Checksum

Product : ISaGRAF V3.3

Date : 24-November-1998

File : Checksum.htm

Subject : Checksums embedded with ISaGRAF TIC code

Keywords: Checksum - TIC

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ISaGRAF Workbench version 3.3 embeds in downloaded code a

list of checksums calculated on programs.

There is one checksum for each program.

A new table in the downloaded code gives the list of

checksums.

The new table is called __PROGCRC.

It is generated only if the target version is higher or

equal to 2.00.

That is the version number attached to the target and

entered in .RSC file.

Below is an example of new .RSC file header:

* resource identification (version 2.00: checksums embedded)

Name=ISA86M

Version=2.00

Comment=TIC code for Intel

If version number is 1.xx, checksums are not embedded,

i.e. the downloaded code is not changed from 3.2 version.

When checksums are embedded, the main pointer to __PROGCRC

table is put at the end of the main header structure, at

the beginning of the downloaded code:

00000000: __MAIN

00000000: 01 00 00 00 * High/Low flag

00000004: 68 00 00 00 * defprod

00000008: 44 00 00 00 * defappli

0000000C: D4 05 00 00 * defusf

00000010: D8 05 00 00 * deffbl

00000014: 80 00 00 00 * deffcnv

00000018: 84 00 00 00 * deftcnv

0000001C: 1C 07 00 00 * defvar

00000020: 14 04 00 00 * codevar

00000024: 88 00 00 00 * defoem

00000028: 90 00 00 00 * codeoem

0000002C: 74 05 00 00 * defhie

00000030: 98 05 00 00 * defprog

00000034: AC 05 00 00 * codeprog

00000038: C0 05 00 00 * linksfc

0000003C: DC 11 00 00 * resources

00000040: 7C 11 00 00 * <<<< THIS IS THE OFFSET OF __PROCRC TABLE

When checksums are embedded, the __PROGCRC table is put after

other tables, before resources.

Resources still are at the very end of the file:

... ISaGRAF tables and TIC code ...

0000117C: __PROGCRC

0000117C: 01 00 00 00 C4 07 00 00 - D0 00 00 00 25 E9 F2 5B

0000118C: 02 00 00 00 70 09 00 00 - 54 01 00 00 F8 71 DA 5A

0000119C: 03 00 00 00 B0 0B 00 00 - FC 01 00 00 B2 D1 AE 40

000011AC: 04 00 00 00 08 0E 00 00 - 10 01 00 00 05 DD 4E 4C

000011BC: 05 00 00 00 5C 0F 00 00 - 20 02 00 00 27 47 67 C5

000011CC: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 00 00

000011DC: __RESOURCE

000011DC: 00 00 00 00

000011E0: __CODECRC

000011E0: A6 F4 DB 09

000011E4: FB 90 3B BF

Note that two other checksums, calculated on the entire

file (including resources and other checksums) are put

at the end of the code.

The first one includes all the files except code generation

date/time stamp and application code version number.

Thus, this checksum is stable from one compiling to another,

if the application is not changed.

The second one includes data/time stamp plus code version

number, and can change even if the application is modified.

This checksum was already available on ISaGRAF V3.23.

Below is the format of __PROGCRC table:

_PROGCRC = <prog_def> .... <prog_def> <end_mark>

<prog_def> = <prog_num> <prog_off> <prog_siz> <prog_crc>

<prog_num> = 32 bit integer = internal program number

(from 1 to N)

<prog_off> = 32 bit integer = offset of program code from

beginning of file

<prog_siz> = 32 bit integer = size (in bytes) of program code

<prog_crc> = 32 bit integer = program checksum

<end_mark> = 16 null characters

Checksum calculation:

All checksums are calculated according to the same algorithm.

Checksum initial value is 0xffffffff.

Then the 'addbyte' function is sequentially called for each

character:

static uint32 CHEKSUM;

void checksum_init (void)

{

CHECKSUM = 0xffffffff;

}

void checksum_addbyte (uchar cbyte)

{

uchar i;

uint32 mem1;

CHECKSUM ^= (uint32)cbyte;

for (i=1; i<= 8; i++) {

mem1 = CHECKSUM;

CHECKSUM /= 2;

if (mem1 & 1) CHECKSUM ^= 0xA000A001;

}

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