diff options
| -rw-r--r-- | firmware/usbdrv/usbdrvasm.S | 47 |
1 files changed, 23 insertions, 24 deletions
diff --git a/firmware/usbdrv/usbdrvasm.S b/firmware/usbdrv/usbdrvasm.S index 32ce8ef..d927564 100644 --- a/firmware/usbdrv/usbdrvasm.S +++ b/firmware/usbdrv/usbdrvasm.S @@ -142,10 +142,14 @@ RTMODEL "__rt_version", "3" #endif + #if USB_USE_FAST_CRC ; This implementation is faster, but has bigger code size -; Thanks to Slawomir Fras (BoskiDialer) for this code! +; This version has been optimized in size compared to the original +; fast CRC by Shay Green, January 2014. + +; Thanks to Slawomir Fras (BoskiDialer) for the original contribution. ; It implements the following C pseudo-code: ; unsigned table(unsigned char x) ; { @@ -174,36 +178,31 @@ RTMODEL "__rt_version", "3" ; scratch r23 ; resCrc r24+r25 / r16+r17 ; ptr X / Z + usbCrc16: - mov ptrL, argPtrL - mov ptrH, argPtrH + movw ptrL, argPtrL ldi resCrcL, 0xFF ldi resCrcH, 0xFF + clr bitCnt ; zero rjmp usbCrc16LoopTest usbCrc16ByteLoop: ld byte, ptr+ - eor resCrcL, byte ; resCrcL is now 'x' in table() - mov byte, resCrcL ; compute parity of 'x' + eor byte, resCrcL ; scratch is now 'x' in table() + mov scratch, byte ; compute parity of 'x' swap byte - eor byte, resCrcL - mov scratch, byte + eor byte, scratch + mov resCrcL, byte lsr byte lsr byte - eor byte, scratch + eor byte, resCrcL inc byte - lsr byte - andi byte, 1 ; byte is now parity(x) - mov scratch, resCrcL - mov resCrcL, resCrcH - eor resCrcL, byte ; low byte of if(parity(x)) value ^= 0xc001; - neg byte - andi byte, 0xc0 - mov resCrcH, byte ; high byte of if(parity(x)) value ^= 0xc001; - clr byte - lsr scratch - ror byte - eor resCrcH, scratch - eor resCrcL, byte + andi byte, 2 ; byte is now parity(x) << 1 + cp bitCnt, byte ; c = (byte != 0), then put in high bit + ror scratch ; so that after xoring, shifting, and xoring, it gives + ror byte ; the desired 0xC0 with resCrcH + mov resCrcL, byte + eor resCrcL, resCrcH + mov resCrcH, scratch lsr scratch ror byte eor resCrcH, scratch @@ -214,8 +213,8 @@ usbCrc16LoopTest: com resCrcL com resCrcH ret - -#else /* USB_USE_FAST_CRC */ + +#else // USB_USE_FAST_CRC ; This implementation is slower, but has less code size ; @@ -258,7 +257,7 @@ usbCrcReady: ret ; Thanks to Reimar Doeffinger for optimizing this CRC routine! -#endif /* USB_USE_FAST_CRC */ +#endif // USB_USE_FAST_CRC ; extern unsigned usbCrc16Append(unsigned char *data, unsigned char len); usbCrc16Append: |