Post by thread:Efficient way to reverse a byte?

Jim, Your choices are likely to be fast execution and big code usage OR slow execution and small code usage. As Wagner suggested, fastest would be a 256 byte lookup table (but it uses up 256 bytes for the lookup table). A small, slower approach is to rotate the source byte right one bit (putting the rightmost bit in the carry flag), then rotate the destination byte left one bit (putting the carry flag in the rightmost bit of your destination register). Repeat this cycle 8 times. Your destination byte will have to be a temp register, then when done move the temp register back to the original source register. I would estimate this could be done in 10 to 15 bytes of code and 50 to 70 instruction cycles. Obviously I am thinking like the C programmer I am and someone like Dimitry could probably give you a method for accomplishing the same thing in 5 bytes of code and 10 cycles. :-) Hope this helps, Adam On Fri, 29 Oct 1999 14:48:54 +0100 Jim Main <spam_OUTj.mainTakeThisOuTNET.NTL.COM> writes: {Quote hidden}> I'm sure I saw an easy way to reverse a byte some time ago in this > list, but > I've forgotten it.. > > I want to change b7 b6 b5 b4 b3 b2 b1 b0 to b0 b1 b2 b3 b4 b5 > b6 b7. > > Quickest & most code efficient?? > > Jim > --- Adam Bryant (age 0x23) .....abryantKILLspam@spam@peaktech.com (work) adamdbKILLspamjuno.com (home) Parker, CO, USA Robotics, RC Airplanes, anything using a PIC ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: dl.http://www.juno.com/dynoget/tagj.

I colleague of mine has extoled the virtues of using lookup tables as a means of improving speed for as long as I can remember. Just about every time I have ever asked him, "How can I make this faster", he usually replies: "use a table". Ever wonder how they got so many of those video games on the 1Mhz "pre-Nintendo-era" game units to animate smoothly? I tend to agree that this technique is often underused. But, Of course, there are plenty of situations where the use of a table may not be practical. Just my $.02 Dan On Fri, 29 Oct 1999 17:12:13 +0200, Douglas Burkett wrote: >If I'm not mistaken you could use a 16 byte lookup for the nybbles and then >use the efficency of the swap instruction, quicker I believe than rotating >all 8 bits and less code space than the full lookup table. > >Doug > >{Original Message removed}

Adam Bryant wrote: > A small, slower approach is to rotate the source byte right one bit > (putting the rightmost bit in the carry flag), then rotate the > destination byte left one bit (putting the carry flag in the rightmost > bit of your destination register). Repeat this cycle 8 times. Your > destination byte will have to be a temp register, then when done move the > temp register back to the original source register. I would estimate > this could be done in 10 to 15 bytes of code and 50 to 70 instruction > cycles. > > Obviously I am thinking like the C programmer I am and someone like > Dimitry could probably give you a method for accomplishing the same thing > in 5 bytes of code and 10 cycles. :-) Adam is correct about the options, but even in assembler: 1) A direct flow routine: 8 instructions shifting bits out 8 instructions shifting bits in 1 instruction swapping bytes Total Program Instructions: 17 Total Instructions Executed: 17 2) A loop routine: 1 instruction setting loop count = 8 1 instruction shifting bit out 1 instruction shifting bit in 1 instruction decrementing count and looping back 1 instruction to swap bytes Total Program Instructions: 5 Total Instructions Executed: 26 3) Lookahead table (16 bytes of nibbles): 1 instruction to set the lookahead base address 1 instruction to save the original byte 1 instruction to AND the original byte lower nibble 1 instruction to get the table nibble 1 instruction to save this table byte 1 instruction to retrieve the original byte 1 instruction to swap original byte nibbles 1 instruction to AND the original byte lower nibble 1 instruction to get the table nibble 1 instruction to OR with previous table nibble 1 instruction to swap final byte nibbles Total Program Instructions: 11 + 16 (table) Total Instructions Executed: 11 4) Lookahead table (256 bytes): 1 instruction to set the lookahead base address 1 instruction to get the table byte Total Program Instructions: 2 + 256 Total Instructions Executed: 2 As you can see, execution speed cost a lot, always.

