Write a note on memory management unit

History[ edit ] Ferranti introduced paging on the Atlasbut the first mass market memory pages were concepts in computer architecture, regardless of whether a page moved between RAM and disk. This zone of memory was called a page. This use of the term is now rare.

Write a note on memory management unit

Every instruction has to be fetched from memory before it can be executed, and most instructions involve retrieving data from memory or storing data in memory or both.

The advent of multi-tasking OSes compounds the complexity of memory management, because because as processes are swapped in and out of the CPU, so must their code and data be swapped in and out of memory, all at high speeds and without interfering with any other processes.

Shared memory, virtual memory, the classification of memory as read-only versus read-write, and concepts like copy-on-write forking all further complicate the issue. The memory hardware doesn't know what a particular part of memory is being used for, nor does it care.

This is almost true of the OS as well, although not entirely. The CPU can only access its registers and main memory. It cannot, for example, make direct access to the hard drive, so any data stored there must first be transferred into the main memory chips before the CPU can work with it.

Device drivers communicate with their hardware via interrupts and "memory" accesses, sending short instructions for example to transfer data from the hard drive to a specified location in main memory. The disk controller monitors the bus for such instructions, transfers the data, and then notifies the CPU that the data is there with another interrupt, but the CPU never gets direct access to the disk.

Memory accesses to registers are very fast, generally one clock tick, and a CPU may be able to execute more than one machine instruction per clock tick. Memory accesses to main memory are comparatively slow, and may take a number of clock ticks to complete.

This would require intolerable waiting by the CPU if it were not for an intermediary fast memory cache built into most modern CPUs. The basic idea of the cache is to transfer chunks of memory at a time from the main memory to the cache, and then to access individual memory locations one at a time from the cache.

User processes must be restricted so that they only access memory locations that "belong" to that particular process. This is usually implemented using a base register and a limit register for each process, as shown in Figures 8. Every memory access made by a user process is checked against these two registers, and if a memory access is attempted outside the valid range, then a fatal error is generated.

The OS obviously has access to all existing memory locations, as this is necessary to swap users' code and data in and out of memory. It should also be obvious that changing the contents of the base and limit registers is a privileged activity, allowed only to the OS kernel.

These symbolic names must be mapped or bound to physical memory addresses, which typically occurs in several stages: Compile Time - If it is known at compile time where a program will reside in physical memory, then absolute code can be generated by the compiler, containing actual physical addresses.

However if the load address changes at some later time, then the program will have to be recompiled. COM programs use compile time binding.

Load Time - If the location at which a program will be loaded is not known at compile time, then the compiler must generate relocatable code, which references addresses relative to the start of the program.

If that starting address changes, then the program must be reloaded but not recompiled. Execution Time - If a program can be moved around in memory during the course of its execution, then binding must be delayed until execution time.

This requires special hardware, and is the method implemented by most modern OSes. Addresses bound at compile time or load time have identical logical and physical addresses.

Addresses created at execution time, however, have different logical and physical addresses. In this case the logical address is also known as a virtual address, and the two terms are used interchangeably by our text. The set of all logical addresses used by a program composes the logical address space, and the set of all corresponding physical addresses composes the physical address space.

The run time mapping of logical to physical addresses is handled by the memory-management unit, MMU.

write a note on memory management unit

The MMU can take on many forms. One of the simplest is a modification of the base-register scheme described earlier. The base register is now termed a relocation register, whose value is added to every memory request at the hardware level. Note that user programs never see physical addresses.

User programs work entirely in logical address space, and any memory references or manipulations are done using purely logical addresses.

Only when the address gets sent to the physical memory chips is the physical memory address generated. The advantage is that unused routines need never be loaded, reducing total memory usage and generating faster program startup times.

The downside is the added complexity and overhead of checking to see if a routine is loaded every time it is called and then then loading it up if it is not already loaded.

With dynamic linking, however, only a stub is linked into the executable module, containing references to the actual library module linked in at run time. This method saves disk space, because the library routines do not need to be fully included in the executable modules, only the stubs.

Each process would have their own copy of the data section of the routines, but that may be small relative to the code segments.Instruction pipelining is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous "pipeline") performed by different processor units with different parts of instructions processed in parallel.

Oracle Enterprise Manager (Enterprise Manager) is a web-based system management tool that provides management of Oracle databases, Exadata database machine, Fusion Middleware, Oracle applications, servers, storage, and non-Oracle .

GDDR5, an abbreviation for graphics double data rate type five synchronous random-access memory, is a modern type of synchronous graphics random-access memory (SGRAM) with a high bandwidth ("double data rate") interface designed for use in graphics cards, game consoles, and high-performance computation.

6 Managing Memory.

write a note on memory management unit

Memory management involves maintaining optimal sizes for the Oracle Database instance memory structures as demands on the database change.

Definition: Virtual memory is the feature of an operating system (OS). It is responsible for memory leslutinsduphoenix.com the Virtual Memory the Physical Memory (Hard Disk) will be treated as the Logical Memory (random access memory (RAM)).

barrier (1). A barrier is a block on reading from or writing to certain memory (2) locations by certain threads or processes. Barriers can be implemented in either software or hardware. Software barriers involve additional instructions around load or store (1) operations, which would typically be added by a cooperative compiler.

Hardware .

Memory Management Glossary: B — Memory Management Reference documentation