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CSE115 / CSE503Introduction to Computer Science I
Dr. Carl Alphonce343 Davis Hall
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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.
