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 CPU performance

Buffered Memory

DIMMs modules are designed to consume 3.3V or 5V that can be buffered or unbuffered.
It is important for installation purposes to know the type of DIMM module required by the system board which can be checked on the product manufacturers manual

(FPM ) DRAM

One of the slowest memory designs in SIMM Module.
Compatible with most PC’s designed to use the standard asynchronous RAM .
Fast Page Ram (FPM ) is faster then standard DRAM
Generally very low performance
Compatible with most old systems
Improves access time by sending row addresses to locations near each other once.

VRAM

It has separate data ports for read and write operations.
Simultaneously reading and writing data from both ports improve the performance
VRAM gives increased performance to video cards but should not be mistaken with video RAM

WRAM

Improved design over VRAM to increase performance by 25%.
Also have 2 separate data ports for reading and writing data.
Predominantly used on graphics cards .

SGRAM Synchronous Graphics RAM

Improves performance by increasing data transfer rates.
Designed to handle the new video card acceleration features.
Single data port design that is faster then VRAM and WRAM

EDO DRAM

It has common Asynchronous DRAM
It is a SIMM Module.
Faster then FPM DRAM
Improved performance by 30% due to modifications in chip design.
Simultaneously set access requests for next address while it outputs data from one address improving performance
Now cheaper then FPM
Good for Pentium class PC’s, not 486
Requires support for PCI chipset
Not good for systems with bus speeds of 75Mhz or higher e.g. Pentium II or III

(BEDO ) DRAM

Burst EDO DRAM allows for higher bus speeds then the ordinary EDO DRAM
Much better performance then EDO with little cost increase in production
Main rival is SDRAM in SIMM Module

Synchronous DRAM

Designed to run at the same speed as the system bus (no system wait state)
Data delivered in high speed bursts
SDRAM does not function on incompatible bus speeds.
Designed for Pentium systems chipset
It is a DIMM module

Double Date Rate (DDR ) SDRAM

The modules look like and operate quite similar to existing SDRAMs but are much faster.
Can be used in 64bit Itanium system servers
Double Date Rate (DDR ) SDRAM technology is supported by VIA chipset , and not Intel chipset.
Doubles bandwidth by transferring data twice in each cycle, once on the rise and once on the fall
DDR technology is also used in AGP (advanced graphics adapter ) boards.
The 266 MHz modules reaches a 2.1 GB /sec bandwidth . Hence they are to be sold as PC2100 RAM .
backed Rambus technology but has same performance

RAMBUS Modules or RIMM

Intel has become committed to the Rambus RAM , which also referred to as RDRAM (Rambus Direct RAM), nDRAM, or RIMM (Rambus Inline Memory Modules) despite serious problems with the technology.
The RAMBUS technology allows the system to prepare the data used in small packets, which can be sent at very high speeds that can free the CPU from extra work.
Narrow 16 bit data path competent of running at speeds of 400 to 800MHz
The 184-pin module works at low voltages (2.5V-0.5V) to reduce power consumption and heat production.
All RAM slots on the system board must be filled in RAMBUS design; Slots not in use must be filled with blank modules called CRIMMs (with a 'C' for continuity).
Dual Rambus channels must be used to benefit from the high-speed possible 1.6GB/s or higher with quad channels.
RIMM modules placed close to CPU to reduce radio noise.
Best memory to use with Pentium 4 system for optimal speeds.

RAM Bandwidth Comparison

Memory Parity

Parity is a method of error checking for RAM memory on older systems.
It can be either ODD parity or EVEN parity.
There is a ninth bit added to the byte of data, which forces the number of ones in the data to a predefined parity.
If the parity check for an even parity does not match an even number, the system halts and an error message is produced.
The same is applied to an odd parity producing an error message.
This technique can only error checks a single bit error.
The more modern systems use ECC error checking.
Parity chips are slightly more expensive then non-parity chips.

Even Parity And Odd Parity

If the parity is EVEN then the number of one’s in the data including the parity bit should be an even number.
should be an odd number.

ECC Error Checking and Correction

ECC stands for Error checking and correction.
Enhanced parity checking which detects single bit error and corrects them without halting the system.
ECC will also detect up to 4 bit errors but cannot correct them and halts the system while producing an error message.
Often used in important systems such as servers.
ECC cannot be mixed with other parity systems.

CMOS

CMOS stands for Complimentary Metal Oxide Semiconductor.
CMOS is soldered on the system board
CMOS is typically 64 bytes in size.
CMOS stores all the current settings of our system including HD , FD , time and date setup.
Requires a battery located on all system boards to hold settings data.

Question

1) What sort of RAM is Cache?

a) DRAM

b) SRAM

c) SDRAM

d) VRAM

e) WRAM

2) Where is L1 cache located?

a) Inside the system

b) Inside the VGA card

c) Inside the RAM

d) Inside the processor

e) Outside the system

3) Where is L2 cache located?

a) Inside the system

b) Inside the VGA card

c) Inside the RAM

d) Inside the processor

e) Outside the system

4) How many pins is a SIMM module ? (choose all that apply)

a) 26

b) 30

c) 168

d) 72

5) How many pins is a DIMM module ? (choose all that apply)

a) 26

b) 30

c) 168

d) 72

6) What modules contain SPD (Serial Presence Detect)?

a) 30 pin SIMM

b) 72 pin SIMM

c) DIMM

d) RIMM

7) Which RAM provides the best performance ?

a) FPM RAM

b) VRAM

c) WRAM

d) EDO RAM

8) Which RAM provides the best performance ?

a) BEDO RAM

b) VRAM

c) SDRAM

d) EDO RAM

9) Which RAM provides the best performance ?

a) SDRAM

b) DDR RAM

c) WRAM

d) EDO RAM

10) Which chip manufacturer supports DDR RAM ?

a) IBM

b) Apple

c) Motorola

d) Intel

e) VIA

11) Which chip manufacturer supports RAMBUS?

a) IBM

b) Apple

c) Motorola

d) Intel

e) VIA

12) Which RAM technology provides full performance when in pairs?

a) DRAM

b) SDRAM

c) DDR RAM

d) RAMBUS

13) What are the different forms of error detection? (choose all that apply)

a) Even parity

b) Odd parity

c) ECC

d) Dual parity

14) Which error detection method also corrects errors detected?

a) Even parity

b) Odd parity

c) ECC

d) Dual parity

15) Which memory within the computer requires the system battery ?

a) RAM

b) CMOS

c) DDR

d) L1 Cache

e) L2 cache

Answer

B
D
A
B,D
C
C
D
C
B
E
D
D
A,B,C
C
B