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The role of memory in computation
The following steps outline the way in which memory interacts with the processor during computational cycles, once the bootstrapping process has completed and the OS kernel is itself loaded into memory.
- A file is loaded from the harddisk into memory.
- The instruction at the first address is sent to the CPU, travelling accross the data bus part of the system bus.
- The CPU processes this instruction and then sends a request accross the address bus part of the system bus for the next instruction to the memory controller within the chipset.
- The chipset finds where this instruction is stored within the DRAM and issues a request to have it read out and send to the CPU over the data bus.
This is a simplified account; it is not the case that only single requests are passed back and forth. This would be inefficient and time-wasting. The kernel sends to the CPU not just the first instruction in the requested file but also a number of instructions that immediately follow it.
Every part of the above process - the journey accross the bus, the lookup in the controller, the operations on the DRAM, the journey back accross the bus - takes muliple CPU clock cycles.
The role of the cache
The cache is SRAM memory that is separate from the DRAM memory which comprises the main memory. It exists in order to boost perfomance when executing the read/request cycles of the steps detailed above.
There are two types of cache memory:
- L1 cache
- Situated on the CPU chip itself
- L2 cache
- Situated outside of the CPU on its own chip
The L1 cache is the fastest since the data has less distance to travel when moving to and from the CPU. This said, the L2 cache is still very fast when compared to the main memory, both because it is SRAM rather than DRAM and because it is closer to the processor than the main memory.
Cache controllers use complex algorithms to determine what should go into the cache to facilitate the best performance, but generally they work on the principle that what has been previously used by the CPU will be requested again soon. If the CPU has just asked for an instruction at memory location 555 it's very likely that it will next ask for the one at 556, and after that the one at 557 and so on. The cache's controller circuits therefore go ahead and fetch these from slow DRAM to fast SRAM.
Relation between cache and buffers
The terms cache and buffer are often used interchangeably but there is a significant difference. Buffer 'size' refers to the amount of physical memory in RAM that is dedicated to a process where this size is measured in terms of the amount of disk blocks it would take up in the harddrive. Cache is also a measure of memory but it is expressed in terms of virtual memory: the page size of the memory.
Buffers and caches are both mechanisms for avoiding writing data to a storage device in the midst of active computation. Every time data is saved, the OS doesn't immediately write it to disk. It keeps it in the most short term memory in case it is edited again or until the processor is free. In this state it is kept in buffers. When written to disk it becomes blocks. This is why you shouldn't immediately remove a disk device without first saving / syncing int.
Similarly with transferring memory to and from the RAM. The OS keeps occurent memory in the processor caches to enable reuse and not to overburden the CPU, this page data is then moved to RAM when convenient.