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Mc (c) 2019 1
COMP 273
Unit 3 – Circuits and System Board Lecture 13
~
RAM and the basic U-Bus
COMP 273 Introduction to Computer Systems

Lecture Outline
● Information Theory
● High level view of RAM
● Low level view of RAM
● The Flip-Flop
● Addressing and the u-bus
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COMP 273 Introduction to Computer Systems

Readings Notes:
• •
2 Intro to Circuits 3 RAM and BUS
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COMP 273 Introduction to Computer Systems

Data is information
Examples:
– Characters(letters, punctuation,spaces)
– Symbols
– Numbers
– Programinstructions
– Webpages
– Images
– Databases
Claude E. Shannon, 1948
Entropy
– Howmuchworkdoesit take to communicate one letter to someone?
The medium
–
?
What is data?
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How can we represent a
single letter
COMP 273 Introduction to Computer Systems

The simplest medium to use A bit
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Coding data as light
Bit = 1 bit Nibble = 4 bits Byte = 8 bits
RAM ~in 1940
COMP 273 Introduction to Computer Systems

Bit structures over time
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COMP 273 Introduction to Computer Systems

Fundamental Unit of Data
• Logical
• The Bit, represents: 1/0, T/F, On/Off, Yes/No, Flags, Switches
• Binary Value: 0 and 1
• No electricity / Has electricity
• Physical representation:
•5V is 1 •2V is 0
Space provided for fluctuations in current
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COMP 273 Introduction to Computer Systems

Three basic things to encode
● Characters
● Numbers
● Program Instructions
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COMP 273 Introduction to Computer Systems

Binary Encoding
Numbers
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3 bit binary math
000 0
001 1
010 2
011 3
COMP 273 Introduction to Computer Systems

Example – Digital Data
If I have 4 bits (known as a “nibble”), how many unique sequences can I make?
Each unique sequence represents one unit of data.
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COMP 273 Introduction to Computer Systems

DRAM
RAM
Random Access Memory
Construction:
– –
An array of bytes
Read & Write control circuitry
– Address Register = integer
Memory Hierarchy consists of: on-die SRAM (static RAM) caches, DRAM (dynamic RAM),
– Mode Register
– Data Register
= flag = byte
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COMP 273 Introduction to Computer Systems

Basic RAM Architecture
32 bits
8 bits
n-1
Cell Address (circuit cell selector)
0
OS
Video Buffer
Program Space
Zero Page
As image
ISA Slots
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Address
Data
COMP 273 Introduction to Computer Systems

Basic RAM Architecture
32 bits
8 bits
n-1
0
Cell Address (circuit cell selector)
OS
Video Buffer
Program Space
Zero Page
Addressing (ie. n = … )
232 = 4,294,967,296
264 = 18,446,744,073,709,551,616
As image
ISA Slots
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COMP 273 Introduction to Computer Systems

Basic RAM Architecture
32 bits 8 bits
How does this function? RAM addressing
With CPU
With system bus
n-1
Cell Address (circuit cell selector)
OS
Video Buffer
0
Addressing machine
Program Space
Zero Page
As image
Address R.
Mode R.
Data R.
TO:
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COMP 273 Introduction to Computer Systems

0
Basic RAM Architecture
4 bits
1001
8 bits
n-1
0111
Mode 1 means “read”:
Read contents of cell 0110
0110
11010010
0101
0100
0011
0010
0001
0000
Address R.
0110
TO:
CPU, or
Bus 15
Mode R.
Data R.
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11010010
McGill
1000
COMP 273 Introduction to Computer Systems

9278 9279
9280
9281 9282 9283 9284 9285 9286
Memory
Main memory is divided into many memory locations (or cells)
Each memory cell has a numeric address, which uniquely identifies it
Memory goes from address
0 to N-1 where N is the amount
Cell
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Address
COMP 273 Introduction to Computer Systems

9278 9279
9280
9281 9282 9283 9284 9285 9286
Each memory cell stores a set number of bits (usually 8 bits, or one byte)
Large values are stored in consecutive memory locations
Storing Information
10011010
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COMP 273 Introduction to Computer Systems

Question
What does this mean: – X = 10;
And this:
– void *p = 100;
– *p = 5;
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COMP 273 Introduction to Computer Systems

The Circuits of RAM
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COMP 273 Introduction to Computer Systems

RS Flip Flop Signal stabilization wait
R = reset and S = set
Mc is the bit saved?
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COMP 273 Introduction to Computer Systems

The Mechanics of Saving R & S = 0 then Nothing changes
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R
S
Clocked RS Flip Flop
(Shows only input but output is also needed)
Similar clocked mechanism on this side.
COMP 273 Introduction to Computer Systems

Basic Flip Flop Circuits
Single input D Flip-Flop
(Register)
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COMP 273 Introduction to Computer Systems

Example
What would the high-level diagram of a full 4-byte read/write memory look like?
Hint:
Data out
Architecture of a row?
Architecture of selecting? (called addressing)
Why to data registers? (can we merge them?)
Row n
Row 2
Row 1
Row 0
Machine to select a row
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Mode
Data In
Address we want
COMP 273 Introduction to Computer Systems

BUS:
The U-Bus
A communication system that transfers information from one location to another
U-BUS:
A specific bus design that facilitates read and write operations
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COMP 273 Introduction to Computer Systems

Data Flow & Control
Memory 1 8-bit memory READ
From clock
From Sequencer
How the bus works
Ground
8-bit CPU bus
The Water-flow Analogy
Memory 2
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WRITE
COMP 273 Introduction to Computer Systems

U-Bus
It is critical to control the flow of electricity. It must move in one direction. However, reading and writing causes electricity to flow in opposite directions. The u-bus helps maintain the correct directional flow for the current.
IN
Memory 1
Memory 2
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Clock and sequencer wires are still needed.
OUT
COMP 273 Introduction to Computer Systems

Example
What circuit can be used to address 4 bytes?
How do we address 4 bytes and connect them through a u-bus for input and output?
How would we move a byte of data from address 0 to address 4?
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COMP 273 Introduction to Computer Systems