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--- en:pfw:crc32_ieee [2023-09-04 18:12] – gelöscht - Externe Bearbeitung (Unbekanntes Datum) 127.0.0.1
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+{{pfw:banner.png}}
+==== CRC, 32bit IEEE CRC ====
+The IEEE 32bit CRC standard was introduced for the ethernet protocol. The essential difference with previous CRC-implementations was that the start-number is 0xFFFFFFFF instead of 0x0. The reason is that a network can easily start with a couple of spurious 0x0's. A CRC with a 0x0 as start-number cannot detect such a network-specific error. By having 0xFFFFFFFF as start number, the first 4 characters received are effectively inverted. And that enables the detection of spurious zero's at the start of a transmission.
+This CRC version is probably the most widely used CRC-version there is. Examples where it is used: ethernet protocol, PKZIP, Gzip, MPEG-2, SATA, PNG, ZMODEM.
+If you look at the implementation below you will see that the CRC is shifted to right and that the polynomial of the CRC-IEEE has been reversed. This avoids having to bit-reverse every received byte, another IEEE peculiarity.
+<code>
+EDB88320 constant crc-polynomial ( reversed IEEE )
+hex
+: CRC32_IEEE ( addr len -- crc )            \ input is address and length
+    FFFFFFFF -rot                           \ FFFFFFFF = start-value CRC
+    bounds do
+        i  c@ xor
+0 do
+            dup 1 and if                    \ if lsb = '1' do rshift and xor
+rshift
+                crc-polynomial xor
+            else                            \ otherwise only a rshift
+rshift
+            then
+        loop
+    loop
+    -1 xor                                  \ invert output
+;
+</code>
+===== Contributions =====
+<html><h2 style="background-color:yellow">Alternative Implementations</h2></html>