1. COMP 1321 Digital Infrastructure
Richard Henson
University of Worcester
October 2013

2. Week 3: The Fetch-Execute Cycle
Explain the instruction set of a typical CPU
Understand the sequential way a CPU works, using its instruction set
Understand how registers and memory addresses are used to process a CPU instruction and store the results

3. CPUs and the SAM SAM is a CPU simulator CPU very, very, very fast
designed to allow you to watch what happens when a CPU works
CPU very, very, very fast
Processes one instruction at a time
Instructions can require several cycles

4. What is “Processing”? Usually calculations:
need data input
from register
from external memory
need to store output
Could also be a command without data

5. CPU types Most frequently used: We’ll focus on Intel 8086 family
Motorola (esp family)
ARM (many mobile phones)
We’ll focus on Intel 8086 family
dates back to original IBM PC…

6. Registers A series of memory stores inside the CPU
usually containing one word of memory
1, 2, 4 bytes i.e. very very small!
CPU reads/writes data very very quickly to/from the registers

7. Registers (from last week…)
high-speed memory on the CPU chip
Parking places for data on the move 1 4 6 8 AX BX
AX and BX registers are used for ALU operations
MARR
MAR is memory address register, 4 in eg.
Result, 6+8=14, will go into memory cell address 4

8. 8086 CPU family registers 8086 chip always used a 16-bit word
SAM simulates an 8-bit word
popular for most early microcomputers
Typical 8086 registers:
general purpose data: AX, BX, CX, DX
specific use e.g.
program counter: instruction address in memory
stack pointer…

9. Data and Addressing A general purpose register could contain
A memory address that points to data
Convention:
data written as hexadecimal equivalent
e.g. 4A
memory location has square brackets
e.g. [4A]

10. Instructions Used to tell the CPU what to do…
MOV is for moving data around…
MOV AX, 4A – move “4A” into AX register
MOV AX, [4A] – move data contained in address 4A into AX register
Other instructions for different operations…
collectively known as an instruction set

11. 8086 in practice Four 16-bit General Purpose registers
each gen register split into upper byte & lower byte:
upper byte
lower byte AX AH AL BX BH BL CX CH CL DX DH DL

12. Another 8086 Instruction: ADD
Takes values from two registers
Adds them together
Deposits results back in one of the registers
Which one?
the register that appears first in the instruction

13. Fetch-Execute Cycle (Organization and Control)
1. Fetch instruction from memory
5. Write back results to registers
add ax , bx
ax <- ALU
4. Do any Memory Access
2. Decode the instruction and read any registers
(Data cache)
ALU <- ax ALU <- bx
None needed
3. Do any ALU operations (execute units)
ax + bx

14. Fetch-Exec : State 1 Instruction Fetch 3 3 1 8 7 1 9 add ax , bx AX BX1 4 3 2
add ax bx AX BX 3
add ax,bx 3 1 8 7 1 9

15. Fetch-Exec : State 2 Decode, Register Operations 3 3 1 8 7 3 1 1 9
add ax , bx 1 4 3 2
add ax bx AX BX 3
add ax,bx 3 1 8 7 3 1 1 9

16. Fetch-Exec : State 3 ALU Operation 3 8 7 3 1 4 1 9 add ax , bx AX BX 11 4 3 2
add ax bx AX BX 3
add ax,bx 8 1 7 3 4 1 9

17. Fetch-Exec : State 4 Memory Access 3 8 7 3 1 4 1 9 add ax , bx AX BX 11 4 3 2
add ax bx AX BX 3
add ax,bx 8 1 7 3 4 1 9

18. Fetch-Exec : State 5 Register Write 3 4 8 7 3 1 4 1 9 add ax , bx BX 11 4 3 2
add ax bx BX 3
add ax,bx 4 8 1 7 3 4 1 9

19. Fetch-Execute Cycle (Organization and Control)
1. Fetch instruction from memory
5. Write back results to registers
mov ax , [1]
Data into ax
4. Do any Memory Access
2. Decode the instruction and read any registers
Read memory at addr ‘1’
Read the ‘1’
3. Do any ALU operations (execute units)
Put ‘1’ into MAR

20. Fetch-Exec : State 1 Instruction Fetch 3 8 7 1 9 mov ax , [1] 1 2 3 41 4 3 2
mov ax 1
mov ax , [1] 3 8 7 1 9

21. Fetch-Exec : State 2 Decode, Register Operations 3 8 7 1 9
mov ax , [1] 1 4 3 2
mov ax 1
mov ax , [1] 3 8 7 1 9

22. Fetch-Exec : State 3 ALU Operation 3 8 7 1 9 mov ax , [1] 1 1 2 3 41 4 3 2
mov ax 1
mov ax , [1] 3 8 7 1 1 9

23. Fetch-Exec : State 4 Memory Access 3 8 8 7 1 9 mov ax , [1] 1 1 2 3 41 4 3 2
mov ax 1
mov ax , [1] 3 8 8 7 1 1 9

24. Fetch-Exec : State 5 Register Write 3 8 8 8 7 1 9 mov ax , [1] 1 1 2 31 4 3 2
mov ax 1
mov ax , [1] 3 8 8 8 7 1 1 9

25. 8088: Brains of the IBM PC

26. Inside the 8088 address bus address adder External buses gen registers
ALU

27. 1
Pentium (same family) 2
Fetch
Decode
ALU
Mem Ops
Reg Write 3 4 5

28. Programming a CPU CPU programming code written as assembly language
Each family has its own instruction set
Programming syntax will depend on the instructions & how they should be used
Intel 8086 assembly language is used for CPUs that support Microsoft platforms…

29. Example 8086 Assembly Language
MOV AH,08
INT 21
MOV DL,AL
MOV AH,02
MOV AH,4C

30. So THAT’S how it all works
So THAT’S how it all works!  now you try it on SAM2… Next week: a focus on writing programs and i/o