Chapter 6: Managing Memory

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RAM on the Motherboard. Memory is divided into two categories:ROM Read-Only Memory (last chapter)-- Retains data when PC is turned offRAM Random Access Memory-- Temporary storage loses data on shutdown. RAM on the Motherboard. Two Subcategories of RAM:DRAM (dynamic RAM) needs to be refreshed (computer must rewrite the data to the chip) by the memory controller (part of the chip set on the motherboard). Most DRAM is stored on DIMMs instead of RIMMs (will get to this later in the chapter)Dynamic = constantly changing!!.

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Chapter 6: Managing Memory

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1. Chapter 6: Managing Memory A+ Guide to Managing and Maintaining your PC

2. RAM on the Motherboard Memory is divided into two categories: ROM – Read-Only Memory (last chapter) -- Retains data when PC is turned off RAM – Random Access Memory -- Temporary storage – loses data on shutdown

3. RAM on the Motherboard Two Subcategories of RAM: DRAM (dynamic RAM) – needs to be refreshed (computer must rewrite the data to the chip) by the memory controller (part of the chip set on the motherboard). Most DRAM is stored on DIMMs instead of RIMMs (will get to this later in the chapter) Dynamic = constantly changing!!

4. RAM on the Motherboard Two Subcategories of RAM: SRAM (static RAM) – acts as a memory cache (a temporary location for data and instructions) – helps speed up access time to main memory – contained on motherboard or inside CPU housing Static = only temporary!!

5. RAM on the Motherboard Types of DRAM: DIMM – dual inline memory module -- up to 2 GB or RAM, newest of the RAM SIMM – single inline memory module -- 8, 16, 32, or 64 MB, much older type of RAM RIMM – not an acronym, made by Rambus, Inc. -- seen mostly on Intel Pentium 4 motherboards

6. RAM on the Motherboard Types of SRAM: Cache on a stick (COAST) – Memory module with multiple chips *NOTE: Most SRAM is now located inside the processor housing (i.e.: you don’t see SRAM sticks in newer computers)

7. RAM on the Motherboard Most common DRAM technologies: Double Data Rate SDRAM – also known as “DDR, DDR SDRAM, DIMM” Direct Rambus DRAM – this is what we refer to as a “RIMM” in a P4 processor *Natural Transition* - SRAM is now hidden, DRAM is generally referred to as “Memory” or “Memory Sticks.” This creates a more “user-friendly” and less confusing environment for less-experienced users

8. RAM on the Motherboard Static RAM Technologies: Most computers have little SRAM and a lot of DRAM – this is typically due to cost, but one most also consider heating and processor size issues Memory Comparison: See Table 6-1 on p. 242

9. RAM on the Motherboard Memory Cache Locations: L1 cache – On the CPU die* L2 cache – Inside CPU housing L2 cache – Motherboard (older systems) L3 cache – Inside CPU housing** L3 cache – Motherboard (L2 in housing) *Circuits of the CPU **L3 caches are farther away than L2’s

10. RAM on the Motherboard Memory Caching (aka: “Guesswork”) – Method to store in SRAM for quick retrieval Cache controller guesses which data or code the CPU will request next and copies that data or code to SRAM If cache guessed correctly, it can satisfy the CPU request without accessing the slower DRAM This results in an average 90% accuracy rate!

11. RAM on the Motherboard Dynamic RAM Technologies: Main differences include the width of the data path and the way the data moves from the system bus to the module DIMM’s, SIMM’s, and RIMM’s See Table 6-3, page 245 Need to know all technologies, even older ones!

12. RAM on the Motherboard SIMM Technologies: Rated by speed in nanoseconds (ns) Common speeds are 60, 70, 80 ns Speed is a measure of access time The smaller the number, the better! EDO (extended data out) – first seen on 72 pin SIMMs on motherboards rated at 33 to 75 mhz – now used as RAM in memory expansion boards (mostly in Compaqs and HPs)

13. RAM on the Motherboard DIMM Technologies: Rated usually by amount of memory 144-pin (older), 168 pin, 184 pin Hold from 8 MB to 2 GB of RAM Older models used EDO and BEDO (burst EDO), newer models use SDRAM (synchronous DRAM) technology Position of notches identifies type of DIMM and Voltage requirement (i.e.: “if it doesn’t fit, don’t force it!”) – see Figure 6-5, p. 247

14. RAM on the Motherboard SDRAM – most popular memory type Rated by system bus speed Operates in sync AND at the same speed as system clock Data path is 64 bits wide 3 flavors: SDRAM, DDR, SLDRAM

15. RAM on the Motherboard DDR SDRAM, SDRAM II, or DDR (Double Data Rate – named for “up/down” beat) Not that game where you dance around Most popular, most widely used Twice as fast as regular SDRAM Can hold up to 2 GB or RAM Processes twice per beat of system clock Regular SDRAM became SDR SDRAM Ex: if motherboard runs at 100mhz, then DDR runs at 200mhz with 64 bit data path

