[Coco] Fwd: IP packets on my coco

[Bill Pierce]

I don't know how useful it would be, but there IS a CRC program (asm) in the Nitros9 repo sources...
I'm pretty sure it's named "crc"...
You give the address range of a block of data and it calculates the crc... it's 24 bit I think (standard for OS9 crcs).
Also, several of the standard OS9 utilities calculate CRCs for their file checking, "ident" comes to mind.

 

 


Bill Pierce
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-----Original Message-----
From: Dave Philipsen <dave at davebiz.com>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Sent: Wed, Jun 8, 2016 3:00 am
Subject: Re: [Coco] Fwd: IP packets on my coco

This is a C code snippet for CRC calculation showing a 256-entry table of unsigned longs (1024 bytes).  I've also read that there are ways to to it with a 16-entry table of 32-bit words (64 bytes) but it requires a little more computation that the 256-entry table.There are also some webpages that have CRC calculators on them so if you are writing an algorithm you can test it.  So I don't think calculating the CRC is going to be all that difficult nor should it be overly CPU intensive.Dave    /* Table of CRCs of all 8-bit messages. */    unsigned long crc_table[256];        /* Flag: has the table been computed? Initially false. */    int crc_table_computed = 0;        /* Make the table for a fast CRC. */    void make_crc_table(void)    {      unsigned long c;      int n, k;          *for (n = 0; n < 256; n++) { *c = (unsigned long) n;        for (k = 0; k < 8; k++) {          if (c & 1)            c = 0xedb88320L ^ (c >> 1);          else            c = c >> 1;        }        crc_table[n] = c;      }      crc_table_computed = 1;    }       /* Update a running CRC with the bytes buf[0..len-1]--the CRC       should be initialized to all 1's, and the transmitted value       is the 1's complement of the final running CRC (see the       crc() routine below). */        unsigned long update_crc(unsigned long crc, unsigned char *buf,                             int len)    {      unsigned long c = crc;      int n;          if (!crc_table_computed)        make_crc_table();      for (n = 0; n < len; n++) {        c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);      }      return c;    }        /* Return the CRC of the bytes buf[0..len-1]. */    unsigned long crc(unsigned char *buf, int len)    {      return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;    }DaveOn 6/8/2016 12:57 AM, Barry Nelson wrote:>   The problem with that approach is that an 8 bit CRC requires a 256 byte lookup table or 2^8 bytes. A 32 bit CRC would need a 2^32 byte table or 4294967296 bytes, that is a 4Gb lookup table.>>> Dave Philipsen dave at davebiz.com>> Wed Jun 8 01:27:51 EDT 2016>> I remember from the CRC calculation in X/Ymodem that there is an easy>> and faster way to get the CRC using lookup tables.  That's how I did it>> with Supercomm.  I believe the CRC for the networking packets is a>> 32-bit CRC so I don't know if lookup tables are still practical or not>> with that size CRC.  What is amazing is that I was able to find that>> information without the internet back then.  I just now did a few>> cursory searches on calculating CRCs and I'm guessing that there is a>> fairly easy method that doesn't require so much overhead.>>>>>> Dave>-- Coco mailing listCoco at maltedmedia.comhttps://pairlist5.pair.net/mailman/listinfo/coco