a+ guide to managing and maintaining your pc fifth edition chapter 6 managing memory
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A+ Guide to Managing and Maintaining Your PCFifth Edition
Chapter 6
Managing Memory
2 A+ Guide to Managing and Maintaining Your PC, Fifth Edition
Memory Types
ROM – Read-Only Memory Permanent – retains data even when power is off System ROM BIOS, device ROM BIOS
RAM – Read-write Memory Volatile – losses data when power is off CPU cache memory, main memory
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EEPROM
Electrically Erasable Programmable ROM – data can be erased under software control
“Flash BIOS”
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RAMStatic RAM
(SRAM)Dynamic RAM
(DRAM)
Speed Faster
Cost more expensive
Size more space
Simplicity no refresh Constant refresh
Usage L1/L2/L3 cache Main memory
Addressing No Yes
% of accesses 90-95% 5-10%
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The Memory Controller Is normally integrated into the chipset
(northbridge) Controls accesses to CPU caches and main
memory CPU caches: predicts and prefetches instructions
and data Main memory:
• specifies the memory address on the address bus
• Interfaces with processor and other devices
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DRAM Refresh
Operates every a few milliseconds A refresh circuit reads the contents of each
memory cell, whether it is being used or not The reading action itself refreshes the memory
contents
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DRAM Memory Accesses
Asynchronous accesses Not synchronized to the system clock For lower-speed memory buses (< 66 MHz)
Synchronous accesses Synchronized to the system clock Much faster than asynchronous DRAM For higher-speed memory systems of newer PCs
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Representative Dynamic RAM Technologies Asynchronous DRAM
EDO (Extended Data Out): Pentium /486 SDRAM (Synchronous DRAM)
Regular SDRAM DDR (Double-data Rate) SDRAM: doubles the
bandwidth by transferring data twice per cycle Rambus DRAM (RDRAM) – not the trend
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Error-detection using Parity Parity
Older method of error-detection An additional parity bit to make an even/odd
number of ones or an odd number of ones
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Error-correcting Code (ECC) Current method of error checking that can
detect and correct an error of a single bit 8 ECC bits for 8 bytes (64 bits)
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DRAM Speed Ratings
Speed in nanoseconds – the minimum access time for a read/write to memory
SDRAM Speed in MHz – reciprocal of the nanoseconds
speed• E.g., 100 MHz (10ns)
Speed rating: PCXXX• PC100-compatible (8ns)
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Memory Packaging
Dual Inline Packages (DIPs) Memory Modules A DIP
A MemoryModule
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Memory Modules
SIMM (Single Inline Memory Module) 30-pin: 8 bits of data (+ 1 bit parity) 72-pin: 32 bits of data (+ 4 bits parity/ECC)
DIMM (Dual Inline Memory Module) 168-pin: 64 bits of data (+ 8 bits parity/ECC) 184-pin DDR: 64 bits of data (+ 8 bits ECC) SODIMM – laptop computers
RIMM: 184-pin
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Examples of Memory Modules
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72-pin SIMM
Intel 486, 586 and some early Pentium desktop 32 bits data width
36 bits with parity EDO and Fast page mode (FPM) technologies 4, 8, 16, and 32 MBytes 60ns – 70ns
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168-pin DIMMs
FPM, EDO, and Synchronous DRAM (SDRAM)
64 bits data width 72 bits (with 8 bits ECC) 80 bits (with 16 bits ECC)
Buffered/unbuffered Serial Presence Detect: an EERPOM that
stores configuration information of the module
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Identifying the DIMM Module
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Memory Banks
PCs read a certain bit width of memory at a time, typically 32 or 64, and the memory making up this width is called a bank
72-pin SIMMs: banks of two for a system with 64-bit system bus data path
168-pin DIMMs: installed as a single module for a system with 64-bit system bus data path
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Gold/Tin Connectors and Sockets 30-pin SIMMs: often with gold contacts 72-pin SIMMs: usually found with tin contacts 168-pin DIMMs: generally found only with
gold contacts
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Memory Size Specifications
D*W-S D: # of memory units for each bit W: data width of the module in bits S: speed of the module in nanoseconds
E.g. 2x32-60 4x9-70 128MB 16x64, 16x72
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Memory Module Quality Factors
DRAM Quality Number of Chips on the Module – beware of
modules that have too many chips on them Module Quality Warranty
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Installing a SIMM Module
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Installing a DIMM Module
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Installing a RIMM Module
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Using Motherboard Documentation to Select Memory
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Using Motherboard Documentation to Select Memory (continued)
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Troubleshooting Memory
What to do if the computer does not recognize new SIMMs, DIMMs, or RIMMs, or memory error messages appear
Recurring errors during normal operations can mean unreliable memory
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DOS Memory Management
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Memory Layout
Conventional Memory
Upper Memory Area (UMA)
1 MB
High Memory Area (HMA) 64 KB
384 KB
640 KB
Extended Memory
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Conventional Memory
The first 640 KB of system memory (00000h to 9FFFFh)
Used by DOS and standard DOS programs (device drivers, user programs, etc……)
Goal: free up as much as possible space to user programs
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Upper Memory Area (UMA)
The next 384 KB of the system memory (A0000h - FFFFFh)
Reserved for system BIOS, device BIOSes, and system devices
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UMA LayoutAddress First 16K
(0000h-3FFFh)
Second 16 K
(4000h-7FFFh)
Third 16K
(8000h-BFFFh)
Fourth 16K
(C000h-FFFFh)
A0000-AFFFFh
Video Card RAM
B0000-BFFFFh
Video Card RAM
C0000-CFFFFh
Video Card BIOS IDE Hard Disk BIOS
D0000-DFFFFh
E0000-EFFFFh
F0000-FFFFFh
System BIOS ROM
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Upper Memory Blocks (UMBs)
The parts of the upper memory that are not used by ROM or for video RAM
EMM386.exe: a UMB provider that enables DOS programs and device drivers to access UMBs
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Load DOS programs in UMBs
Config.sys must includedevice = himem.sys
device = emm386.exe
dos = UMB
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Load Device Drivers in UMBs
Load device drivers in config.sysdevicehigh = mydrive.sys e.g, DBLSPACE.SYS, PRINTER.SYS,
DISPLAY.SYS,RAMDRIVE.SYS Load DOS programs in autoexec.bat
loadhigh program e.g., DOSKEY, DOSSHELL, KEYB
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Extended Memory
All of the memory above the first megabyte The first 64 KB (high memory area HMA) can
be accessed by DOS programs Beyond 64 KB can only be accessed by
Windows programs Himem.sys: manages extended memory
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High memory Area (HMA)
The first 64 KB after 1 MB of the memory (100000h-10FFEFh)
HIMEM.SYS enables DOS programs (practically the DOS itself) to used this memory
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Load DOS High
Open config.sys Type in
device = himem.sys
DOS = high
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A Typical Config.sys
device = himem.sys
device = emm386.exe
dos = high,UMB
devicehigh = XXX.sys
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Windows Virtual Memory Management
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Virtual Memory
Each application has 2 GB of virtual memory space
2 GB Virtual MemoryApplication 1
Application 2
2 GB Virtual Memory
WindowsVM
Manager
RAMMemory
SwapFile
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Virtual Memory Settings in Windows 9x Start->Settings->Control Panel->System-
>Performance tab->Virtual Memory Change the size of the swap file Check c:\windows\win386.swp
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Virtual Memory Settings in Windows 2000/XP Figure 6-26 Open a MS-DOS window, type “dir /ah” to see
the swap file
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