System Memory

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Contents 1 System memory 2 Desktop RAM table 3 Discuss This

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[edit section] System memory

• RAM (Random Access Memory) – Stores all running programs on a computer, which are copied to ram once loaded from mass storage. Any one byte is accessed as easily as another
• DRAM (Dynamic RAM) – Volatile & needs periodic refreshing so as not to loose its contents. Delivered data to the CPU in 8 bit chunks.
• Access Speeds – The amount of time it takes RAM to deliver data to the Northbridge once it has been requested. Measured in ns
• FPM (Fast Page Mode) – Northbridge requests data from DRAM and waits for a response
• EDO (Extended data out) – Like FPM but RAM needed to be refreshed less often, thus reducing wait states and allowing for faster data access: 60-80ns
• Banking – One bank of RAM needs to provide information the full width of the data bus. Example – on a Pentium system with a 64-bit FSB 2 72-pin SIMMS are used to fill one bank as they are 32-bits wide each & 32x2=64

One bank = Width (# bits) CPU FSB

    Width (#bits) of RAM stick

• Parity – A form of error correcting that uses 1 extra bit in each 8-bit row of RAM width, turning each row into 9-bits. The parity bit is set to 1 when the Northbridge places data on RAM. If the parity bit changed fro a 1 to 0 between data access and storage then and error was displayed.
• ECC (Error Correcting Code) – A major advance in DRAM error checking. It was able to detect problems in RAM and fix most of them on the fly.
• SDRAM (Synchronous DRAM) – Has a clock speed that is tied to the system clock (synchronised) therefore enabling the Northbridge to know when data is ready. SDRAM pipeline instructions from the Northbridge enabling commands to be ready as soon as the Northbridge takes the previous command. The SDRAM used must be of the same speed, or higher, of that of the FSB
• PC100/133 – Defines the construction standard of SDRAM. Each RAM stick contains a SPD (Serial Presence Detect) chip that supplies the system with information about itself (size, speed etc)
• Double pumping – An increased speed between the CPU and Northbridge that was provided by the Athlon and Pentium 4 CPUs. The Athlon was capable of sending two bits of data for every clock cycle, effectively doubling the speed of the FSB, from say 133MHz to 266MHz. The Pentium 4 worked on the same principle but quadrupled the FSB

• SIMM (Single Inline Memory Modules) – Came in two different types: 30-pin and 72-pin. 30-pin SIMMs use FPM technology, are 8-bits wide and had capacities of 1MB-16MB. 72-pin SIMMs used FPM, and then EDO, technology, where 32 bits wide and came in capacities of up to 64MB. Access speeds ranged from 120-50 ns

• DIMM (Dual Inline Memory Module) – are used in many technologies and come in all shapes and sizes. Desktops commonly use: 168 & 184-pin sticks, & laptops use: 68, 144 or 172-pin micro-DIMMS. All have a 64-bit width apart from 72-pin Small Outline DIMM.

• DDR (Double Data Rate) SDRAM – Doubles the throughput of SDRAM, making two processes for every clock cycle, synchronising well with the Athlon's double pumped FSB. Desktops use 184-pin DIMM while laptops use either 200MHz SODIMM or 172-pin micro-DIMM sticks. Speeds come in 200, 266, 333 & 400MHz.The naming convention used is based on the number of bytes per second.

PC DDR name = #MHz x 8

200MHz x 8 = PC1600

• R (Rambus) DRAM – Uses sticks called RIMMS & come in two types: 184 – pins (Desktops) & 160-pin SORIMM (Laptops). Can handle speeds of 600, 700, 800 & 1066MHz, & also allows to double the channel width for speeds up to 1.6GHz/ Capacities range from 64-512MB. RDRAM runs very hot and needs a heat spreader attached to it to disperse the heat. All RIMM slots must be populated by a RIMM or terminated by a CRIMM (Continuity RIMM).

[edit section] Desktop RAM table

Technology	EDO	SDRAM	DDR SDRAM	RAMBUS
Package	SIMM, DIMM	DIMM	DIMM	RIMM
Pins	72-SIMM, 168-DIMM	168, 184	184	184
Bit Width	32-SIMM, 64-DIMM	64	64	64
Speed	50-60ns	66, 100, 133MHz	200, 266, 333 400MHz	600, 700, 800 1066MHz
Capacity	Up to 64MB, 64-512MB	64-512MB	64MB-1GB	64-512MB

[edit section] Discuss This

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