1. 1
Microprocessor Operation and
Execution of Instructions
Prof. Yusuf Leblebici
Microelectronic Systems Laboratory (LSM)
yusuf.leblebici@epfl.ch

2. 2
8085 Functional Block Diagram

3. 3
Basic Processor Operation
 All instructions (program steps) are stored in memory.
 To run a program, the individual instructions must
be read from the memory in sequence, and executed.
 Detailed sequence:
 Instruction fetch (read from memory)
 Decode instruction
 Get operands
 Execute operation
 Save result
get
next
instruction

4. 4
Basic Processor Operation
 We will first demonstrate the main steps of execution
using a very simple processor “model” :
 Few instructions (commands)
 Simple memory access
 Easy to understand
 Then, we will start examining the actual 8085
processor instruction set and operations.

5. The OTC (Our Tiny Computer)
• Only 4 Instructions
– Add
• sets condition of result: negative, zero, positive
– Load, Store
• Absolute Reference
– Branch
• PC Relative branch
• 4 Registers (r0, r1, r2, r3)
• 16 Storage (Memory) Locations

6. Example OTC Instructions
load r1,12 // r1 = M[12]
add r1,r2,r2 // r1 = r2 + r2
br pos,-3 // if (result > 0) PC = PC – 3
st r0,3 // M[3] = r0
Illegal instructions
load r4,17 // no register 4, no location 17
add r1,19 // add requires register references

7. OTC
Operation
Load
Store
Branch
Add
Register
File
Memory
Program Counter
Inst Reg
Load r0,b
Load r1,c
Add r2,r1,r0
Store r2,a
7
6 B
C
A
0
Load r0,b
0
6
Load r0,b
0
6
Load r0,b
1
6
Load r1,c
1
6
Load r1,c
1
7
6
Load r1,c
2
7
6
Add r2,r1,r0
2
7
13
6
Add r2,r1,r0
2
7
+
13
6
Add r2,r1,r0
3
7
+
13
6
Store r2,a
3
7
+
13
6
Add r2,r1,r0
3
7
+
13
r0
r1
r2
r3
0
1
2
3

8. OTC Instruction Encoding
0 1 2 3 4 7
op reg loc
op reg reg reg
0 1 2 3 4 5 6 7
op cond target
0 1 2 4 5 7
Load/Store
Add
Branch
Opcodes
Load
Store
Branch
Add
00
01
10
11

9. Load
Store
Branch
Add
Register
File
Memory
Program Counter
Inst Reg
0 0 0 0 1 0 0 1
0 0 0 1 1 0 1 0
1 1 1 0 0 1 0 0
0 1 1 0 1 0 0 0
1 10 0 0 0 0 0
1 10 0 0 0 0 1
00
0 0 0 0 1 0 0 1
1 10 0 0 0 0 0
0
0 0 0 0 1 0 0 1
1 10 0 0 0 0 0
1
0 0 0 0 1 0 0 1
1 10 0 0 0 0 0
1
0 0 0 1 1 0 1 0
1 10 0 0 0 0 1
1 10 0 0 0 0 0
1
0 0 0 1 1 0 1 0
1 10 0 0 0 0 1
1 10 0 0 0 0 0
2
0 0 0 1 1 0 1 0
1 10 0 0 0 0 1
1 10 0 0 0 0 0
2
1 1 1 0 0 1 0 0
1 00 0 0 0 1 1
1 10 0 0 0 0 1
1 10 0 0 0 0 0
2
1 1 1 0 0 1 0 0
+
1 00 0 0 0 1 1
1 10 0 0 0 0 1
1 10 0 0 0 0 0
3
1 1 1 0 0 1 0 0
+
0 1 1 0 1 0 0 0
1 00 0 0 0 1 1
1 10 0 0 0 0 1
1 10 0 0 0 0 0
3
+
0 1 1 0 1 0 0 0
1 00 0 0 0 1 1
0 1 1 0 1 0 0 0
1 00 0 0 0 1 1
1 10 0 0 0 0 1
1 10 0 0 0 0 0
3
+
OTC
Operation

