6. The CPU and Memory
Chapt. 7
ITEC 1011
Introduction to Information Technologies
Introduction
• The Little Man Computer (LMC) is cute,
but…
• Real computers…
• Memory is separate from the CPU
• Data are in binary (not decimal)
ITEC 1011
Introduction to Information Technologies
Central
Processing
Unit
Components of a CPU
• ALU (arithmetic and logic 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
ITEC 1011
Introduction to Information Technologies
LMC Counterparts
ALU
Input/output
interface
123
456
00
01
02
03
04
05
123
500
199
500
399
Control unit
95
96
97
98
99
789
123
05
Program counter
p. 167
ITEC 1011
Introduction to Information Technologies
System Block Diagram
CPU
ALU
Input/output
interface
Memory
Control unit
Program counter
ITEC 1011
Introduction to Information Technologies
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
ITEC 1011
Introduction to Information Technologies
Memory Unit
Memory cell
bit 0
bit 1
0
1
2
3
4
Address decoder
Memory address register
n bits
2n-1
bit n - 1
0
1
2
m-1
Memory data register
m bits
p. 170
ITEC 1011
Introduction to Information Technologies
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
ITEC 1011
Introduction to Information Technologies
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
ITEC 1011
Introduction to Information Technologies
Exercise – Memory Capacity
• Q: How many bits of memory are contained
in a memory unit with 2MB of memory?
A:
Skip answer
ITEC 1011
Introduction to Information Technologies
Answer
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
ITEC 1011
Introduction to Information Technologies
Memory Implementations
• RAM – random access memory
• Static RAM
• Dynamic RAM
• ROM – read-only memory
p. 175
ITEC 1011
Introduction to Information Technologies
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
ITEC 1011
Introduction to Information Technologies
The Store Instruction
Fetch
PC  MAR
MDR  IR
IR[address]  MAR
Execute
A  MDR
PC + 1  PC
ITEC 1011
Introduction to Information Technologies
time
The Add Instruction
Fetch
PC  MAR
MDR  IR
IR[address]  MAR
Execute
A + MDR  A
PC + 1  PC
ITEC 1011
Introduction to Information Technologies
time
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…
ITEC 1011
Introduction to Information Technologies
bus
Printer
Mouse
Ports
Keyboard
Modem
CPU
Disk
controller
Graphics
card
Monitor
Sound
card
Speakers
RAM
Network
card
Computer
ITEC 1011
Introduction to Information Technologies
Types of Buses (1 of 3)
• Point-to-point
Serial
port
Control
unit
ITEC 1011
Modem
ALU
Introduction to Information Technologies
Types of Buses (2 of 3)
• Multipoint
Computer
Computer
Computer
CPU
Disk controller
ITEC 1011
Computer
Memory
Video controller
Introduction to Information Technologies
Types of Buses (3 of 3)
• Daisy chain
Device controller
Device
Device
Device
Terminator
ITEC 1011
Introduction to Information Technologies
Buses Inside a Computer
Data bus
Address bus
Control bus
CPU
I/O Module
Memory
I/O Device
ITEC 1011
Introduction to Information Technologies
Motherboard
• Many
configurations
possible
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.
ITEC 1011
Introduction to Information Technologies
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.
ITEC 1011
Introduction to Information Technologies
Control Bus
• Collection of signals for coordinating CPU
activities
• Each signal has a unique purpose
• Typical sizes: 10-20 lines
• Signals are output, input, or bi-directional
• Typical signals
•
•
•
•
•
ITEC 1011
/RD (read)
/WR (write
CLK (clock)
/IRQ (interrupt request)
etc.
Introduction to Information Technologies
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…
ITEC 1011
Introduction to Information Technologies
Memory
Map
7 6 5 4 3 2 1 0
FFFF
Data bit
position
Hexadecimal
address
ITEC 1011
0002
0001
0000
Introduction to Information Technologies
The “bottom”
of memory
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…
ITEC 1011
Introduction to Information Technologies
Memory
Map
FFFFFF
14 MB
empty
224 bytes = 16 MB
“capacity”
200000
1FFFFF
1 MB RAM
100000
0FFFFF
1 MB RAM
000000
ITEC 1011
Introduction to Information Technologies
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
ITEC 1011
Introduction to Information Technologies
Answer
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
ITEC 1011
Introduction to Information Technologies
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
ITEC 1011
Introduction to Information Technologies
Answer
Answer
Memory
Map
7FFFFFFF
1,952 MB
empty
231 bytes = 2 GB
“capacity”
06000000
05FFFFFF
04000000
03FFFFFF
Note:
2 GB = 2,048 MB
02000000
01FFFFFF
00000000
ITEC 1011
32 MB RAM
32 MB RAM
32 MB RAM
Introduction to Information Technologies
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)
ITEC 1011
Introduction to Information Technologies
Thank you
ITEC 1011
Introduction to Information Technologies