CODEWITHSUNDEEP
operating-systemcpusystems-programming
hyumilitary
hyumilitary

Reputation: 47

Why program is executed on Memory not HardDisk?

when I study in Computer Architecture and System Programming, some question rises up.

First of all, program is in SSD or Hard Disk but when it executed, this load to memory(RAM). Why program is not executed on HardDisk directly?? Why need to load on RAM?

Thanks

Upvotes: 1

Views: 1632

Answers (1)

theblackips
theblackips

Reputation: 809

This is simply done because your RAM is way faster than your Hard Disk.

When your computer executes a program, the CPU reads all the instructions from memory one after another and executes them. The CPU itself cannot store the whole program while executing it, so it has to be read from somewhere else. If the CPU had to read the instructions from a hard disk, it would be crazy slow.

Now that we have SSDs this is becoming somewhat less relevant, but in the old days the difference between RAM ("Random Access Memory") and HDD ("Hard Disk Drive") was that the RAM could access any memory address at any point in time, thus "Random Access". The HDD would have to rotate the hard disk your data is stored on to read from a certain address. Accessing random memory addresses is very hard for an HDD.

When the CPU executes a program it has to jump around all the time. It also has to store the program's memory somewhere and access that as quickly as possible whenever needed. An HDD is very bad at those two things, a RAM is very good.

So why did we use HDDs at all? Because RAM

  1. is way to expensive
  2. does not persist data when turned off

And what about SSDs? They are a lot better at random access that HDDs, but they're still considerably slower than RAM.

Also, you have to take swap files into account. The computer can use some of your HDD or SSD storage as system memory if it needs to. This can be very useful if the data that's using up your RAM does not get accessed by the CPU very often.

Upvotes: 5

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