1. COMP 1321 Digital Infrastructure
Richard Henson
University of Worcester
October 2018

2. Week 4: Programming a CPU – The Fetch-Execute Cycle
Explain the instruction set required for a typical CPU
Understand the sequential way a CPU uses its instruction set to run programs
Understand how registers and memory addresses are used to process a CPU instruction and store the results

3. Programming a CPU! Overview: Back to Babbage and Lovelace…
Back to Babbage and Lovelace…
CPU is a machine
machines are stupid
CPU needs instructions it can understand..

4. (identify & name components)
The computer so far…
(identify & name components) ip
Data Memory
Instruction
Memory 1 4
mar

5. A couple of extra registers..
Memory Data Register
Instruction Register
Data Memory 1 4 1. 2
add ax,bx 2. 8 34
Instruction
Memory
Data
Energize ax
Energize bx
Select ALU “add" 2
Address 34
Line of code goes in…
Electrical bit signals come out

6. Moving data into Registers (ie from specified location)
mov ax , [1]
for example …
Instruction
Memory 1 2
mar 3 4 AX
mov ax , [1] BX
mov bx , [2] 5
mov bx , [2] 8 7 8 7 6 1

7. Moving data into Memory
For example …
mov [3] , ax
Instruction
Memory 1 2
mar 3 4
mov [0], bx AX
mov [3] , ax BX 5 7
mov [0] , bx 8 7 8 7 6 8 1

8. 8086 CPU family registers 8086 chip always used a 16-bit word
SAM simulates an 8-bit word
popular on most early microcomputers…
Typical 8086 registers (stores):
general purpose data: AX, BX, CX, DX
specific use e.g.
program counter (PC): instruction address in memory
stack pointer SP): address of the top of the “stack”

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

10. CPU 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
Many other instructions; range of operations…
collectively known as an instruction set
each CPU family has its own unique codes

11. 8086 in practice Four 16-bit General Purpose registers
each gen register (e.g. AX) can be read/written to upper (AH) & lower (AL) 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 appeared first
e.g. “MOV, AX, BX” puts result in AX

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 (8086 family) 2
Fetch
Decode
ALU
Mem Ops
Reg Write 3 4 5

28. Intel Multi-core

29. Programming a CPU CPU programming code written as assembly language
each family has its own instruction set
Programming syntax depends on the
CPU/instructions
how they should be used
Intel 8086 assembly language used for CPUs that support PC platforms

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

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