245 lines
11 KiB
Plaintext
245 lines
11 KiB
Plaintext
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If you encounter the message "parity error" while operating your PC, a
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malfunction has occurred somewhere in the PC's memory (RAM). In many
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instances, a parity error can be traced to a defective RAM chip or one
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that is seated poorly in its socket.
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To find the cause of the parity error, turn off the PC, wait about five
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seconds, and turn it on again. After flipping the power switch on, the
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PC automatically runs a series of diagnostics which tests different
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parts of the system. If a memory problem exists, a PARITY CHECK 1 or
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PARITY CHECK 2 error message will appear. This article will help you
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interpret these error messages, isolate defective memory chips, and
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correct the situation.
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System Board Memory
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Early PCs have system boards which hold only 64K of RAM. These system
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boards have four parallel rows (or banks) of nine 16K 4116 RAM chips.
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Newer PCs and XTs with 256K system boards have four parallel rows of
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nine 64K 4164 RAM chips. In both cases, each row has eight chips for
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memory (data bits) and one for parity checking (parity bit).
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- More - <C>ontinue, <S>top, <N>onStop?
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When facing the PC chassis from the front, the row of chips closest to
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the front is designated row 3, with the next row being row 2, then row
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1, and row 0 next to the expansion slots. In each row or bank, the
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chip on the extreme left, slightly separated from the data bits, is the
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parity bit chip.
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Expansion Board Memory
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The amount of memory contained on expansion boards varies widely from
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manufacturer to manufacturer. While the early expansion memory boards
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only contained 64K memory, the current standard seems to be memory
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boards in one of three configurations: 256K, 384K, or 512K. Most
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memory boards still use the 64K 4164 RAM chip, although use of the
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newer 256K 41256 RAM chip has become more prevalent as its
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availability has increased and unit cost has dropped substantially
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(from $60/chip to $7/chip during the past six months). The
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introduction of 80286-based computers like the AT has also impacted the
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popularity of mega-memory expansion cards because the 80286 can address
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up to 16 megabytes of RAM, as compared to the 1 megabyte limitation of
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8088-based PCs.
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To simplify matters, we will limit our discussion here to those
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- More - <C>ontinue, <S>top, <N>onStop?
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expansion memory boards that use the 64K RAM chips. In general, these
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expansion boards comprise from four (256K) to eight (512K) parallel
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rows of nine 64K memory chips. The rows are usually arranged
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horizontally, numbered from 1 to 8, and are populated sequentially from
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right to left (row 1 is on the extreme right; row 8 on the extreme
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left).
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Memory Error Messages
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Now that you know which row is where and its "numeric" position
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indicator, you can begin to isolate which chip may be causing the
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parity error.
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If your computer has a memory problem, a memory error message will
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appear on the monitor during the start-up diagnostic (Power-On Self
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Test) or when performing system diagnostics. This memory error message
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consists of two parts: a four-digit error code followed by the numbers
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201, e.g. 3040 201; and either a PARITY CHECK 1 or PARITY CHECK 2
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message.
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A PARITY CHECK 1 message indicates that the memory error was detected
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on the system board memory; a PARITY CHECK 2 message identifies a
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- More - <C>ontinue, <S>top, <N>onStop?
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memory error on a memory expansion board.
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PARITY CHECK 1 -- System Board Memory Errors
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The first number of the memory error code indicates which 64K bank of
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memory is involved. On PCs with 256K system boards, this can be rows
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0, 1, 2, or 3. On 64K system board PCs, the number 0 represents the
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entire 64K bank of 36 16K chips.
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For PCs with 64K system boards, the second digit can be 0, 4, 8, or C,
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and points to the 16K bank within the 64K which is failing; 0 is row
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0, 4 is row 1, 8 is row 2, and C is row 3. In contrast, for PCs with
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256K system boards, the second digit of the error message identifies
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the 4K page in the memory chip that is failing; this number is not
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needed to identify the problem chip and should be ignored.
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The third and fourth digits represent which bit position (or RAM chip)
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in the 64K row is causing the error condition. This number represents
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the hexadecimal address of the chip within the row.
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Table 1 provides the address for each of the chips on a 64K or 256K
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system board:
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- More - <C>ontinue, <S>top, <N>onStop?
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Bank 64K 256K Parity Bit Bit Bit Bit Bit Bit Bit Bit
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Number System System Bit 0 1 2 3 4 5 6 7
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0 00 0x 00 01 02 04 08 10 20 40 80
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1 04 1x 00 01 02 04 08 10 20 40 80
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2 08 2x 00 01 02 04 08 10 20 40 80
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3 0C 3x 00 01 02 04 08 10 20 40 80
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M E M O R Y C H I P S
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F R O N T O F C O M P U T E R
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Table 1. System Board Memory Addresses.
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Using the error message example given earlier, 3040 201, we can
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quickly identify the faulty chip as the eighth chip (Bit 6) in the
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first row from the front (Bank 3) of a 256K system board.
