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⬅️ Previous capture (2024-05-10)
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I've been a bit distracted lately with my astronomy studies, but I have still been working with Mecrisp Stellaris Forth here and there. I asked in the #mecrisp IRC channel if anybody had a software implementation of the CRC32 checksum algorithm available for Mecrisp Stellaris. They told me no, but that it would be a great idea for me to make one and contribute it. So, I did.
The main appeal of CRC32 is that it is very simple to implement, especially if you store the polynomial lookup table in memory. Here are links to my code:
https://codeberg.org/infrared/mf-rp2040/src/commit/82ff68e023d84e563fa33c4b54ec329595fd492e/ARRAY.FS
https://codeberg.org/infrared/mf-rp2040/src/commit/82ff68e023d84e563fa33c4b54ec329595fd492e/CRC32.FS
ARRAY.FS is just a few helper words to make storing arrays of data look nicer. CRC32.FS is just the polynomial lookup table (256 32-bit values) and these two words:
\ ******************************************************************
\ PARAMETERS:
\ W1: CURRENT CHECKSUM (32 BITS)
\ C: NEXT BYTE OF INPUT DATA
\ OUTPUT:
\ W2: NEXT CHECKSUM
\ ******************************************************************
: CRC32_STEP ( W1 C -- W2 )
OVER $FF AND XOR CELLS LOOKUP + @ SWAP 8 RSHIFT XOR ;
\ ******************************************************************
\ PARAMETERS:
\ U: NUMBER OF BYTES
\ A: START ADDRESS OF DATA
\ OUTPUT:
\ W: CHECKSUM
\ ******************************************************************
: CRC32 ( U A -- W )
$FFFFFFFF -ROT DUP ROT + SWAP
DO I C@ CRC32_STEP LOOP $FFFFFFFF XOR ;
The first word is a helper word. CRC32 is the loop which takes a byte count and a starting memory address, and then outputs the checksum for the data in that memory area.
As a word of caution, the code just uses C@ to fetch each byte, so I'm not sure if that works for all use cases. More specifically, I don't know if all Stellaris targets support reading firmware with C@.