10. Another Simple Program
Acc = 1
Count = -n
Loop
Acc += Acc
Count += 1
Until (Count = 0)
L Ra,1
L Rc,-n
L Ri,1
Loop
Add Ra,Ra,Ra
Add Rc,Rc,Ri
Br Neg,Loop

11. Load
Store
Branch
Add
Register
File
Memory
Program Counter
Inst Reg
Load R0,14
Load R1,14
Load R2,15
Add R0,R0,R0
Add R2,R2,R1
BR Neg,3
1
-5
00
Load R0,14
0
Load R0,14
1
1
Load R0,14
1
1
Load R1,14
1
1
Load R1,14
1
1
2
Load R2,15
1
1
2
Load R2,15
1
1
-5
3
Load R2,15
1
1
-5
3
Add R0,R0,R0
1
1
-5
3
Add R0,R0,R0
2
1
-5
4
Add R0,R0,R0
2
1
-5
+
4
Add R2,R2,R1
2
1
-5
+
4
Add R2,R2,R1
2
1
-4
-
5
Add R2,R2,R1
2
1
-4
-
5
BR Neg,3
2
1
-4
-
3
BR Neg,3
2
1
-4
-
3
Add R0,R0,R0
2
1
-4
-
3
Add R0,R0,R0
2
1
-4
-
3
Add R0,R0,R0
4
1
-4
+
OTC
Operation
0
1
2
3
4
5
14
15
r0
r1
r2
r3

12. 12
Now:
Back to the 8085 Microprocessor

13. 13
The 8085 Microprocessor

14. 14
The 8085 Microprocessor
address
(8 bits)

15. 15
The 8085 Microprocessor
address
and data
(8 bits)
dual - use
pins !!

16. 16
The 8085 Microprocessor
Demultiplexing the
address / data bus
16
8

17. 17
Example: Instruction Fetch Operation

18. 18
Example: Instruction Fetch Operation
Instruction
(opcode)
reaches the
instruction
decoder now !
It takes four
clock cycles
to get one
instruction
into the CPU.

19. 19
Execution of an Instruction
Now consider the execution of a simple instruction:
 Instruction 3E (hex) means: Load a data byte into
the accumulator
 The instruction is followed by the data byte 32 (hex)
 Two-byte instruction !

20. 20
Execution of an Instruction

21. 21
Execution of an Instruction
Put the first
memory
location on
the address
bus (2000 h)

22. 22
Execution of an Instruction
Get the
instruction
(opcode)
byte from
memory

23. 23
Execution of an Instruction
Interpret the
instruction:
Wait for the
data byte !

24. 24
Execution of an Instruction
Put the next
memory
location on
the address
bus (2001 h)

25. 25
Execution of an Instruction
Get the data
byte from the
memory
Put into
accumulator

26. 26
Execution of an Instruction
How long does it take to execute this two-byte
instruction (op-code) ?
It is quite possible to accurately predict the time
that is required to run each instruction, and to
run the entire program !

27. 27
8085 Instruction Set
The 8085 instructions can be classified as follows:
 Data transfer operations
 Arithmetic operations (ADD, SUB, INR, DCR)
 Logic operations
 Branching operations (JMP, CALL, RET)
• Between registers
• Between memory location and a register
• Direct write to a register / memory
• Between I/O device and accumulator

28. 28
8085 Instruction Types

29. 29
8085 Instruction Types

30. 30
8085 Instruction Types

31. 31
A VERY Simple Program
Add two hexadecimal numbers:
 Load register A (accumulator) with 32 (hex)
 Load register B with 48 (hex)
 Add the two numbers and save the sum in A
 Display accumulator (A) contents at port (01)
 End

32. 32
A VERY Simple Program