Post by thread:[PICLIST] Challenge: Shifting by a variable amount

In C: unsigned char x; unsigned char y; ... x <<= y; How would you do this in PIC assembly? This has been discussed here before (but I couldn't find it in the archives) and IIRC a 12 cycle solution was found (by Dmitry?). I've got a 12-cycle solution, but thought it would be interesting to propose this as a challenge. (The winner may simply find the answer in the archives :). Scott -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> movlw amount_to_shift > movwf y > shiftit > rlf x, f > decfsz y, f > goto shiftit I think you may want to add: shiftit bcf status,c ... ... goto shiftit The rlf rotates through the carry, and we just want a shift, so it's necessary to clear the carry bit before each rotate. -Steve -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Wed, 26 Sep 2001, Regulus Berdin wrote: Reggie, welcome back. {Quote hidden}> Hi Scott, > > I'll try (untested): > > ;x <<= y; > > clrc > btfsc y,0 > rlf x,f ;x = x*2 > > clrc > rlf x,w ;w = x*2 > addwf x,w ;w = w+x = x*2+x = x*3 > btfsc y,1 > addwf x,f ;x = w+x = x*3+x = x*4 > > swapf x,w > andlw 0xF0 > btfsc y,2 > movwf x ;x=x*8 Interesting. Mine is slightly different and is suitable for this type of operation: unsigned char x; unsigned char y; unsigned char z; z = x << y; swapf x,w andlw 0xf0 btfss y,2 movf x,w movwf z btfss y,0 addwf z,f rlf z,w andlw 0xfe addwf z,w btfsc y,1 addwf x,f ----------------- Dmitry, The "unsigned char" implies an 8-bit register (at least on all of the C compilers that target a PIC that I know of). Only the lower three bits of the shift count are examined. It looks like an additional 4 instructions would be required to verify that the upper bits are not set. ------ Now, to do the same thing in a looping snippet: ; if x and z are different registers movf x,w movwf z comf y,w ; the shift count ; note w= 0-y+1 rrf z,f loop: rlf z,f addlw 1 skpc goto loop Scott -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Now thats mere rotation, shift is needed,so add a clrc. Even then, Scott was talking of the number of Cycles. Your thing would take 2 + ( (5 * n) - 1 ) cycles. Where n is the number of shifts. 21 cycles for shifting 4 bits. I've got a small little optimization to this. When you shift more than or eqal to 4 times, a swapf would make things a lot simpler. Herez my way of doing it. <code> movlw amount_to_shift movwf y movlw 0x04 subwf y,w skpc goto slow_shift ; 8 cycles ; 6 cycles swapf x,w andlw 0xF0 movwf x movlw 0x04 subwf y,f ; 11 cycles slow_shift tstf y skpnz goto shift_done ; 16 cycles clrc rlf x,f decf y,f goto slow_shift shift_done </code> It takes 16 cycles for shift by 4. Not the best solution. But hey I just coded it on the fly. Jeethu Rao > {Original Message removed}

Hi guys. ;) {Quote hidden}> > Hi Scott, > > > > I'll try (untested): > > > > ;x <<= y; > > > > clrc > > btfsc y,0 > > rlf x,f ;x = x*2 > > > > clrc > > rlf x,w ;w = x*2 > > addwf x,w ;w = w+x = x*2+x = x*3 > > btfsc y,1 > > addwf x,f ;x = w+x = x*3+x = x*4 > > > > swapf x,w > > andlw 0xF0 > > btfsc y,2 > > movwf x ;x=x*8 This will not work. swapf block has to be first. Otherwise you'll be possibly destroying right sequency of bits. {Quote hidden}> Interesting. Mine is slightly different and is suitable for this type of > operation: > > > unsigned char x; > unsigned char y; > unsigned char z; > > > z = x << y; > > > swapf x,w > andlw 0xf0 > btfss y,2 > movf x,w > > movwf z > btfss y,0 > addwf z,f > > rlf z,w > andlw 0xfe > addwf z,w > btfsc y,1 > addwf x,f I think not x but z is suggested in last operation. > Dmitry, The "unsigned char" implies an 8-bit register (at least on all of > the C compilers that target a PIC that I know of). Only the lower three > bits of the shift count are examined. It looks like an additional 4 > instructions would be required to verify that the upper bits are not set. Yep. I was asking this sort of question to clarify is this arithmetical shift operation (left shift with incoming zeroes) or logical (from carry rolled over from highest bit) Anyway number 12 is still looking magical doesn't it ? ;) WBR Dmitry. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

On Thu, 27 Sep 2001, Dmitry Kiryashov wrote: {Quote hidden}> > > > unsigned char x; > > unsigned char y; > > unsigned char z; > > > > > > z = x << y; > > > > > > swapf x,w > > andlw 0xf0 > > btfss y,2 > > movf x,w > > > > movwf z > > btfss y,0 > > addwf z,f > > > > rlf z,w > > andlw 0xfe > > addwf z,w > > btfsc y,1 > > addwf x,f > > I think not x but z is suggested in last operation. You're right, of course. > Anyway number 12 is still looking magical doesn't it ? ;) Mystical! Scott -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

On Thu, 27 Sep 2001, Jeethu Rao wrote: > Hey Scott, are you doing this for SDCC, > Since shifts are very essential in C ? > I just got this thought now! :). void shift_left_var(void) { achar0 <<= achar1; } produces: (copious debugging junk left in...) ;_shift_left_var: _shift_left_var ;Function start ; 1 exit point ;;ic ;; rotate2.c 25 ;; *** resultRemat 2320 ;; *** genLeftShift 6032 ;; 607 ;; 630 - true symop ;; *** aopForSym 341 ;; 427 sym->rname = _achar1, size = 1 ;mov b,_achar1 ;inc b ;; 607 ;; 630 - true symop ;; *** aopForSym 341 ;; 427 sym->rname = _achar0, size = 1 ;; 607 ;; 623 SWAPF _achar0,W ANDLW 0xf0 BTFSS _achar1,2 MOVF _achar0,W MOVWF _achar0 BTFSS _achar1,0 ADDWF _achar0,F RLF _achar0,W ANDLW 0xfe ADDWF _achar0,W BTFSC _achar1,1 ADDWF _achar0,F Yes I'm doing this for SDCC. This code has been tested in the regression suite as well. I thought about posting the snippets for shifting by a constant amount too. Scott -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body