> 3) Lookahead table (16 bytes of nibbles): > 1 instruction to set the lookahead base address > 1 instruction to save the original byte > 1 instruction to AND the original byte lower nibble > 1 instruction to get the table nibble > 1 instruction to save this table byte > 1 instruction to retrieve the original byte > 1 instruction to swap original byte nibbles > 1 instruction to AND the original byte lower nibble > 1 instruction to get the table nibble > 1 instruction to OR with previous table nibble > 1 instruction to swap final byte nibbles > Total Program Instructions: 11 + 16 (table) > Total Instructions Executed: 11 I made it Total Program Instructions: 7 + 19 (table) &Total Instructions executed: 7 eg. movf original,w call rev_nibble movwf result swapf result,f swapf original,w call rev_nibble iorwf result,f rev_nibble andlw 0x0F addwf pcl,f retlw 00 retlw 08 ....etc... Thanks for all the suggestions.. jim

>I'm sure I saw an easy way to reverse a byte some time ago in this list, but >I've forgotten it.. > >I want to change b7 b6 b5 b4 b3 b2 b1 b0 to b0 b1 b2 b3 b4 b5 b6 b7. I've seen a lot of replies, but to me, the simple 'brute force and ignorance' technique seems to make the most sense. clrf dest btfsc source,0 bsf dest,7 btfsc source,1 bsf dest,6 btfsc source,2 bsf dest,5 btfsc source,3 bsf dest,4 btfsc source,4 bsf dest,3 btfsc source,5 bsf dest,2 btfsc source,6 bsf dest,1 btfsc source,7 bsf dest,0 17 instructions, 17 cycles dwayne Dwayne Reid <.....dwaynerKILLspam.....planet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 15 years of Engineering Innovation (1984 - 1999) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. My posting messages to Usenet neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.

How about: rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in rlf arg1,1 ;shift out a bit rrf arg2,1 ;shift it in Kind of long, but this should reverge arg1 and store it in arg2. At the same time, it will also reverse arg2 and store it in arg1. this is untried... give it a go egh? Terry {Quote hidden}>>I'm sure I saw an easy way to reverse a byte some time ago in this list, but >>I've forgotten it.. >> >>I want to change b7 b6 b5 b4 b3 b2 b1 b0 to b0 b1 b2 b3 b4 b5 b6 b7. > >I've seen a lot of replies, but to me, the simple 'brute force and >ignorance' technique seems to make the most sense. > > clrf dest > btfsc source,0 > bsf dest,7 > btfsc source,1 > bsf dest,6 > btfsc source,2 > bsf dest,5 > btfsc source,3 > bsf dest,4 > btfsc source,4 > bsf dest,3 > btfsc source,5 > bsf dest,2 > btfsc source,6 > bsf dest,1 > btfsc source,7 > bsf dest,0 > >17 instructions, 17 cycles > >dwayne > > >Dwayne Reid <EraseMEdwaynerspam_OUTTakeThisOuTplanet.eon.net> >Trinity Electronics Systems Ltd Edmonton, AB, CANADA >(780) 489-3199 voice (780) 487-6397 fax > >Celebrating 15 years of Engineering Innovation (1984 - 1999) > >* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * >Do NOT send unsolicited commercial email to this email address. >My posting messages to Usenet neither grants consent to receive >unsolicited commercial email nor is intended to solicit commercial >email. > > ----------------------------------------------------------------- Sent from the desk of: Terry Allen Steen, EE engineeringspam_OUTmarinapower.com 332 McLaws Circle, Ste 111 757-258-8800 (Voice) Williamsburg, Va 23185 757-258-8805 (FAX) ----------------------------------------------------------------- !I AM A WHALE MAIL USER! If you have a large file to send, goto http://www.whalemail.com send them to: mplengineer Give me your account and I will use it also ----------------------------------------------------------------- Visit our web-site: http://www.marinapower.com -----------------------------------------------------------------

John Payson, Are you sure this works? I've tried it with CC5X and haven't sucess. uns8 source; MOVLW (0b.1101.1001); // this value is just for test MOVWF (source); BTFSC (source.0); XORLW (0x41); // 0100.0001 BTFSC (source.6); XORLW (0x41); // 0100.0001 BTFSC (source.1); XORLW (0x22); // 0010.0001 BTFSC (source.5); XORLW (0x22); // 0010.0001 BTFSC (source.2); XORLW (0x14); // 0001.0100 BTFSC (source.4); XORLW (0x14); // 0001.0100 It is very ingenious, but seens to have a bug somewhere. Regards, Brusque ___________________________________________________________________________ | | || |\| | || || |\|\ Edson Brusque :-^= (@spam@brusqueKILLspamflynet.com.br) | | || ||| | || || |||| Musician, Tech Consultant, Programmer, Developer | |_||_||| |_||_||_|||| Blumenau / SC / Brazil / Earth / Solar Syst / Milk... | \_\\_\|| \_\\_\\_\||| Giro In'Italia homepage: http://www.flynet.com.br/giro | | | | | | | || C.I.Tronics Lighting Designers: http://www.citronics.com.br |__|__|__|__|__|__|__||----------------- ICQ# 15937748 --------------------- \__\__\__\__\__\__\__\| The SoundFont Users Group Mailing List is at ----------------------| www.geocities.com/SiliconValley/Port/6619/ ----------------------------------------------------------------------------

