1012 B
1012 B
Input / Output Subsystem
Handles devices that allow the computer system to:
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Communicate and interact with the outside world
- Screen, keyboard, printer
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Store information
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The Control unit
Program stored in memory. (machine language in binary)
Task of control unit is to execute programs by:
- Fetch: from memory the next instruction
- Decode: instruction to determine what to do
- Execute: issuing signals to ALU, memory and IO systems.
Repeats until HALT instruction.
Typical Machine Instructions
MOV: move data from a to b ADD: add numbers SUB: subtract numbers JMP: transfers program control flow to the indicated instruction
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CPU Time vs IO Time
Total time required to run program is: CPU Time + IO Time
Typical time to read from IO: 20ms Typical time to run instruction: 20ns CPU can execute up to 200 million instructions while a block is read from a hard disk.
Conclusion: While I/O is executed, CPU is idling.
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