System Memory

Introduction to system memory

There exist two types of memory : RAM and ROM , which perform different tasks on the computer.
RAM is the workspace within the computer and comes in two types, DRAM and SRAM .
SRAM is fast and expansive and is used in small quantities or a computer where as DRAM is slow and cheap and used in large quantities on a computer.
In order to create the optimal system both in terms of response time and price the RAM used within the computer is dispersed throughout our system from the motherboard to the CPU in different quantities.

Key Topics

What is memory

SRAM or Cache

SIMMs and DIMMs

SIMM and DIMM Slots

FPM DRAM

VRAM &WRAM SGRAM

EDO & BEDO DRAM

SDRAM

DDR and RAMBUS

RAM bandwidth Comparison

Memory parity

ECC error checking

CMOS

What is Memory

Memory is solid-state chips with no moving parts, which stores data as 1’s and 0’s.
Memory is distributed throughout the computer system and cannot be compartmentalized .

Volatile Memory

Memory is either sold as chips, or modules, which contain many chips.
Memory can be volatile or non-volatile.
Volatile memory loses its data when power is lost

What is Memory

Memory comes in all sizes speeds and prices reflecting its utilization within our system.
Most devices have both ROM and RAM on board to improve performance e.g. SCSI , VGA , and HD .
Two memory types, RAM and ROM

RAM Details

RAM = Random Access Memory
RAM used as workspace by PC and is volatile (losses data when power is switched off)
RAM hardware Can be upgraded
PC require large quantities of RAM

RAM Speed

Memory is one of the faster components of your system which has speeds measured in nanoseconds (ns).
Memory access time is how fast the data within the memory can be made available.
Typical access times for RAM are between 50 to 70 ns that are marked on the RAM chips.

ROM Details

ROM = Read Only Memory
ROM chips are non volatile memory
ROM contains information on hardware configuration limitations e.g. BIOS
BIOS = Basic Input Output System
ROM chips are soldered on system board
PC require small quantities of ROM
ROM chips are used in older PC (up to 486 systems) and cannot be overwritten.
The ROM chips contain programs , which are specific for testing that particular system board design called Power On Self Test (POST ).
These programs are referred to as the BIOS (Basic Input Output System), which are the startup instructions , or Firmware .

Flash RAM

Flash RAM is used in more modern system (Pentiums and above) to store the BIOS settings.
The advantage of Flash RAM over ROM chips is that the BIOS can be upgraded from the system board manufacturers via the internet.
This upgrade feature enables us to update our hardware support on the motherboard to the latest hard ware in the market, e.g. greater hard drive capacity, greater RAM capacity.
This feature of flash RAM chips makes them vulnerable to viruses from infected disks.

ROM BIOS

The BIOS will remain unchanged on older system ROM chips.
ROM Chips are not susceptible to viruses.
The BIOS can be updated on more modern system by overwriting the programs in the Flash RAM .

Power On Self Test

The BIOS instructions include POST (Power On Self Test), which allows it to connect to various hardware devices after checking with the CMOS , and making sure they all operate.

Boot Instruction

The BIOS also contains Boot instructions , which call the operating system of choice (DOS , Windows, etc)

SRAM or Cache Memory

Cache is very fast static RAM memory that interacts with the CPU when performing instruction to enhance the systems response time.
Typical RAM has access times of 50-70ns but cache access times are 7-15ns
Obviously the more cache we have within our system, the faster the performance of the system.

L1 and L2 Cache Memory

Cache is very expensive and a balance has to be reached on a system between system performance and cost, There 2 categories of cache , L1 (built on the CPU chip) L2 (external to CPU).
Typical quantities of L1 cache range from 8KB to 32KB and typical quantities of L2 cache range from 128KB to 4MB.

Dynamic RAM

DRAM = (Dynamic Random Access Memory)
DRAM is comparatively slow and cheap with a variety of technologies.
DRAM requires the data to be refreshed frequently
DRAM is used in large quantities as main system memory

Memory Modules

Parity format can detect errors and are expensive mainly used on older systems
Non parity format cannot detect errors and are cheap
Parity format has an extra bit added to the byte of data for parity information , odd or even parity
ECC (Error Checking and Correction) enhanced checking and correction technique for Pentium generation systems.

DIMM and SIMM

DRAM are placed on boards called SIMM (Single Inline Memory Module) and DIMM (Dual Inline Memory Module)
Memory modules are miniature circuit boards with the memory chips soldered on them

SIMMs

SIMM = Single Inline Memory Module
SIMMs are sold with capacities ranging from 512KB to 64MB
SIMMs are designed in 30 Pin and 72 Pin formats
SIMMs usually need to be installed in pairs
Could mix 60ns and 70ns modules without any problems up to 486 system
Could not mix speeds on Pentium systems
Ideal for systems up to the 486 computer.

DIMMs

DIMM = Dual Inline Memory Module
DIMM modules are 64bit wide running at a speeds of 133Mhz and above
DIMMs are designed in 168 pin format
• Each module can hold from 8MB to 512 MB of data
• DIMMs can be installed in single units
• Modules are synchronized with system bus for increased performance
• DIMMs must match with the system Board requirements.
• SODIMM (Small Outline DIMMs) are small DIMMs used on Notebook systems.

Serial Presence Detect

Modern DIMM modules also carry a ROM chip on them called the SPD (Serial Presence Detect) holding information about the module , This will inform system BIOS of itself and the system will tune itself to memory bus speeds optimal RAM