cse115 / cse503 introduction to computer science i dr. carl alphonce 343 davis hall...

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CSE115 / CSE503Introduction to Computer Science I

Dr. Carl Alphonce343 Davis Hall

alphonce@buffalo.edu

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Announcements

• Recitations begin this week.– “preparatory” lab exercises

• Name sign judging today

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cell phones off

laptops away

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name signs out

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Today

• Representations

• Gates

• Fetch-Decode-Execute cycle

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Interpretation

• QUESTION:– What does the bit string 1101

represent?

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Interpretation

• QUESTION:– What does the bit string 1101

represent?

• ANSWER:– Whatever we want it to represent!

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Roadmap

• A computer is a very simple machine– it manipulates voltages– gates are used to control voltage flow– circuits are combinations of gates– a flip-flop is a circuit that remembers

• Instructions are encoded as bit strings• Machine/assembly/high-level languages• Fetch-Decode-Execute cycle• Requirements for a programming language

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physical vs. logical perspectives

• Physical reality:

• Logical view:

WIRE

Carries a HIGH voltage or a LOW voltage

WIRECarries a 1 or a 0

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Idea

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Idea

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http://commons.wikimedia.org/wiki/File:Relay_principle_horizontal.jpgUser:Bisgaard,

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http://commons.wikimedia.org/wiki/File:Electronic_component_relays.jpg

User:FDominec

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• Electro-mechanical computers – relays: Mark I (1944)

• Electronic computers– vacuum tubes: ENIAC (1946)– transistors: U of Manchester (1953)– integrated circuits (~1963)– microprocessors: Intel 4004 (1971)

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Controlling flow

0

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Controlling flow

1

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Two in a row?

0

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Two in a row?

0

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Two in a row?

0

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Two in a row?

1

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AND gate

For which input values is output 1?

For which input values is output 0?

inputs are on leftoutput is on right

0 or 1

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Truth table for AND

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 0

1 1 1

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OR gate

For which input values is output 1?

For which input values is output 0?

inputs are on leftoutput is on right

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Truth table for OR

Input 1 Input 2 Output

0 0 0

0 1 1

1 0 1

1 1 1

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NOT gate

For which input value is output 1?

For which input value is output 0?

input is on left output is on right

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Truth table for NOT

Input Output

0 1

1 0

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Flip-flop (a bit of memory!)

S (set)

R (reset)

remembered value

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Setting the flip-flop

The normal value of R and S is zero.

S (set) = 0

R (reset) = 0

remembered value

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Setting the flip-flop

To store 1 in the flip-flop, we “raise” S to 1…

S (set) = 1

R (reset) = 0

remembered value

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Setting the flip-flop

…which makes the output of the OR gate 1.

S (set) = 1

R (reset) = 0

remembered value

1

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Setting the flip-flop

The NOT gate inverts this 1 value to 0, which becomes the second input to the upper OR gate.

S (set) = 1

R (reset) = 0

remembered value

1 0

0

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Setting the flip-flop

Since both inputs of the upper OR gate are zero, its output is zero.

S (set) = 1

R (reset) = 0

remembered value

1 0

0

0

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Setting the flip-flop

The NOT gate inverts this 0 to a 1; this value becomes the second input to the bottom OR.

S (set) = 1

R (reset) = 0

remembered value

1 0

0

0 1

1

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Setting the flip-flopBecause the output of the bottom OR gate will now stay at 1, we can lower S to zero, and the circuit will stay in a stable state, with 1 as the

remembered value!

Resetting the flip-flopResetting the remembered value to zero is similar, except we raise, then

lower, the value on R.

S (set) = 0

R (reset) = 0

remembered value

1 0

0

0 1

1

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Recap

• Bit string by itself does not carry meaning.

• Bit string can be interpreted under a given representation scheme, which allows us to recover the encoded meaning.

• Circuits made from simple gates let us store and manipulate bit strings.