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Hardware and software - memory (CPU memory (RAM -Random Access Memory: a…
Hardware and software - memory
Categories of memory
Reading from memory
Memory Unit
the computer memory unit is made up of a number of partitions.
each partition consists of an
Address
and its
Contents
.
The address will uniquely identify every
Location
in the memory and the contents will be the binary value of every location
The example shown here uses 8 bits for each address and 8 bits for content however in a real computer the address and its contents are actually much larger than this.
writing to memory
CPU memory
RAM
-Random Access Memory: a type of memory that is read-write and volatile. Used for Main Memory.
ROM
-Read Only Memory: memory that is hard-coded at the time of manufacture. Stores the start up programs, the boot-strap loader.
Volatile
- Describes memory that loses its contents when the power is turned off. E.g. main memory
Non-Volatile
- Describes memory that does not lose its contents when the power is turned off. E.g. hard disk.
Secondary Memory
- Long term, non-volatile storage media such as hard disks, memory sticks, magnetic tapes & CDs.
Virtual Memory
- Part of the hard disk that is configured to behave as an extension to main memory.
RAM
Volatile/temporary memory
Used to store
Data
Programs
Files
parts of the operating system that are currently in use
It can be written to or read from and the contents of the memory can be changed.
the larger the size of the RAM the faster the computer will operate, never runs out of memory
As the RAM becomes full, the processor has to continually access the hard disk drive to overwrite old data on RAM with new data. By increasing the RAM size, the number of times this access operation is carried out is reduced, making the computer run faster.
RAM is much faster to write to or read from than other types of memory; but its main drawback is its volatility.
DYNAMIC RAM (DRAM)
Each chip consists of a number of transistors and capacitors. Each of these parts is tiny since a single RAM chip will contain millions of transistors and capacitors. The function of each part is:
capacitor
– this holds the bits of information (0 or 1)
transistor
– this acts like a switch; it allows the chip control circuitry to read the capacitor or change the capacitor’s value.
This type of RAM needs to be constantly REFRESHED (that is, the capacitor needs to be recharged every 15 microseconds otherwise it would lose its value). If it wasn’t refreshed, the capacitor’s charge would leak away very quickly, leaving every capacitor with the value 0.
Advantages over SRAM
less expensive than SRAM
consume less power
higher storage capacity
Static RAM (SRAM)
doesn’t need to be constantly refreshed.
SRAM is much faster than DRAM when it comes to data access
the preferred technology
the processor’s MEMORY CACHE is the high-speed portion of the memory; it is effective because most programs access the same data or instructions many times
ROM - Read Only Memory
Non-Volatile/ permanent memory
used to store the start-up instructions when the computer is first switched on
The data or contents of a ROM chip can only be read; they cannot be changed.
Storage Devices
size of a text file
Suppose we typed in the following message:
“This is text from the computer science text book”
and then saved it as a text file, we would expect it to be 48 bytes as each ASCII character is a byte.
size of an image file
• This is slightly more complicated as there is ‘header’ information stored but we can work out roughly what it should be if we know the dimensions and colour depth.
If we have a 24 bit colour depth each pixel will need 3 bytes (24/8).
If we have an image which is 2000x1000 pixels it will roughly require:
2000x1000x3 bytes = 6000000 bytes = 6MB
The bootstrap loader
• It is the first program that is loaded into main memory from ROM by the BIOS when a computer is switched on.
The Bootstrap Loader loads the operating system from secondary storage. Once the operating system is loaded it takes care of the rest. The term comes from the idea that when you’re not doing very well in life you can “pull yourself up by your bootstraps”! In this day and age you’re more likely to hear “sort yourself out” but the meaning is the same - you are in effect getting yourself restarted.
