The most common fields found in instruction formats are:

  1. An operation code field(opcode)
  2. An address field that designates a memory address or register.
  3. A mode field (addressing mode field)

Operations specified by computer instructions are executed on data stored in memory or processor register. Number of address fields in the instruction can vary depending on the internal organization of its registers. There are mainly three type of CPU organizations:

  1. Single accumulator organization
  2. General register organization
  3. Stack organization

Some notations to remember

ADD X   # AC <- AC + M[X]  
# AC is accumulator register
# M[x] is the word located at address X

ADD R1, R2, R3   # R1 <- R2 + R3
ADD R1, R2       # R1 <- R1 + R2
ADD R1, X        # R1 <- R1 + M[X]
MOV R1, R2       # R1 <- R2 (or R2 -> R1) Depending on the assembly style
PUSH X           # push the word at address X to the top of the stack
ADD              # Perform add on the two items that are on the top of the stack

Three address instructions

  • can use each address field to specify either a processor register or memory operand.

Advantage

  • It results in short progranwhen evaluating arithmatic expressions.

Disadvantage

  • Instructions require too many bits

Example

Let’s evaluate X = (A + B)*(C + D) with three address

ADD R1, A, B    # R1 <- M[A] + M[B] 
ADD R2, C, D 
MUL X, R1, R2   # M[X] <- R1 * R2

Two address instructions

  • can use each address field to specify either a processor register or memory operand.

Example

Let’s evaluate X = (A + B)*(C + D) with two address

MOV R1, A
ADD R1, B
MOV R2, C
ADD R2, D
MUL R1, R2
MOV X, R1

One address instructions

  • Uses an implied accumulator (AC)

Example

Let’s evaluate X = (A + B)*(C + D) with one address

LOAD  A  # AC <- M[A]
ADD   B
STORE T  # M[T] <- AC, here T is the address of a temporary memory location
LOAD  C
ADD   D
MUL   T  # AC <- AC * M[T]
STORE X  # M[X] <- AC
NOTE

LOAD is used to load data from memory to accumulator.

STORE is used to store the accumulator result in memory.

Both LOAD and STORE are 1-address instructions.

MOV is used to move contents of a register to memory and vice versa or from one register to another

MOV is a 2-address instruction.

Zero address instructions

  • A stack organized computer does not use an address field for the instruction ADD and MUL.
  • PUSH & POP instructions need an address field to specify the operand
  • TOS stands top of the stack
  • It’s called Zero-address because of the absence of an address field in the computational instructions(ADD, MUL).

Example

Let’s evaluate X = (A + B)*(C + D) with Zero address

PUSH A      
PUSH B
ADD
PUSH C
PUSH D
ADD
MUL
POP X       # M[X] <- TOS