This document discusses instruction execution and different addressing modes used in computer architecture. It explains straight-line sequencing of instructions and branching. It then describes various addressing modes like immediate, direct, indirect, register, register indirect, displacement, relative and their uses. It also discusses condition codes, conditional branching and how indexed addressing is useful for accessing arrays or finding sum of scores in a list.

1. Instruction Execution: Straight-
line sequencing and branching

2. Instruction Execution and Straight-Line
Sequencing
R0,C
B,R0
A,R0
Movei + 8
Begin execution here Movei
ContentsAddress
C
B
A
the program
Data for
segment
program
3­instruction
Addi + 4
A program for C ← [Α] + [Β].
Assumptions:
- One memory operand
per instruction
- 32-bit word length
- Memory is byte
addressable
- Full memory address
can be directly specified
in a single-word instruction
Two-phase procedure
-Instruction fetch
-Instruction execute

3. Branching
NUMn
NUM2
NUM1
R0,SUM
NUMn,R0
NUM3,R0
NUM2,R0
NUM1,R0
A straight­line program for adding n numbers.
Add
Add
Move
SUM
i
Move
Add
i 4n+
i 4n 4­+
i 8+
i 4+
•
•
•
•
•
•
•
•
•

4. Branching
N,R1Move
NUMn
NUM2
NUM1
R0,SUM
R1
"Next" number to R0
Using a loop to add n numbers.
LOOP
Decrement
Move
LOOP
loop
Program
Determine address of
"Next" number and add
N
SUM
n
R0Clear
Branch>0
•
•
•
•
•
•
Branch target
Conditional branch

5. Condition Codes
• Condition code flags
• Condition code register / status register
• N (negative)
• Z (zero)
• V (overflow)
• C (carry)
• Different instructions affect different flags

6. Conditional Branch Instructions
• Example:
A: 1 1 1 1 0 0 0 0
B: 0 0 0 1 0 1 0 0
A: 1 1 1 1 0 0 0 0
+(−B): 1 1 1 0 1 1 0 0
1 1 0 1 1 1 0 0
C = 1
N = 1
V = 0
Z = 0

7. Status Bits
ALU
V Z N C
Zero Check
Cn
Cn-1
Fn-1
A B
F

8. Addressing Modes

9. Generating Memory Addresses
• How to specify the address of branch target?
• Can we give the memory operand address
directly in a single Add instruction in the loop?
• Use a register to hold the address of NUM1;
then increment by 4 on each pass through the
loop.

10. Addressing Modes
• Immediate
• Direct
• Indirect
• Register
• Register Indirect
• Displacement (Indexed)
• Implicit

11. Immediate Addressing
• Operand is part of instruction
• Operand = address field
• e.g. ADD 5
– Add 5 to contents of accumulator
– 5 is operand
• No memory reference to fetch data
• Fast
• Limited range

12. Immediate Addressing Diagram
OperandOpcode
Instruction

13. Direct Addressing
• Address field contains address of operand
• Effective address (EA) = address field (A)
• e.g. ADD A
– Add contents of cell A to accumulator
– Look in memory at address A for operand
• Single memory reference to access data
• No additional calculations to work out
effective address
• Limited address space

14. Direct Addressing Diagram
Address AOpcode
Instruction
Memory
Operand

15. Indirect Addressing (1)
• Memory cell pointed to by address field
contains the address of (pointer to) the
operand
• EA = (A)
– Look in A, find address (A) and look there for
operand
• e.g. ADD (A)
– Add contents of cell pointed to by contents of A to
accumulator

16. Indirect Addressing (2)
• Large address space
• 2n
where n = word length
• May be nested, multilevel, cascaded
– e.g. EA = (((A)))
• Draw the diagram yourself
• Multiple memory accesses to find operand
• Hence slower

17. Indirect Addressing Diagram
Address AOpcode
Instruction
Memory
Operand
Pointer to operand

18. Register Addressing (1)
• Operand is held in register named in address
filed
• EA = R
• Limited number of registers
• Very small address field needed
– Shorter instructions
– Faster instruction fetch

19. Register Addressing (2)
• No memory access
• Very fast execution
• Very limited address space
• Multiple registers helps performance
– Requires good assembly programming or compiler
writing
– N.B. C programming
• register int a;
• c.f. Direct addressing

20. Register Addressing Diagram
Register Address ROpcode
Instruction
Registers
Operand

21. Register Indirect Addressing
• C.f. indirect addressing
• EA = (R)
• Operand is in memory cell pointed to by
contents of register R
• Large address space (2n
)
• One fewer memory access than indirect
addressing

22. Register Indirect Addressing Diagram
Register Address ROpcode
Instruction
Memory
OperandPointer to Operand
Registers

23. Displacement Addressing
• EA = A + (R)
• Address field hold two values
– A = base value
– R = register that holds displacement
– or vice versa

24. Displacement Addressing Diagram
Register ROpcode
Instruction
Memory
OperandPointer to Operand
Registers
Address A
+

25. Relative Addressing
• A version of displacement addressing
• R = Program counter, PC
• EA = A + (PC)
• i.e. get operand from A cells from current
location pointed to by PC
• c.f locality of reference & cache usage

26. Base-Register Addressing
• A holds displacement
• R holds pointer to base address
• R may be explicit or implicit
• e.g. segment registers in 80x86

27. Indexed Addressing
• A = base
• R = displacement
• EA = A + (R)
• Good for accessing arrays
– EA = A + (R)
– R++

28. Combinations
• Postindex
• EA = (A) + (R)
• Preindex
• EA = (A+(R))
• (Draw the diagrams)

29. Implicit Addressing
• Operand is (implicitly) on top of stack
• e.g.
– ADD Pop top two items from stack
and add

30. Addressing Modes
• The different
ways in which
the location of
an operand is
specified in an
instruction are
referred to as
addressing
modes.
Name Assem bler syn tax Addressing function
Immediate #Value Operand = Value
Register Ri EA = Ri
Absolute(Direct) LOC EA = LOC
Indirect (Ri ) EA = [Ri ]
(LOC) EA = [LOC]
Index X(Ri) EA = [Ri ] + X
Basewith index (Ri ,Rj ) EA = [Ri ] + [Rj ]
Basewith index X(Ri,Rj ) EA = [Ri ] + [Rj ] + X
and offset
Relative X(PC) EA = [PC] + X
Autoincrement (Ri )+ EA = [Ri ] ;
Increment Ri
Autodecrement (Ri ) Decrement Ri ;
EA = [Ri]
−

31. Branching
N,R1Move
NUMn
NUM2
NUM1
R0,SUM
R1
"Next" number to R0
Using a loop to add n numbers.
LOOP
Decrement
Move
LOOP
loop
Program
Determine address of
"Next" number and add
N
SUM
n
R0Clear
Branch>0
•
•
•
•
•
•
Branch target
Conditional branch

32. Utility of using
Indirect
Addressing

33. Using Auto-increment Mode

34. Utility of Indexed Addressing
A list of students’ marks

35. Find sum of Test1, Test2 and Test 3
scores