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PARITY CHECK 2 -- Expansion Board Memory Errors
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The procedures for diagnosing errors in memory contained on expansion
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- More - <C>ontinue, <S>top, <N>onStop?
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boards is similar to that used for system board memory diagnosis.
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The first number of the memory error code indicates which 64K bank of
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memory is involved. On PCs with 256K system boards, this can be row 4
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or greater; on 64K system board PCs, row 1 or greater.
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For both 64K and 256K system board PCs, the second digit is not used
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and should be ignored. The third and fourth digits contain the
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hexadecimal address (within the row) of the problem memory chip.
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Because there are numerous third-party manufacturers of expansion
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memory boards and because each manufacturer may use a different
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configuration and/or bank numbering scheme, the rules for
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identifying errant memory chip addresses for your expansion board might
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be different. You should check the documentation provided with your
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expansion board as reference for proper diagnosis of expansion board
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memory problems.
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For demonstration purposes, however, the following example uses a 384K
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memory expansion board (AST Six Pak Plus), configured with six banks of
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chips, numbered 1 to 6, from right to left.
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Table 2 provides the address for each of the chips on the expansion
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- More - <C>ontinue, <S>top, <N>onStop?
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memory card. Notice that the starting address for the rows of memory
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chips is different for the 64K and 256K system boards. This is due to
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the fact that the expansion board rows begin addressing where the
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system board stops: for the 64K board, starting address is 1x; the
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starting address for the 256K system board is 4x (x can be any number).
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Bank No. 6 5 4 3 2 1
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64K System 6x 5x 4x 3x 2x 1x
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256K System 9x 8x 7x 6x 5x 4x
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____________________________________________
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Parity Bit 00 00 00 00 00 00 M
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Bit 7 80 80 80 80 80 80 E
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Bit 6 40 40 40 40 40 40 M
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Bit 5 20 20 20 20 20 20 O
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Bit 4 10 10 10 10 10 10 R
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- More - <C>ontinue, <S>top, <N>onStop?
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Bit 3 08 08 08 08 08 08 Y
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Bit 2 04 04 04 04 04 04 C
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H
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Bit 1 02 02 02 02 02 02 I
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P
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Bit 0 01 01 01 01 01 01 S
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BOTTOM OF EXPANSION BOARD
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Table 2. Expansion Board Memory Addresses.
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Using the above table, we can see that a 7120 201 error code identifies
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the errant memory chip as the sixth chip from the bottom (Bit 5) in the
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fourth bank from the right (Bank 4) in a 256K system board machine.
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Correcting Memory Errors
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Now that we have identified the problem chip, we should verify it by
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replacing it with a spare chip and run the diagnostics again and see
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- More - <C>ontinue, <S>top, <N>onStop?
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whether the error is corrected. If no spare chip is available,
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exchange the suspect chip with another one in an adjacent bank. If we
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have correctly identified the problem chip, the diagnostics will
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display a different memory code -- that of the location where we put
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the suspected chip.
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If, however, the error code continues to identify the original
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location, a problem may exist with the socket and you should contact
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your dealer for assistance.
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If the system board or expansion board switches are not set properly or
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a chip is missing, the bit position in the error code may be AA, FF,
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55, or 01. If you are experiencing problems with more than one memory
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chip, the bit position code displayed will be the sum (in Hexadecimal)
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of the problem chip locations and consequently, will not match any of
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the values in Table 1 or 2. When this happens, the resultant error
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code could be any number from FF to 00. Diagnosis of the errant chips
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will involve a trial and error process of switching several chips from
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the identified row to an adjacent row.
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Reseat Chips Before Switching
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- More - <C>ontinue, <S>top, <N>onStop?
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Before switching chips in the "suspect" row, however, remove all chips
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and reseat them in their sockets. Run the diagnostics again. Because
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many parity problems are due to poor contacts between chips and
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sockets, this trick may eliminate the parity error. RAM chip failures
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are rather rare.
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The most difficult parity error or memory error to locate is one which
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occurs "intermittently". For example, heat generated by expansion
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boards, disk drives or other add-ons may cause a memory chip or some
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other memory-related component to shift sufficiently to break the
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electrical contact. You may never find this particular problem since
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the same set of conditions may not be duplicated exactly during
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diagnostics.
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A Simple, Do-It-Yourself Remedy
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What has been discussed here is a simple method to troubleshoot your
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PC. If in doubt, professional service technicians have the tools to
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diagnose memory problems instantly. But performing these simple checks
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yourself can save you money, especially if the cause is a poorly
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socketed chip.
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You have been on for 36 minutes, with 24 remaining.
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Enter Bulletin #, <L>ist, or <Q>uit to Main Menu?
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You have been on for 36 minutes, with 24 remaining.
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Enter Bulletin #, <L>ist, or <Q>uit to Main Menu? ????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
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