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| author | Christian Starkjohann <cs+github@obdev.at> | 2013-11-22 15:48:48 +0100 |
|---|---|---|
| committer | Christian Starkjohann <cs+github@obdev.at> | 2013-11-22 15:48:48 +0100 |
| commit | e1a453ce38d77f80c171b2752bedd1be54fd0eb4 (patch) | |
| tree | beabb15589e776df64ed77460d7b6b678c825151 | |
| parent | 4b174f6ed96d82dc8c76209057626abc8ceea338 (diff) | |
| download | v-usb-e1a453ce38d77f80c171b2752bedd1be54fd0eb4.tar.gz v-usb-e1a453ce38d77f80c171b2752bedd1be54fd0eb4.tar.bz2 v-usb-e1a453ce38d77f80c171b2752bedd1be54fd0eb4.zip | |
Optimized fast CRC routine to be shorter and faster.
| -rw-r--r-- | usbdrv/usbdrvasm.S | 37 |
1 files changed, 16 insertions, 21 deletions
diff --git a/usbdrv/usbdrvasm.S b/usbdrv/usbdrvasm.S index 32ce8ef..6fa575d 100644 --- a/usbdrv/usbdrvasm.S +++ b/usbdrv/usbdrvasm.S @@ -145,7 +145,8 @@ RTMODEL "__rt_version", "3" #if USB_USE_FAST_CRC ; This implementation is faster, but has bigger code size -; Thanks to Slawomir Fras (BoskiDialer) for this code! +; Thanks to Slawomir Fras (BoskiDialer) for this code and to Shay Green for +; even further optimizations! ; It implements the following C pseudo-code: ; unsigned table(unsigned char x) ; { @@ -175,35 +176,29 @@ RTMODEL "__rt_version", "3" ; 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 reg 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 |