16. RAM on the Motherboard SyncLink DRAM (SDRAM) Developed by consortium of 12 DRAM manufacturers Increased number of memory banks (locations on the motherboard that contain slots for memory modules) that can be accessed simultaneously from 4 to 16 Now considered obselete

17. RAM on the Motherboard RIMM Technologies – Direct Rambus DRAM (also known as RDRAM, Direct RDRAM) Named after Rambus, Inc. (developer) Can travel on 16 or 32 bit data path Plan on releasing 64 bit soon?? Runs at internal speeds of 800–1200mhz Uses 400-600mhz system bus Rambus does not manufacture RIMMs

18. RAM on the Motherboard **IMPORTANT INTERJECTION** With RIMM, every socket must be filled to maintain continuity – if you do not have enough RIMMs, you must have a placeholder module, called a C-RIMM (Continuity RIMM) to maintain continuity. It contains no memory chips - think of it as a wall that “bounces” data off of it to the actual RIMMs in the sockets. This is important, as you will learn later in the chapter when you learn how to “balance” your RAM.

19. RAM on the Motherboard Error Checking and Parity: In older machines, RAM existed as individual chips socketed to the motherboard in banks or rows of nine chips each (8 at one bit each for a total of 1 byte, 1 for parity bit) Parity bit was usually a full chip length away from the other 8 chips

20. RAM on the Motherboard Parity is an “integrity check”: Odd Parity – number of 1’s in bits is odd Even Parity – number of 1’s in bits is even When data written to RAM, computer calculates how many ON (1) bits are in the eight bits. The ninth bit is set to 1 or 0 to preserve the type of parity.

21. RAM on the Motherboard Example: [1] [0] [1] [0] [1] [0] [1] [0] [?] 4 1’s, 4 0’s In an odd parity system, bit 9 would = 1 In an even parity system, bit 9 would = 0 Let’s talk ASCII - what is “A” in even parity? Odd?

22. RAM on the Motherboard When byte is read back, computer checks odd or even state – if number of bits is not odd in an odd parity system (or even in an even parity system) a parity error occurs – this always halts the system Parity Error 1 – parity error on motherboard Parity Error 2 – parity error on expansion board

23. RAM on the Motherboard Older DRAM memory used parity checking, but today’s memory uses ECC (error-correcting code) that auto-corrects errors First ECC – detects and corrects an error in one bit of the byte Newer ECC – detects errors in two bits of a byte but cannot correct them

24. RAM on the Motherboard Some older motherboards use parity memory, most newer ones use nonparity memory (saves processing time/money) To identify – count the number of chips in the SIMM stick if odd, it is most likely parity memory if even, it is most likely nonparity memory

25. RAM on the Motherboard Some SDRAM, DDR, and RIMM memory modules support ECC. DIMMs that support ECC have an odd number of chips (#9 is the ECC chip) DIMMs are normally 64 bit modules, but ECC makes them 71 or 72 bits – the extra bits are used to verify the integrity of every 8 bits stored on the module and to correct errors when possible. ECC is more expensive and slower (because of the time needed to verify data), but is more reliable and generally used in servers

26. RAM on the Motherboard Which memory do I use? When buying memory, check motherboard documentation and CMOS/BIOS setup Older boards supported parity/nonparity Newer boards support ECC/non-ECC Motherboard must support parity or ECC for it to be active – however, sometimes you can use nonparity memory in parity systems and non-ECC memory in ECC systems and the error checking will not be enabled

27. RAM on the Motherboard CAS (column access strobe) Latency and RAS (row access strobe) Latency – two other memory features Both refer to the number of clock cycles it takes to write or read a column or row of data Values are two or three clock cycles CAS used more, CL2 (CAS Latency 2) is a little faster than CL3 (CAS Latency 3) Remember to use the memory type the motherboard manufacturer recommends!

28. RAM on the Motherboard Memory Speeds: SIMMs – measured in nanoseconds (ns) SDRAM/DDR/RIMM – measured in MHz or a PC rating. DRR is usually described with the MHz speed in its’ name (example: DDR333). A PC rating is a measure of the total bandwidth of data moving between the module and the CPU (Ex: DDR266 has a 64-bit, or 8-byte, data path. The transfer rate is 8 bytes * 266 MHz = 2128 MB/sec. Its PC rating would then be PC2100). Other current ratings include PC1600 (200 MHz), PC2700 (333 MHz), and PC3200 (400 MHz)

29. RAM on the Motherboard Factors to consider (see Figure 6-8, p. 251) Speed of memory (ns, MHz, or PC rating) How much memory is installed Memory technology CL (CAS latency) rating – the lower the better! ECC/parity or non-ECC/nonparity Always use the “best” that the board supports!

