Computer Structure – Primary Storage Devices

Computer Structure – Primary Storage Devices
Primary Storage Devices
Primary storage, also known as main memory, or internal memory, is memory that is accessed directly by the CPU.
Random Access Memory (RAM)
Computer memory is made up of silicon chips and is the computer’s working area. This is where software instructions and data are stored. This type of memory is known as volatile memory as it requires a power source to maintain the stored data. Once the power is cut, the data is lost.

For example if you are typing a document in Microsoft Word, both Microsoft Word and your document are loaded into and stored in the computer’s memory while you are working on it. This is not to be confused with the Hard Disk.
DRAM
The first type of RAM we’ll take a look at is DRAM. DRAM stands for Dynamic Random Access Memory and is the memory used to store data in

personal computers.
DRAM stores each bit of information in a cell composed of a capacitor and a transistor. The transistor admits current to the capacitor during writes, and discharges the capacitor during reads.

If there is a charge in the capacitor, it is read as a 1, if there is no charge, it is read as a 0.
The capacitor in a DRAM cell can hold a charge for only a few milliseconds, and is said to be ‘dynamic’, because the cell must be constantly recharged (or refreshed) in order to retain its data.
SDRAM
The next type of RAM is called SDRAM (or synchronous DRAM). This type of DRAM is called synchronous because its operation is synchronized with an external clock signal (such as the computer’s internal clock). SDRAM uses only the rising edge of the clock signal to transfer data

DDR SDRAM
DDR stands for Double Data Rate and is a memory technology that works by allowing operations to occur on both the rising and falling edge of the clock cycle, thereby effectively doubling the data rate without increasing the clock frequency.

The data is stored in memory cells arranged in a grid like pattern called a memory array. The rows are called wordlines and the columns are called bitlines. Each memory array can contain thousands of cells or even millions, but to simplify the design so we can understand what’s going on, we’ll use an 8x8 memory array.

During a read cycle:

1. During a read cycle:
1. The row address is placed on the address pins via the address bus.
2. The RAS pin is activated. This places the row address into the Row Address Buffer.
3. The Row Decoder selects the row which is sent to the Sense Amp.
4. The Write Enable pin is deactivated.
5. The column address is placed on the address pins via the address bus.
6. The CAS pin is activated. This places the column address into the Column
Address Buffer.
1. The Column Decoder selects the data from the selected column in the Sense Amp, and places it on the Data Out pin, which is connected to the data bus.
2. Refresh row with data from Sense Amp.
3. The RAS and CAS pins are both deactivated and the cycle begins again.
​
During a write cycle:
1. The row address is placed on the address pins via the address bus.
2. The RAS pin is activated. This places the row address into the Row Address Buffer.
3. The Row Decoder selects the row which is sent to the Sense Amp.
4. The input value placed on the Data In pin, then the Write Enable pin is activated.
5. The column address is placed on the address pins via the address bus
6. The CAS pin is activated. This places the column address into the Column Address Buffer.
7. The value from the Data In pin is loaded into the Sense Amp according to
8. Refresh row (with the modified data from Sense Amp).
9. The RAS and CAS pins are both deactivated and the cycle begins again.

​A single memory array would just store and retrieve one bit of data. To store a byte, we would need an 8 of these arrays

A set of memory arrays is called a bank.
On a stick of RAM, you’ll usually find 8 banks on a side. This is called a rank.

A stick of RAM is called a DIM (Dual Inline Memory Module). DDR SDRAM comes in different forms such as: DDR, DDR2, DDR3 and DDR4.

Laptops have their own type of memory. It’s more or less the same except for the physical size. These memory modules are called SO-DIMMs

SRAM
SRAM or static RAM is a lot faster than DRAM because it doesn’t need to be refreshed and is reserved for cache memory.

Cache Memory
The cache is usually an extremely fast memory chip that stores data so that subsequent requests for that data can be served faster.
Data read from a hard disk drive can be stored in a cache, so when a program requests that data again, it can be read from the cache instead of from the hard disk drive.

Read Only Memory (ROM)
Read Only Memory is non-volatile memory or storage containing data that cannot be changed.
Read Only Memory is useful for storing a program that very rarely change. An example is the BIOS program needed to start a PC, sometimes known as firmware.