1. Intel 8080 Processor The 8080 was an 8-bit processor
Registers hold 8-bits of information
The ALU is capable of doing computations on 8-bit data values
Used 16 address lines
Address Bus = 16 wires
Data Bus = 8 wires
System Bus
CPU
Memory
Unit
Control Bus

2. 64K Intel 8080 Processor Question:
How much memory could be installed in a computer system which uses the 8080 processor?
64K
16 address lines
216 = 65,536 = 64K
First address is 0 and the last address is 65,535
So range of addresses is 0H to FFFFH (0FFFFH)
Why 64K?
Most computers had about 4K of memory (memory was expensive!)
Wanted to allow room for growth in the industry
“64K is more memory than anyone will ever need!”

3. Intel 8080 Processor The addressing was simple!
Example: Read the contents of memory cell #24,263
Put the address into the MAR
This selects the appropriate memory cell
Issue the read command to the memory unit
Memory unit responds by putting cell’s contents on data bus
Memory tells CPU data is ready (using an interrupt)
CPU picks up data and stores it in MBR
Memory Unit
CPU
0000
0001
0002
5EC5
5EC6
5EC7
FFFD
FFFE
FFFF A6
Other
Registers
MAR
MBR 7E
Address bus
Data bus
Control bus F0
5EC6 49 2C 2C 2C 9D
ALU 8A
Internal
Bus 51 9B

4. General CPU Architecture
MAR
General Purpose
Registers
Address Bus
to memory &
I/O devices 1 2 3 4
MBR
Data Bus 1 2 3 4 5 6 7 8 9
Program Counter
Instruction Register
ALU

5. General CPU Architecture
MAR
General Purpose
Registers
Address Bus 1 2 3 4
MBR
Data Bus 1 2 3 4 5 6 7 8 9
Program Counter
Instruction Register
ALU

6. Program to add 4 + 7 Instruction 1: Fetch 1 2 3 4 1 1 2 3 4 5 6 7 8 9
MAR
General Purpose
Registers
Address Bus 1 2 3 4
MBR
Data Bus
Put 4 into 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 1
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Put 4 into
in mem 9
Stop

7. Instruction 1: Fetch - Complete! 1 1 2 3 4 1 2 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 1 1 2 3 4
MBR
Data Bus
register 1 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 1 2
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Put 4 into
register 1
in mem 9
Stop

8. Instruction 1: Execute - Complete! 1 2 3 4 4 2 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 1 2 3 4 4
MBR
Data Bus 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 2
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Put 4 into
register 1
in mem 9
Stop

9. Instruction 2: Fetch 2 1 2 3 4 4 3 2 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 2 1 2 3 4 4
MBR
Data Bus
Put 7 into 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 3 2
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Put 7 into
in mem 9
Stop

10. Instruction 2: Fetch - Complete! 3 1 2 3 4 4 3 4 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 3 1 2 3 4 4
MBR
Data Bus
register 2 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 3 4
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Put 7 into
register 2
in mem 9
Stop

11. Instruction 2: Execute - Complete! 1 2 3 4 4 7 4 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 1 2 3 4 4
MBR
Data Bus 7 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 4
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Put 7 into
register 2
in mem 9
Stop

12. Instruction 3: Fetch 4 1 2 3 4 4 7 4 5 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 4 1 2 3 4 4
MBR
Data Bus 7
Add reg 1 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 4 5
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Add reg 1
in mem 9
Stop

13. Instruction 3: Fetch - Complete! 5 1 2 3 4 4 7 6 5 1 2 3 4 5 6 7 8 9
MAR
General Purpose
Registers
Address Bus 5 1 2 3 4 4
MBR
Data Bus 7
and reg 2 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 6 5
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Add reg 1
and reg 2
in mem 9
Stop

14. Instruction 3: Execute - Complete! 1 2 3 4 4 7 11 6 1 2 3 4 5 6 7 8 9
MAR
General Purpose
Registers
Address Bus 1 2 3 4 4
MBR
Data Bus 7 11 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 6
register 2
Add reg 1
Instruction Register
ALU
and reg 2
ADD
Store reg 3
Add reg 1
and reg 2
in mem 9
Stop

15. Instruction 4: Fetch 6 1 2 3 4 4 7 11 6 7 1 2 3 4 5 6 7 8 9 MAR
General Purpose
Registers
Address Bus 6 1 2 3 4 4
MBR
Data Bus 7
Store reg 3 11 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 6 7
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Store reg 3
in mem 9
Stop

16. Instruction 4: Fetch - Complete! 7 1 2 3 4 4 7 11 8 7 1 2 3 4 5 6 7 8
MAR
General Purpose
Registers
Address Bus 7 1 2 3 4 4
MBR
Data Bus 7
in mem 9 11 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 8 7
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Store reg 3
in mem 9
in mem 9
Stop

17. Instruction 4: Execute - Complete! 9 1 2 3 4 4 7 11 11 8 11 1 2 3 4 5
MAR
General Purpose
Registers
Address Bus 9 1 2 3 4 4
MBR
Data Bus 7 11 11 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 8
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Store reg 3
in mem 9
in mem 9
Stop 11

18. Instruction 5: Fetch - Complete! 8 1 2 3 4 4 7 11 8 11 1 2 3 4 5 6 7 8
MAR
General Purpose
Registers
Address Bus 8 1 2 3 4 4
MBR
Data Bus 7
Stop 11 1 2 3 4 5 6 7 8 9
Put 4 into
Program Counter
register 1
Put 7 into 8
register 2
Add reg 1
Instruction Register
ALU
and reg 2
Store reg 3
Stop
in mem 9
Stop 11