1. TOPICS:-
Instruction and Instruction Sequencing
•Register Transfer Notation(RTN)
•Assembly Language Notation(APN)
•Basic instruction type
•Instruction execution & straight line sequencing
•Branching
•Condition codes
•Generating memory address

2. Addressing modes
•Types of addressing modes
•Implementation of variables and constants
•Indirection and pointer
•Indexing and array
•Relative addressing
•Additional modes

3. Computer program consist of sequence of small steps,
such as adding two numbers, testing for a particular
condition , reading a character from the keyword
Four types of operation.
1) Data transfers between the memory and the
processor registers.
2) Arithmetic and logical operations on data.
3) Program sequencing and control.
4) I/O Transfers

4. •Transfer of information from one location in the
computer to another.
•Possible locations that may be involved in such transfers
are memory locations, processor registers and registers
in the I/O subsystem.
•Example –
• Add the contents of the register R1 and R2 and then
places their sum into register R3.
R3[R1]+[R2]
 Right side denoted the value.
 Left side denoted the name of the location where the
value is to be placed.

5. •It represent machine instructions and program.
•For this, we use assembly language format.
•Example :-
1) Move LOC ,R1
• The contents of LOC are unchanged by the execution of
this instruction but the old contents of register R1 are
overwritten.
2) Add R1,R2,R3

6. Three types of basic instruction:-
1) 3 –Address instruction:-
• Add A,B,C
• A &B are called Source operands.
• C is called Destination operand.
Operation Source1,Source2,Destination
• 3-address instruction is too large to fit
in one word for a reasonable word
length.

7. 2) 2-Address Instruction:-
• Add A,B
• [B][A]+[B]
• A is source operand and B is destination
operand
Operation Source, Destination
• A single 2-address instruction cannot be
used to solve original problem A and B ,
without destroying either of them ,and to
place the sum in location C.
• Move B,C
C[B]

8. 3) 1-Adress Instruction:-
• Add A
• Accumulator[A]+Accumulator
Operation Destination
• Using 1-address instructions , the operation
[C][A]+[B] can be performed by execution
the sequence the instruction:-
• Load A
• Add B
• Store C

9. •Executing a given instruction is a two-phase procedure:-
• 1) Instruction fetch:-
•Instruction fetched from the main memory location whose
address in the PC.
•This instruction placed in the IR in the processor.
•2)Instruction execute:-
•The instruction in IR is examined to determine which
operation is to be performed.
•The specified operation is performed by processor.
•Perform arithmetic and logic operation.
•Store the result in destination location.

10. •PC holds the address of the instruction to be execute
next.
•The address of 1st instruction placed into PC, then
•The processor control circuits use the information in the
PC to fetch and execute instruction ,one at a time , in the
order of increasing addresses . It is called straight line
sequencing.
•During each instruction PC incremented by 4 to point to
the next instruction

11. • It is loads a new value into the PC.
•Processor fetch and executes the instruction at the new
address , called branch target.
•It follows the branch instruction in sequential address
order .
•A conditional branch instruction causes a branch only if
a specified condition is satisfied.
•If the condition is not specified ,the PC is incremented.
•And the next instruction in sequential order is fetched
and executed.
• Branch>0 LOOP

12. •The processor keep the track of information about the
result of various operation for use by subsequent
conditional branch instructions.
•This is accomplished by recording the required
information in individual bits, often called condition code
flags.
•These flags are usually grouped together in a special
processor register called condition code register or status
register.

13. •Four commonly used flags are:-
•N(negative):-Set to 1 if the result is negative; otherwise
cleared to 0
•Z(zero):-Set to 1 if the result is 0;othrrwise cleared to 0
•V(overflow):-Setto 1 if the arithmetic overflow occurs ;
otherwise cleared to 0
•C(carry):-Setto 1 if a carry-out results from the
operation ; otherwise cleared to 0

14. •The N and Z flags are affected by instructions that
transfer data such as Move ,Load , or Store.
•TheInstruction Branch>0 tests one or more condition
codes.
•Insome computers ,the condition code flags are affected
automatically by instruction that perform arithmetic or
logic operation

15. •Theinstruction set of a computer typically provides a
number of such method, called addressing modes.
•Types of addressing modes:- EA=effective address
Name Assembler syntax Addressing function
Immediate # value Operand=value
Register Ri EA=Ri
Absolute(Direct) LOC EA=LOC
Indirect (Ri) , (LOC) EA=[Ri] ,EA=[LOC]
Index X(Ri) EA=[Ri]+X
Base with Index (Ri,Rj) EA=[Ri]+[Rj]
Base with Index and X(Ri,Rj) EA=[Ri]+[Rj]+X
offset
Relative X(PC) EA=[PC]+X
Auto increment (Ri+) EA=[Ri] ;Increment Ri
Auto decrement -(Ri) Decrement Ri
EA=[Ri];

16. •A variable is represented by allocating a register or a
memory location to hold its value.
•Two addressing modes to access variables:-
1) Register mode:-The operand is the contents of a
processor register .
•Operand is held in register named in address file
•EA = R
•Limited number of registers
•Very small address field needed
Shorter instructions
Faster instruction fetch
•Move LOC,R1

17. 2) Absolute mode:-
•The operand is in a memory location.
•The address of this location is given explicitly in the
instruction .
•It is also called Direct mode.
•Declaration such as
• Integer A,B

18. Address and data constants can be represented in
assembly level language using Immediate mode
•Immediate mode:- The operand is given explicitly in the
instruction.
•Example – Move 200immediate ,Ro
•The common convention is to be used the sharp sign(#)
in front of the value of the use as in immediate operand.
• Move #200, Ro

19. •The register or memory location that contains the
address of an operand is called a pointer.
•Indirection is the ability to reference something using a
name, reference, or container instead of the value itself.
•The most common form of indirection is the act of
manipulating a value through its memory address.
•For example, accessing a variable through the use of a
pointer.
•Indirection
and use of pointers are important and
powerful concepts in programming.

20. •Indirectmode:-
• The effective address of the operand is the contents of
register or memory location whose address appears in
the instruction.
•Memory cell pointed to by address field contains the
address of (pointer to) the operand.
•e.g. ADD (A)
Add contents of cell pointed to by contents of A
to accumulator
• Add (R2),R1

21. •Indexing provide the different kind of flexibility for
accessing operands.
•It is useful in dealing with lists and array.
•IndexMode:-
•The effective address of the operand is generated by
adding a constant value to the contents of a register.

22. •The register used may be special register provided for
this purpose or general –purpose register called index
register.
• X(Ri)
• EA=X+[Ri]
•Where X denoted the constant value contained in the
instruction,
•And Ri is the name of the register involved.

23. •Relative Mode:-
•The effective address is determined by the index mode using
the PC in the place of general purpose register Ri.
• EA=[Ri]+PC
•i.e. get operand from Ri cells from current location pointed to
by PC
•The mode can be used access data operands.
•Most common used in to specify the target address in branch
instruction.
• Branch>0 LOOP

24. • Auto increment mode:- (Ri)+
• The effective address of the operand is the contents of a
register specified in the instruction.
• After accessing the operand ,the contents of this register
are automatically incremented to point to the next item.
• Auto Decrement mode:- -(Ri)
• The contents of a register specified in the instruction are
first automatically decremented.
• After that used as the effective address of the operand.