AKT211 – CAO 08 – Computer Memory (2)
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Transcript of AKT211 – CAO 08 – Computer Memory (2)
AKT211 – CAO
08 – Computer Memory (2)
GhifarParahyangan Catholic University
Okt 31, 2011
Last Course ReviewComputer Memory System
Memory Characteristics Memory Hierarchy
RAM Basic Technology Semiconductor SRAM vs DRAM
Advanced RAM Organization SDRAM vs DDR-RAM
OutlineError Correction
Single error correctionDouble error correction
THE BASIC OF ERROR CORRECTION
Semiconductor System Error• Hard Failures
– Permanent physical defect so that it can’t reliably store data
– Stuck at 0 or 1 or switch erratically between 0 and 1
• Soft Error– Random, nondestructive event that alters the
contents of one or more cell without damaging the memory
– Caused by power supply problems or alpha particles
• Most modern main memory systems include logic for both detecting and correcting errors
Single-bit error• Only 1 bit in data unit has changed
Error Correcting Code (ECC) Function
Simples form of Error Detection• Using a parity bit
– A bit that is added to ensure that the number of bits with the value ‘1’ in a set of bits is even or odd
– Only for detecting 1-bit error, not more, nor correcting !– E.g.: no error
– E.g.: 1 bit error
A wants to transmit: 1001A computes parity bit value : 1^0^0^1 = 0A adds parity bit and sends : 10010B receives : 10010B computes parity : 1^0^0^1 = 0B reports correct transmission after observing expected result
A wants to transmit: 1001A computes parity bit value : 1^0^0^1 = 0A adds parity bit and sends : 10010*** TRANSMISION ERROR ***B receives : 11010B computes parity : 1^1^0^1 = 1B reports incorrect transmission after observing unexpected result
Hamming Error-Correcting Code• linear error-correcting code• can detect up to d-1 bit errors• can correct (d-1)/2• d is the minimum hamming distance
between all pairs in the code words
Hamming (7, 4) Code• encodes 4 data bits (d1, d2, d3, d4) into 7
bits by adding 3 parity bits (p1, p2, p3)• single error correction
Hamming (7,4) Example
HAMMING ALGORITHMGENERALIZATION FOR
SINGLE ERROR CORRECTION
Generalization of the Hamming Single Error Correction
• The comparison logic receives as input two K-bit values
• A bit-by-bit comparison is done by taking the XOR
• The result is called the syndrome word– The value 0 indicates that no error
was detected and otherwise– We can determine the position from
that syndrome word
Required criteria for Hamming Error Correction
• If the syndrome contains all 0s, no error has been detected
• If the syndrome contains one and only one bit set to 1, then an error has occurred in one of the n check bits. No correction is needed
• If the syndrome contains more than one bit set to 1, then numerical value of the syndrome indicates the position of the data bit in error
SEC Step-by-Step1. Determine how long the code
(check bits) must be2. Determine the stored position
for each bit in M data bits and K check bits
3. Construct the appropriate XOR function that match with the required criteria
1. Determine how long the code must be
• M : number of bits in data bits• K : number of bits in code bits• Because an error could occur on any
of the M data bits or K check bits, we must have :
• e.g.: for a word of 8 data bits (M=8), we have
2K – 1 ‹ M + K
K=3 : 23 – 1 < 8 + 3K=4 : 24 – 1 > 8 + 4
2. Determine the stored position
Let’s see the explanation !
3. Construct the XOR Function
Again, let’s see the explanation !
Hamming SEC-DED Code• Nowadays, more commonly, semiconductor
memory is equipped with a single-error-correcting, double-error-detecting (SEC-DED) code
• Needs 1 extra parity bit that indicates whether the total number of 1s is even or odd
• Enhances the reliability of the memory, but adds the cost of complexity
• E.g. :– The IBM 30xx implementations used an 8-bit SEC-
DED code for each 64 bits of data in main memory• The size is actually about 12% larger than is apparent to
the user
Hamming SEC-DED Code (2)
Any Question ?
Reference• Chapter 5.2: Error Correction
(Stallings, William. Computer Organization and Architecture, 8th ed. Prentice Hall. 2010)
Exercises1. Dengan penggunaan algoritma Hamming,
berapakah jumlah check bit yang dibutuhkan jika data bit berukuran 1024-bit ?
2. Terdapat data bit sebanyak 8-bit tersimpan di dalam memori yang isinya 11000010. Dengan menggunakan algoritma Hamming, tentukan nilai check bit yang akan tersimpan pada memori.
3. Untuk data word 8-bit 00111001, check bit yang tersimpan adalah 0111. Anggap terjadi error pada pembacaan memori. Ketika data bit tersebut di baca ulang dari memori, nilai check bit yang terhitung adalah 1101. Berapakah sebenarnya nilai data bit yang error?
Week 8 Assignment• Bentuklah persamaan XOR untuk
menentukan SEC code (check bit) dengan menggunakan algoritma Hamming untuk data bit berukuran 16-bit. Bagaimana hasil check bit apabila menerima masukan data bit 0101000000111001 ? Simulasikan bagaimana algoritma Hamming dapat mengoreksi error apabila terjadi error di data bit posisi ke-5 (0101000000101001). Jelaskan jawaban Anda selengkap-lengkapnya.
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