1. 6. The CPU and Memory
Chapt. 7

2. Introduction The Little Man Computer (LMC) is cute, but…
Real computers…
Memory is separate from the CPU
Data are in binary (not decimal)
Central Processing Unit

3. Components of a CPU ALU (arithmetic and logic unit) Control unit
Perform arithmetic and logic operations
Arithmetic: add, subtract, multiply, divide, etc.
Logic: AND, OR, NOT, Shift, etc.
Control unit
Interprets instructions
Controls the flow of information within the CPU
Works with a “program counter” (address of next instruction)
Input/output interface
Provides mechanism for input and output of data
Many variations possible
p. 166

4. LMC Counterparts ALU Input/output interface Control unit00
123
123 01
500 02
199 03
500 04
399
456 05
Control unit 95 96 97 98
789 99
123 05
Program counter
p. 167

5. Input/output interface
System Block Diagram
CPU
Memory
ALU
Input/output interface
Control unit
Program counter

6. Registers A register is a single storage location within the CPU
Unlike memory, which is “outside” the CPU
Examples of registers:
Accumulator (ACC)
Program counter (PC)
Instruction register (IR)
Memory address register (MAR)
Memory data register (MDR)
Status register
General purpose registers (R0, R1, …)
Included on some CPUs
Used for high-speed temporary storage of program variables

7. Memory address register
Memory Unit
n bits
Memory cell
bit 0 1 2 3 4
2n-1
bit 1
Memory address register
Address decoder
bit n - 1
m - 1
Memory data register
m bits
p. 170

8. Memory Capacity 2n x m n address bits = 2n addresses m data bits
m is the “width” of the data path
Typical values:
n: 16, 17, 18, 19, 20, 21, 22, etc.
m: 8, 16, 32, 64

9. Question
Q: How many bits of memory are contained in a memory unit with 512KB of memory?
A: 512 = 29, K = 210, B = byte = 8 = 23
29 x 210 x 23 = 222 = 4,194,304

10. Exercise – Memory Capacity
Q: How many bits of memory are contained in a memory unit with 2MB of memory? A:
Skip answer
Answer

11. Exercise – Memory Capacity
Answer
Q: How many bits of memory are contained in a memory unit with 2MB of memory?
A: 2 = 21, M = 220, B = byte = 8 = 23
21 x 220 x 23 = 224 = 16,777,216

12. Memory Implementations
RAM – random access memory
Static RAM
Dynamic RAM
ROM – read-only memory
p. 175

13. Fetch-Execute Cycle
Two steps, or cycles, in the execution of every instruction
Fetch – fetch the code for the instruction from memory and place it in the IR (instruction register)
Execute – execute the instruction
A pretty picture sometimes helps…
Fetch Execute
time

14. The Store Instruction PC  MAR MDR  IR Fetch IR[address]  MAR
A  MDR
PC + 1  PC
Fetch
time
Execute

15. The Add Instruction PC  MAR MDR  IR Fetch IR[address]  MAR
A + MDR  A
PC + 1  PC
Fetch
time
Execute

16. From our first lecture…
Buses
Definition: a collection of wires with a common purpose
Each wire is called a line
Typically, buses carry information from one place to another
From our first lecture…

17. bus
Ports
Printer
Mouse
Keyboard
Modem
Disk controller
Graphics card
Monitor
Speakers
CPU
Sound card
RAM
Network card
Computer

18. Types of Buses (1 of 3) Point-to-point Serial port Modem Control unit
ALU

19. Types of Buses (2 of 3) Multipoint Computer Computer Computer Computer
CPU
Memory
Disk controller
Video controller

20. Types of Buses (3 of 3) Daisy chain Device controller Device Device
Terminator

21. Buses Inside a Computer
Motherboard
Many configurations possible
CPU
Data bus
Address bus
Control bus
Memory
I/O Module
I/O Device

22. Data Bus Carries data between the CPU and memory or I/O devices
Bi-directional
Data transferred “out of” the CPU for write operations
Data transferred “into” the CPU for read operations
Typical sizes: 8, 16, 32, 64 lines
Signal names:
D0, D1, D2, D3, etc.

23. Address Bus Carries an address from the CPU to Memory or I/O devices
Unidirectional
The address is always supplied by the CPU
(There is one exception to this, which we’ll discuss later.)
Typical sizes: 16, 20, 24 lines
Signal names:
A0, A1, A2, A3, etc.

24. Control Bus Collection of signals for coordinating CPU activities
Each signal has a unique purpose
Typical sizes: lines
Signals are output, input, or bi-directional
Typical signals
/RD (read)
/WR (write
CLK (clock)
/IRQ (interrupt request)
etc.

25. Memory Maps
The usage of memory space on a system is commonly depicted in a “memory map”
The height of the map is determined by the number of addresses
The width of the map is usually 8 bits
E.g.,
a system with a capacity of 216 bytes…

26. Memory Map Data bit position The “bottom” of memory
FFFF
0002
0001
0000
Data bit position
The “bottom” of memory
Hexadecimal address

27. Use of Memory Maps
Memory maps are usually drawn to show “what is where” on a system
The possibilities for “what”
RAM, ROM, I/O, nothing
The possibilities for “where”
Determined by the starting/ending addresses for each “block” of RAM, ROM, I/O, nothing
E.g.,
a memory map for a system with a capacity of 224 bytes with two 1 MB RAM modules residing consecutively at the bottom of memory…

28. Memory Map 14 MB empty 224 bytes = 16 MB “capacity” 1 MB RAM 1 MB RAM
FFFFFF
200000
1FFFFF
100000
0FFFFF
000000
14 MB
empty
224 bytes = 16 MB “capacity”
1 MB RAM
1 MB RAM

29. Exercise – Memory Space
Q: A system with a memory capacity of 128 GB has four 32 MB memory modules installed consecutively at the bottom of memory. The rest of the memory is unused. How much memory space is available for future expansion? (Give your answer in decimal in megabytes.)
A: ?
Skip answer
Answer

30. Exercise – Memory Space
Answer
Q: A system with a memory capacity of 128 GB has four 32 MB memory modules installed. The rest of the memory is unused. How much memory space is available for future expansion? (Give your answer in decimal in megabytes.)
A: 128 GB – 4 x 32 MB
= 27 x 210 MB - 22 x 25 MB
= (217 – 27) MB
= (131,072 – 128) MB
= 130,944 MB

31. Exercise – Memory Maps
Draw a memory map for a system with a capacity of 2 GB. Assume the system has three 32 MB memory modules residing consecutively at the bottom of memory. Illustrate the size of each block in MB and the starting and ending address of each block of memory in hexadecimal.
Skip answer
Answer

32. Memory Map 1,952 MB empty 231 bytes = 2 GB “capacity” 32 MB RAM
Answer
Memory Map
7FFFFFFF
05FFFFFF
03FFFFFF
01FFFFFF
1,952 MB
empty
231 bytes = 2 GB “capacity”
32 MB RAM
32 MB RAM
Note: 2 GB = 2,048 MB
32 MB RAM

33. Memory vs. I/O
Our system block diagram shows memory and I/O connected to the same buses (data, address, & control)
This raises the question…
How is I/O differentiated from memory?
(We’ll answer this in the next topic)

34. Thank you