The piece of code that Tracy posted (from John Payson) does what it was originally meant to do BUT is NOT correct for the present problem. Here we require 76543210 --> 01234567 John's solution is for a 7 bit swap ignoring B7. ie x6543210 --> x0123456 As a consequence B7 & B3 is untouched and there are only 3 swaps needed making the code shorter. Rewriting it for an 8 bit swap as required here, produces - movwf source btfsc source,0 xorlw h'81' btfsc source,7 xorlw h'81' btfsc source,1 xorlw h'42' btfsc source,6 xorlw h'42' btfsc source,2 xorlw h'24' btfsc source,5 xorlw h'24' btfsc source,3 xorlw h'18' btfsc source,4 xorlw h'18' 17 instructions, time varies depending on data but fairly quick. Operation may not be initially obvious. It implements For N = 0 to 3 If bN = 1 then invert bits bN and b(7-N) If b(7-N) = 1 then invert bits bN and b(7-N) Next N Worst case, if both bits in a pair are set it inverts them both twice. Best case, if neither is set it takes no action. This suggests that a more cunning test for "both bits set" may allow a small reduction in run time but intuition suggests that the testing overhead would be excessive.. John's solution is elegant but a little hard on the brain. I prefer Dwayne Reids more brute force - {Quote hidden}> clrf dest > btfsc source,0 > bsf dest,7 > btfsc source,1 > bsf dest,6 > btfsc source,2 > bsf dest,5 > btfsc source,3 > bsf dest,4 > btfsc source,4 > bsf dest,3 > btfsc source,5 > bsf dest,2 > btfsc source,6 > bsf dest,1 > btfsc source,7 > bsf dest,0 because it's much easier for trhe average person to see what is happening at a glance (or a few glances :-) - YMMV). A full 8 bit lookup table is much quicker and much much less code space efficient. A dual nibble lookup table is not much quicker on average and rather less code efficient. Lookup tables have the possible advantage of having a constant implementation time. Russell McMahon _____________________________ >From another world - http://www.easttimor.com What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-)) From: Tracy Smith <KILLspamdot_loKILLspamYAHOO.COM> {Quote hidden}>>I want to change b7 b6 b5 b4 b3 b2 b1 b0 to b0 >b1 b2 b3 b4 b5 b6 b7. >> >>Quickest & most code efficient?? > >How about this... (John Payson) > > movwf source > btfsc source,0 > xorlw h'41' > btfsc source,6 > xorlw h'41' > btfsc source,1 > xorlw h'22' > btfsc source,5 > xorlw h'22' > btfsc source,2 > xorlw h'14' > btfsc source,4 > xorlw h'14'

> John Payson, <blush> I'm NOT John Payson </blush> {Quote hidden}> Are you sure this works? I've tried it with CC5X > and haven't sucess. > > uns8 source; > MOVLW (0b.1101.1001); // this value is just > for test > MOVWF (source); > BTFSC (source.0); > XORLW (0x41); // 0100.0001 > BTFSC (source.6); > XORLW (0x41); // 0100.0001 > BTFSC (source.1); > XORLW (0x22); // 0010.0001 > BTFSC (source.5); > XORLW (0x22); // 0010.0001 > BTFSC (source.2); > XORLW (0x14); // 0001.0100 > BTFSC (source.4); > XORLW (0x14); // 0001.0100 > > It is very ingenious, but seens to have a bug > somewhere. Well it doesn't have a bug, it was intended to reverse seven bits and not eight. I'm no Payson, but it looks like for one more cycle you could do this: rrf source,w rrf source,w btfsc source,0 xorlw h'41' btfsc source,6 xorlw h'41' btfsc source,1 xorlw h'22' btfsc source,5 xorlw h'22' btfsc source,2 xorlw h'14' btfsc source,4 xorlw h'14' .lo ===== __________________________________________________ Do You Yahoo!? Bid and sell for free at http://auctions.yahoo.com

