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Weighted Checksum to Detect and Restore Altered Bits in Computer Memory

IP.com Disclosure Number: IPCOM000074197D
Original Publication Date: 1971-Mar-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 72K

Publishing Venue

IBM

Related People

Glickstein, IS: AUTHOR

Abstract

A relatively small weighted checksum routine can be used to detect and correct any single picked or dropped bit and any series of altered bits confined to a single word. The method does not require a parity bit for each word. In addition to allowing self-restoration of computer memory, the technique has application in systems as a means of detecting and correcting altered bits which arise from line noise.

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Weighted Checksum to Detect and Restore Altered Bits in Computer Memory

A relatively small weighted checksum routine can be used to detect and correct any single picked or dropped bit and any series of altered bits confined to a single word. The method does not require a parity bit for each word. In addition to allowing self-restoration of computer memory, the technique has application in systems as a means of detecting and correcting altered bits which arise from line noise.

A regular checksum is the simple addition of all words in protected memory:
(1) Regular Checksum = Word 1 + Word 2 + Word 3 + ...Word N.

If bits in a single word are altered, the new checksum will differ from the correct checksum by an error: (2) Regular Checksum Error = Altered Word - Correct Word.

A weighted checksum is the sum of each word in protected memory multiplied by a number related to its address: (3) Weighted Checksum = (delta +
1) x (Word 1) + (delta W+ 2) x (Word 2) + (delta + 3) x (Word 3)...(delta + N) x (Word N) (Where delta is any constant which exceeds the highest address in memory).

If a single bit is altered, the new weighted checksum will differ from the correct weighted checksum by an error: (4) Weighted Checksum Error = (delta + ADR) 2 (Where BP is the bit position of the altered bit: LSB =0, MSB = 15, etc.).

For single bit alterations the weighted checksum error contains sufficient information to determine the address, the bit position, and whether the bit was picked...