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.

   

   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.

   

   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.

   

   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	Branch and Save Return Address
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
SZE	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
ION	F080		Interrupt On
IOF	F040		Interrupt Off

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