On Fri, 29 Oct 1999 15:10:14 -0700 Keith Causey <RemoveMEffightTakeThisOuTGEOCITIES.COM> writes: >This would probably waste too much port space but you could reverse >the 'w' >register in two instructions if you wired two ports together like >this: >RA0->RB7 >RA1->RB6 >RA2->RB5 >RA3->RB4 >RA4->RB3 >RA5->RB2 >RA6->RB1 >RA7->RB0 >and executed the code sequence >mov RA,W >mov w,RB Aha! A hardware solution! Just like adding a multiplier to the chip! Here's bit reverser hardware... I normally am squeezing everything I can out of I/O though... Harold Harold Hallikainen spamBeGoneharoldspamBeGonehallikainen.com Hallikainen & Friends, Inc. See the FCC Rules at http://hallikainen.com/FccRules and comments filed in LPFM proceeding at http://hallikainen.com/lpfm ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: dl.http://www.juno.com/dynoget/tagj.

> John's solution is elegant but a little hard on the brain. > I prefer Dwayne Reids more brute force - > because it's much easier for trhe average person to see what is happening at > a glance (or a few glances :-) - YMMV). The shift-out-shift-in method is also pretty easy to follow. It is also 1 instruction shorter because you don't have to clear the destination register at the beginning. > Lookup tables have the possible advantage of having a constant > implementation time. The test-each-bit method and the shift-out-shift-in method also have constant execute times. --- Peace, William Kitchen TakeThisOuTbillEraseMEspam_OUTiglobal.net The future is ours to create.

Greetings everybody. ;-) Tracy Smith wrote: > > >I want to change b7 b6 b5 b4 b3 b2 b1 b0 to b0 > b1 b2 b3 b4 b5 b6 b7. > > > >Quickest & most code efficient?? > > How about this... (John Payson) > > movwf source movfw source ;otherwise you put W to source > btfsc source,0 > xorlw h'41' > btfsc source,6 > xorlw h'41' ok. swap 0 with 6 > btfsc source,1 > xorlw h'22' > btfsc source,5 > xorlw h'22' ok. swap 1 with 5 > btfsc source,2 > xorlw h'14' > btfsc source,4 > xorlw h'14' ok. swap 2 with 4 bit 3 is already on right place. Here is another reduced version: swapf source,w ;3210.654 btfsc source,3 ;swap bit 3 back xorlw 0x44 ;bit 7 is expected = 0 btfsc source,6 xorlw 0x05 btfsc source,4 ; xorlw 0x05 btfsc source,2 xorlw 0x50 btfsc source,0 xorlw 0x50 I believe that there is more quick and compact version of this (without ever table lookups) but I've no time to discover that. WBR Dmitry.

Keith Causey wrote: > This would probably waste too much port space but you could reverse the 'w' > register in two instructions if you wired two ports together like this: Alternately, wait for this beast: http://www.atmel.com/atmel/products/prod39.htm (it's an AVR with dual UART, no ADC, FPGA on chip) looks good for custom protocol interfacing at high speed, multiple channel, Video processing .... I want one to play with now, it would make a beaut logic analyser, just add SIMM & graphics LCD regards, Graham Daniel.

part 0 1626 bytes - 16 cycles - 16 words - uses 1 addition byte or RAM. - Enter with byte to be reversed in WREG, ends with reversed byte in WREG. - Isochronous. rlcf WREG,f ; W = d6 d5 d4 d3 d2 d1 d0 C {C = d7} rrcf Reverse,f ; Rev = d7 R7 R6 R5 R4 R3 R2 R1 {C = R0} rlcf WREG,f ; W = d5 d4 d3 d2 d1 d0 C R0 {C = d6} rrcf Reverse,f ; Rev = d6 d7 R7 R6 R5 R4 R3 R2 {C = R1} rlcf WREG,f ; W = d4 d3 d2 d1 d0 C R0 R1 {C = d5} rrcf Reverse,f ; Rev = d5 d6 d7 R7 R6 R5 R4 R3 {C = R2} rlcf WREG,f ; W = d3 d2 d1 d0 C R0 R1 R2 {C = d4} rrcf Reverse,f ; Rev = d4 d5 d6 d7 R7 R6 R5 R4 {C = R3} rlcf WREG,f ; W = d2 d1 d0 C R0 R1 R2 R3 {C = d3} rrcf Reverse,f ; Rev = d3 d4 d5 d6 d7 R7 R6 R5 {C = R4} rlcf WREG,f ; W = d1 d0 C R0 R1 R2 R3 R4 {C = d2} rrcf Reverse,f ; Rev = d2 d3 d4 d5 d6 d7 R7 R6 {C = R5} rlcf WREG,f ; W = d0 C R0 R1 R2 R3 R4 R5 {C = d1} rrcf Reverse,f ; Rev = d1 d2 d3 d4 d5 d6 d7 R7 {C = R6} rlcf WREG,f ; W = C R0 R1 R2 R3 R4 R5 R6 {C = d0} rrcf Reverse,w ; W = d0 d1 d2 d3 d4 d5 d6 d7 {C = R7} - Andy. --------------------------------------------------------- Andrew David, Software Manager, Ultronics Ltd, Cheltenham RemoveMEakdavidTakeThisOuTultronics.co.uk http://www.ultronics.com

