – These instructions perform operations on registers rather than memory addresses. The IR(14-12) is 111 (differentiates it from memory reference) and IR(15) is 0 (differentiates it from input/output instructions). The rest 12 bits specify register operation.

    register reference instruction

    Example –
    IR register contains = 0111001000000000, i.e. CMA after fetch and decode cycle we find out that it is a register reference instruction for complement accumulator.

    Hence, AC <- ~AC
  3. Input/Output – These instructions are for communication between computer and outside environment. The IR(14-12) is 111 (differentiates it from memory reference) and IR(15) is 1 (differentiates it from register reference instructions). The rest 12 bits specify I/O operation.

    i/o instruction

    Example –
    IR register contains = 1111100000000000, i.e. INP after fetch and decode cycle we find out that it is an input/output instruction for inputing character. Hence, INPUT character from peripheral device.

The set of instructions incorporated in16 bit IR register are:

  1. Arithmetic, logical and shift instructions (and, add, complement, circulate left, right, etc)
  2. To move information to and from memory (store the accumulator, load the accumulator)
  3. Program control instructions with status conditions (branch, skip)
  4. Input output instructions (input character, output character)
Symbol Hexadecimal Code Description
AND 0xxx 8xxx And memory word to AC
ADD 1xxx 9xxx Add memory word to AC
LDA 2xxx Axxx Load memory word to AC
STA 3xxx Bxxx Store AC content in memory
BUN 4xxx Cxxx Branch Unconditionally
BSA 5xxx Dxxx Add memory word to AC
ISZ 6xxx Exxx Increment and skip if 0
CLA 7800 Clear AC
CLE 7400 Clear E(overflow bit)
CMA 7200 Complement AC
CME 7100 Complement E
CIR 7080 Circulate right AC and E
CIL 7040 Circulate left AC and E
INC 7020 Increment AC
SPA 7010 Skip next instruction if AC>0
SNA 7008 Skip next instruction if AC<0
SZA 7004 Skip next instruction if AC>0
SE 7002 Skip next instruction if E=0
HLT 7001 Halt computer
INP F800 Input character to AC
OUT F400 Output character from AC
SKI F200 Skip on input flag
SKO F100 Skip on output flag
IEN F080 Interrupt On


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