Separation of Memory
In modern computers there are several memory systems and hardware components that are used for the internal use and storage of data. These memory components are separated by different types of physical and programming partitions.
The principle types of system memory include, but are not limited to, random-access memory (RAM memory), read-only memory (ROM memory), programmable read-only memory (PROM), specialized types of PROM memory, and virtual memory. System memory is further separated into several component areas, each with their own unique block of addresses. The types of separation are, in many cases, remnants of earlier computer hardware and software designs (i.e., they are traditional designations) made necessary by the small memory capacity of early computer systems.
Conventional memory designates the first 640 KB of system memory, and it is this memory that is utilized by DOS-based programs. Upper memory is the next 384 KB located next to, but above, conventional memory addresses. In modern systems, upper memory is utilized by system drivers and devices. A High-memory area designates approximately the first 64 KB of the second megabyte of system memory. Extended memory (in some operational schemes, also including the High-memory area) consists of the remainder of system memory.
Although the traditional functional distinctions between these separations of memory are often ignored by modern programs (e.g., modern systems can use extended memory areas for running drivers and programs), almost all programs require that some part of the code reside in conventional memory while the code is running.
Memory located on chips (e.g. RAM memory) is linked with memory on disk storage mechanisms (e.g., hard disk drives used for ROM memory) via virtual memory. This separation of memory allows an overall greater storage capacity for computers. A virtual memory bridge between RAM and disk systems allows a rapid interchange of data. Virtual memory is--in accord with its name--not a hardware component or physical form of memory (e.g., a chip, disk, etc.), but rather, a system process. As a process, virtual memory links the separated forms of physical memory contained in RAM and ROM memory systems. The smooth and rapid exchange of data across the virtual memory bridge is essential because, before operations or programming instructions can be execute by a system, data blocks carrying those instructions must be loaded into the operational RAM memory. The exchange of program code and data between the RAM and ROM systems is termed swapping.
The separation of memory allows for the cost efficient storage of data. The separation allows for vastly differing sizes of memory. Large amounts of data may be maintained in larger, less expensive ROM variant systems because virtual memory systems ensure that needed blocks of programming code or data can be constantly called and put into the smaller and more expensive RAM memory as needed. Accordingly, the separation of memory allows efficient and high speed computers to be designed with much smaller allocations of relatively expensive RAM memory.
Prior to the development of modern virtual memory mediated exchanges between separated memory systems, program execution speeds and reliability were often hampered by the need for physical file swapping and the extensive fragmentation of programs.
In addition to the functional differences between the physical separation of RAM and ROM systems (RAM memory systems differs in that it is a readable and writeable form of memory), the separated forms of memory require specialized forms of maintenance, especially with regard to power supply. Ram memory chips provide a volatile memory component that requires a constant supply of electrical current to maintain the integrity of the system. RAM systems experience a steady decay in charge state and, without a steady current input, readily degrade, or corrupt. Accordingly, an interruption in current or high variation in the power supply will result in a loss of the data or programming code maintained in RAM memory.
Apart from the functional differences in ROM memory, the hard disks housing ROM provide a long term stable (non-volatile) form of memory able to maintain it's integrity even after an interruption or loss of power. Although by strict definition, ROM systems are "read-only" forms of memory, there are variations of ROM that reside on programmable read-only memory (PROM) chips that may be erased and reused.
The separation of memory has been an important feature of computing systems since first utilized in the Atanasoff-Berry Computer, constructed by John V. Atanasoff and graduate student Clifford Berry in the late 1930s and early 1940s.
Some modern computer security systems rely on the separation of memory to fragment code. In such systems, the algorithms themselves never exist as a single file, but rather exist in fragmented states that reside in separate memory components.
This is the complete article, containing 766 words
(approx. 3 pages at 300 words per page).
