2.3 KiB
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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Were waiting a lot on IO time to finish, very inefficient. We can solve this using multitasking!
Multitasking
To speed up computers we overlap CPU time and IO Time.
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.
Multitasking overlaps IO time of a program with CPU time of other program.
Multitasking with multiple processors
If computer has a single CPU, programs should take turns in using it.
If computer has multiple CPUS, each task can be given a different CPU. - If the number of tasks is more than available cpu's, we still take turns.
when program issues IO command, cpu is granted to the next program.
Concurrent vs Parallel
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
Parallel: If the computer has multiple CPU's or Cores, the programs run in parallel. -> Physically simultaneous
Sharing Resources
Sometimes diffrent programs (or same program) may share resources.
If data is sequentially accessible, programs should take turns accessing resource/data.
Fast shared resource access can create concurrent access.