30. Upgrading Memory Many computers have empty slots on the motherboard when first purchased, allowing you to add DIMMs or RIMMs What to look for when buying Memory: Use fastest speed supported! Check your motherboard documentation

31. Upgrading Memory When a computer first boots, the system must detect the type of memory installed Two methods to do this: PPD (Parallel Presence Detect) – uses resistors to communicate types present SPD (Serial Presence Detect) – stores information about the memory type in the EPROM (erasable programmable read-only memory – a memory chip that retains its data when its power supply is switched off) *If there is no documentation, assume PPD

32. Upgrading Memory Chips can be high-grade, low-grade, remanufactured, or used. Poor-quality memory chips can cause frequent General Protection Fault (GPF) errors in Windows, application errors, and errors that hang the system

33. Upgrading Memory Tin or Gold Leads? Memory modules and the banks that hold them can be either tin or gold, as are the edge connectors on the memory modules. Tin leads should go to Tin connectors Gold leads should go to Gold connectors This will prevent chemical reactions/corrosion

34. Upgrading Memory Remanufactured vs. Used: On each chip or a RAM module there is a chipID that identifies the date the chip was manufactured (in YY/WW format) Example: 0410 = 10th week of 2004 Date stamps older than one year indicate the chip is probably used memory If some chips on a stick are old and some are new, the module is probably remanufactured It is wise to look for chipID’s close together and fairly new (less than a year old is good)

35. Upgrading Memory Re-Marked Chips: New chips have a protective coating that gives them a polished, reflective surface. If the chip’s surface is dull or matted, or you can scratch off the markings with a fingernail or knife, suspect that the chips have been re-marked (used, returned to the factory, marked again, and resold)

36. Upgrading Memory How much memory fits on a motherboard? **ALWAYS Read documentation!** Not all sizes fit! Remember the slots! Must consider type as well as number – often SIMMs must be “balanced,” meaning you must have the same amount of memory in each slot of a system! (ex: 2 slots in a 512 system – both slots must hold 256 sticks!)

37. Upgrading Memory 30-Pin SIMMs – on older motherboards, installed in groups of four – SIMMs in each group or bank must be the same type AND size – check documentation! 72-Pin SIMMs – to accommodate a 64-bit bus, these have 32-bit data paths and are installed in groups or banks of two. Most older motherboards have 1-3 banks that can be filled with 2, 4, or 6 SIMMs, respectively. The two SIMMs in each bank must match in speed AND size – check documentation!

38. Upgrading Memory DIMMs - 64-bit data path, so one can be installed by itself instead of in a pair DIMMs come as Single-sided (8, 16 , 32, 64, 128 MB – chips on one side of module) or Double-sided (32, 64, 128, 256, and 512 MB, 1 GB, and 2 GB – chips on both sides of the module)

39. Upgrading Memory Example Scenario: A Pentium motherboard might use 168-pin DIMM modules – the documentation says to use unbuffered, 3.3V, PC100 DIMM SDRAM modules PC100 – modules should be rated to work with motherboard that runs at 100MHz Three DIMM sockets are on the board, each represents one bank What the maximum total System Memory?

40. Upgrading Memory Another Scenario: A more recent Pentium motherboard allows you to use three different speeds of DDR DIMMs in one to four sockets Board supports 4 GB of unbuffered 184-pin non-ECC memory running at PC3200, PC2700, or PC2100. Documentation says system bus can run at 800MHz, 533MHz, or 400MHz What speed of memory can be installed?

41. Upgrading Memory RIMM Modules: All RIMM slots filled - RIMMs or C-RIMMs To upgrade, replace C-RIMMs with RIMMs Once again, do this using documentation! See Table 6-4 on p. 256 for an example motherboard memory configuration

42. Upgrading Memory Match Memory Modules to Motherboard (say that three times fast…) Notes to remember: Only use speeds motherboard supports Avoid mixing speeds on same motherboard Better to buy same SIMM brand and speed Kingston is good memory, and will match for you!

