How a Computer Works (2015)

14. Memory

Memory chips retain and store data from the CPU. Data is held in the memory while power is on. Once the computer is switched off data is lost. Some types of memory chips like the ROM BIOS retain their data with a battery.

These memory chips are called non-volatile types as they retain their data even when the computer is switched off.

DRAM, EDO RAM, SRAM

These are the commonest forms of memory chips. DRAM (Dynamic random access memory). This is most likely to be in your computer.

Dynamic means the storage capacitors in the chip have to be refreshed every thousandth of a second or data will be lost. EDO RAM (Extended Data out random access memory).

This type of memory can send data out while receiving another address so making it faster than DRAM.

SRAM (Static random access memory) SRAM is usually faster than DRAM as it doesn’t need to be refreshed, but is more expensive.

VRAM

Data being read into Memory (VRAM)

VRAM (Video random access memory) Found on the display adapter VRAM chips include two ports so data can be written into memory while the card reads the address locations to refresh the screen.

SIMM, DIMM,ECC

SIMM (single in-line memory modules) Memory chips are mounted on a small PCB with connecting pins on one edge that plug into a special socket on the motherboard.

DIMM (Dual in-line memory module) Similar to SIMMs but have memory on both sides of the circuit board. ECC (Error-correcting code) Memory that uses extra data bits to check for errors.

Connections

There are four main types of connections to memory chips. One provides power to the chip. Several connections are required for read and write operations.

Most data bit lines are associated with sending an address, which reads or writes data from a specific location in the memory chip.

Address and Data Lines

Typically a memory chip has many address and data lines. The address is made up of parallel data bits travelling to a group of memory chips. With this address “1010110010101100” each bit travels to the memory board along its own dedicated line.

When the CPU along the system bus sends the address, the CPU also sends the actual data bits it wishes to store in the memory. These data bits are stored in memory at a position pointed to by the address.

Access Time

The number of locations in memory, where data bits can be stored, is governed by the number of different address number combinations that can be made with a binary number. When reading from memory the access time of the memory chip is important.

The access time (measured in nanoseconds) is the total time the memory chip takes to internally retrieve data.

If the memory chip is slow then overall time for performing tasks with the CPU will increase. The main memory RAM is contained on the motherboard in DIMM (Dual in-line Memory Modules).

CPU

Data is transferred from the CPU to the GMCH Controller

GMCH Controller sends address to Memory

The CPU issues data and a memory address for the data to be stored at. These two are placed on the system bus that connects to the Graphics and memory controller hub chip (GMCH).

The data is placed onto the SDRAM bus and routed to the main system memory RAM. Once inside the memory chips data is stored at that specific address.

Graphics and Memory Controller hub

Address is transferred from the CPU to the GMCH Controller

GMCH Controller sends address to Memory

When the CPU wishes to retrieve data from memory it issues an address that is placed on the system bus that connects to the Graphics and memory controller hub chip.

The address is routed to the main system memory RAM. Depending on the access time of the chip (measured in nanoseconds) the data is retrieved and placed on the SDRAM bus which connects the memory to the Graphics and memory controller hub chip.

The data is then placed on the system bus ready to be read in by the CPU.

Silicon Circuit

Inside the memory chip are thousands of transistors and capacitors etched onto a silicon circuit.