73 lines
2.3 KiB
Markdown
73 lines
2.3 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, other programs can use CPU. This solution requires multiple programs to be loaded into memory, hence the name multitasking.
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Multitasking overlaps IO time of a program with CPU time of other program.
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# Multitasking with multiple processors
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If computer has a single CPU, programs should take turns in using it.
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If computer has multiple CPUS, each task can be given a different CPU.
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- If the number of tasks is more than available cpu's, we still take turns.
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when program issues IO command, cpu is granted to the next program.
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# Concurrent vs Parallel
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**Concurrent**: Fast switching from a program to the next program (context switch) can create the illusion that they are being executed at the same time. -> **Logically Simultaneous**
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**Parallel**: If the computer has multiple CPU's or Cores, the programs run in parallel. -> **Physically simultaneous**
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# Sharing Resources
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Sometimes diffrent programs (or same program) may share resources.
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If data is **sequentially** accessible, programs should take turns accessing resource/data.
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Fast shared resource access can create **concurrent** access.
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