43. Upgrading Memory Reading ads about Memory Modules: See Figure 6-12 on page 257 Density listed – tells us the width of the data bus, whether the module supports error checking, size of module Example: 16 x 64 – 64 = width of bus (bits) If it was 16 x 72 – this implies a parity bit When calculating size, ignore 9th bit – convert to bytes (divide by 8 – 64/8 = 8) Multiply 8 by left number (16) to find size 16 x 64 (or 72) = 16*(64/8) = 16*8 = 128 MB

44. Upgrading Memory Installing Memory - SIMMs: ALWAYS wear ground bracelet Power OFF at ALL TIMES during process DO NOT STACK sticks – chips come loose SIMMs go in at angle Line up the notches – do not force! Check POST count on boot to make sure To remove, rotate at 45 degree angle

45. Upgrading Memory Installing Memory – DIMMs: ALWAYS wear ground bracelet Power OFF at ALL TIMES during process DO NOT STACK sticks – chips come loose Line up the notches – do not force! Latches on outside need to point outward Pushing down will bring latches in If latches don’t lock, memory is not in Check POST count on boot to make sure

46. Upgrading Memory Installing Memory – RIMMs: ALWAYS wear ground bracelet Power OFF at ALL TIMES during process DO NOT STACK sticks – chips come loose Start at Bank 0, then Bank 1 If C-RIMM in a slot, remove and insert RIMM Line up the notches – do not force! Latches on outside need to point outward Pushing down will bring latches in If latches don’t lock, memory is not in Check POST count on boot to make sure

47. Windows Memory Management OS Memory Management Evolution Early CPUs had only 20 lines on the bus available to handle addresses, so the largest memory address the CPU could use was 11111111111111111111, or 1,048,575, or 1024K, or 1MB of memory

48. Windows Memory Management How DOS/Windows 9x used memory: 0K to 640K – conventional or base memory -- used by DOS and applications 640K to 1024K – upper memory -- used by BIOS and device drivers 1024K and up – extended memory -- newer models were developed with 24 address lines and more so that memory addresses above 1024K became available **Windows 9x still uses these same divisions**

49. Windows Memory Management How Memory Addresses are Used: Once addresses are assigned to memory, they can be used for communication with all software layers Think of them as a “card catalog” Applications pass information to the OS, which then passes information to the device driver. Also works in reverse. All 3 layers know the location of addresses

50. Windows Memory Management VDM (Virtual DOS machine) – environment that a 32-bit, protected mode OS (like Windows 9x) provides for a real mode program to operate in This is necessary because in Real Mode, one program has direct access to hardware, while in Protected Mode, multiple programs depend on the OS to access the hardware – in this mode, the OS has options on how to use RAM

51. Windows Memory Management OS options in Protected Mode: Virtual Memory – using the hard drive as though it were RAM. Data stored in virtual memory is stored in a file on the hard drive called the swap file or page file Windows NT requires that all device drivers be 32-bit – it does not support real mode

52. Windows Memory Management Quick Summary: DOS – real mode only Windows 9x – real or protected mode Windows NT – protected only (stable) Windows ME – protected only, yet very unstable! Avoid like the plague!

53. Windows Memory Management Windows 9x/DOS memory addressing: On startup, many programs demand memory addresses (BIOS, drivers, OS) Assigning memory addresses to programs is called “memory mapping” Some BIOS and drivers must use certain addresses and will not work otherwise Windows 9x is committed to backwards compatibility with those programs – this is a great limitation of the 9x series!

54. Windows Memory Management OS manages several types of memory: Conventional (base) memory (0-640K) Upper memory (640K-1024K) Extended memory (1024K and up) **The first 64K or extended memory is called the high memory area (HMA)

55. Windows Memory Management Conventional Memory: 640K = grossly inadequate today Problem of restricting the number of memory addresses to 640K could have been solved by providing more addresses in future DOSs Programs always expect data to be written to memory directly above the addresses for the program itself. Eventually, the program will “hit the ceiling,” which is the beginning of upper memory (above 640K – assigned to video ROM) – 32-bit drivers/apps don’t have this problem because of extended memory

56. Windows Memory Management Upper Memory: Video ROM and RAM - stored in first part (the A, B, C areas of memory) 16-bit BIOS programs for other legacy expansion boards are assigned to rest (the D, and E areas of memory) The BIOS on the motherboard is assigned to the top-most part of upper memory (the F area of memory) *Upper memory often has unassigned addresses

57. Windows Memory Management Extended Memory and HMA: Memory above 1 MB First 64K is HMA, exists because a bug in the programming for the older Intel 286 CPU produced it - OS started to store part of itself there to free conventional memory (called “loading DOS high”) Extended Memory is managed by the OS as a device with its own driver!

58. Windows Memory Management Utilities that Manage Memory: himem.sys – driver for memory above 640k -- automatically loaded during boot, can also be loaded with config.sys entry emm386.exe – contains software that loads programs into upper memory -- not loaded automatically, but can be loaded through entry in config.sys

59. Windows Memory Management The Page File: The Page File is created during the Windows XP installation and resides on the hard drive. Page files are measured in megabytes. The size of the page file is based on how much RAM is installed in the computer. By default, XP creates a page file which is 1.5 times the amount of installed RAM and places it on the hard drive where XP is installed. Other than plugging the RAM into the motherboard, there is little than can be done to alter its performance characteristics. The page file is a different story. Because it's located on a hard drive, it's subject to a number of factors that can hinder its performance.

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