47 lines
1.2 KiB
Markdown
47 lines
1.2 KiB
Markdown
# Input / Output Subsystem
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Handles devices that allow the computer system to:
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* Communicate and interact with the outside world
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- Screen, keyboard, printer
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* Store information
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![[io-subsystem.png]]
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# The Control unit
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Program stored in memory. (machine language in binary)
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Task of control unit is to execute programs by:
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* Fetch: from memory the next instruction
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* Decode: instruction to determine what to do
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* Execute: issuing signals to ALU, memory and IO systems.
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Repeats until HALT instruction.
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# Typical Machine Instructions
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MOV: move data from a to b
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ADD: add numbers
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SUB: subtract numbers
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JMP: transfers program control flow to the indicated instruction
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![[example-machine-lang.png]]
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# CPU Time vs IO Time
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Total time required to run program is: CPU Time + IO Time
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Typical time to read from IO: 20ms
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Typical time to run instruction: 20ns
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CPU can execute up to 200 million instructions while a block is read from a hard disk.
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Conclusion: While I/O is executed, CPU is idling.
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![[cpu-vs-io.png]]
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Were waiting a lot on IO time to finish, very inefficient. We can solve this using multitasking!
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# Multitasking
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To speed up computers we overlap CPU time and IO Time.
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While a program waits for IO, o
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