--- Andy David <akdavidEraseME.....ULTRONICS.CO.UK> wrote: > - 16 cycles > - 16 words > - uses 1 addition byte or RAM. > - Enter with byte to be reversed in WREG, ends with > reversed byte in > WREG. > - Isochronous. This is a variation of something Steve Hardy posted several years ago. Except Steve's version would reverse two bytes and worked on the Midrange pics as well. IIRC: ; reverse r1 r2 rlf r1,f ; get carry bit and put r1.7 into carry rrf r2,f ; get r1.7 and put r2.0 into carry rlf r1,f ; get r2.0 and put r1.6 into carry rrf r2,f rlf r1,f rrf r2,f rlf r1,f rrf r2,f rlf r1,f rrf r2,f rlf r1,f rrf r2,f rlf r1,f rrf r2,f rlf r1,f rrf r2,f rlf r1,f ; get r2.7 and put the original ; carry bit back. 17 cycles to reverse two bytes .lo ===== __________________________________________________ Do You Yahoo!? Bid and sell for free at http://auctions.yahoo.com

Andy David wrote: > - 16 cycles > - 16 words 17 if you want to reverse two bytes at once. > - Enter with byte to be reversed in WREG, ends with reversed byte in > WREG. No, it ends with the reversed byte in W, not WREG as written, or in Reverse if you specify F as destination. Unless you're talking of a PIC with which I'm not familiar. {Quote hidden}> - Isochronous. > > rlf WREG,f ; WR = d6 d5 d4 d3 d2 d1 d0 C {C = d7} > rrf Reverse,f ; Rev = d7 R7 R6 R5 R4 R3 R2 R1 {C = R0} > rlf WREG,f ; WR = d5 d4 d3 d2 d1 d0 C R0 {C = d6} > rrf Reverse,f ; Rev = d6 d7 R7 R6 R5 R4 R3 R2 {C = R1} > rlf WREG,f ; WR = d4 d3 d2 d1 d0 C R0 R1 {C = d5} > rrf Reverse,f ; Rev = d5 d6 d7 R7 R6 R5 R4 R3 {C = R2} > rlf WREG,f ; WR = d3 d2 d1 d0 C R0 R1 R2 {C = d4} > rrf Reverse,f ; Rev = d4 d5 d6 d7 R7 R6 R5 R4 {C = R3} > rlf WREG,f ; WR = d2 d1 d0 C R0 R1 R2 R3 {C = d3} > rrf Reverse,f ; Rev = d3 d4 d5 d6 d7 R7 R6 R5 {C = R4} > rlf WREG,f ; WR = d1 d0 C R0 R1 R2 R3 R4 {C = d2} > rrf Reverse,f ; Rev = d2 d3 d4 d5 d6 d7 R7 R6 {C = R5} > rlf WREG,f ; WR = d0 C R0 R1 R2 R3 R4 R5 {C = d1} > rrf Reverse,f ; Rev = d1 d2 d3 d4 d5 d6 d7 R7 {C = R6} > rlf WREG,f ; WR = C R0 R1 R2 R3 R4 R5 R6 {C = d0} > rrf Reverse,w ; W = d0 d1 d2 d3 d4 d5 d6 d7 {C = R7} ; 17th instruction tidies everything up: rlf WREG,f ; WR = R0 R1 R2 R3 R4 R5 R6 R7 {C = C¡} -- Cheers, Paul B.