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Bit Shifting
Posted: Tue Feb 07, 2023 2:54 pm
by interfind
How can i shift the first left single Bit in rax on Bit to Left
eax = 000000100011111 --> 00000100011111 --> 0000100011111 --> 000100011111 an so on.
If rax = 10000000011111 then the next 1-Bit goes to Left --> 0000000001101111 an so on.
This should done for all Bits set to 1.
The result an the End must be 1111110000000000.
What is the fastest way to do it?
I know i can get the first left Bit in rax with BSR rcx, rax
I hope i have explain it well
Re: Bit Shifting
Posted: Tue Feb 07, 2023 3:42 pm
by juergenkulow
Code: Select all
;BSR rcx,rax
Debug Hex( %000000100011111)
Debug Hex( %00000100011111)
Debug Hex( %0000100011111)
Debug Hex( %000100011111)
Debug Hex( %10000000011111)
Debug Hex(%0000000001101111)
Debug Hex(%1111110000000000)
; 32109876543210
Define regrcx.q, regrax.q=%10000000011111
DisableDebugger:EnableASM
MOV rax,[v_regrax]
BSR rcx,rax
MOV [v_regrcx],rcx
EnableDebugger
Debug regrcx
; 11F
; 11F
; 11F
; 11F
; 201F
; 6F
; FC00
; 13
What do you want to achieve with BSR?
Re: Bit Shifting
Posted: Tue Feb 07, 2023 3:55 pm
by NicTheQuick
interfind wrote: Tue Feb 07, 2023 2:54 pm
How can i shift the first left single Bit in rax on Bit to Left
eax = 000000100011111 --> 00000100011111 --> 0000100011111 --> 000100011111 an so on.
That's all the same numbers.
If rax = 10000000011111 then the next 1-Bit goes to Left --> 0000000001101111 an so on.
This should done for all Bits set to 1.
The result an the End must be 1111110000000000.
I also did not understand what you mean with that.
I hope i have explain it well
Unfortunately not.
Re: Bit Shifting
Posted: Tue Feb 07, 2023 4:14 pm
by NicTheQuick
I can only imagine you mean something like this:
Code: Select all
EnableExplicit
Procedure highestBit(i.i)
i | (i >> 1)
i | (i >> 2)
i | (i >> 4)
i | (i >> 8)
i | (i >> 16)
ProcedureReturn i & ~(i >> 1)
EndProcedure
Define a = %100011111
Define hb
While a
Debug RSet(Bin(a), 32, "0")
hb = highestBit(a)
a = ((hb << 1) | (a & (hb -1))) & ($ffffffff)
Wend
But there are also ASM instructions which can return the highest bit in O(1) instead of writing an extra procedure for that. I just don't know how to use them.
Re: Bit Shifting
Posted: Tue Feb 07, 2023 5:34 pm
by interfind
Your Code is good, but all 1 Bits from the right side
must go to the left side an stay here at the end
of the Procedur.
I like do set the max left Position by myself-
00001111
00010111
00100111
01000111
10000111
00011011
00101011
01001011
10001011
00110011
01010011
10010011
01100011
10100011
11000011
00011101
Final should be
11110000
Re: Bit Shifting
Posted: Tue Feb 07, 2023 5:43 pm
by NicTheQuick
That needs a lot more logic then or I did not get the pattern behind it.
Why do you need that? What's the reason?
Re: Bit Shifting
Posted: Tue Feb 07, 2023 5:53 pm
by NicTheQuick
Does the sequence always start with n · zeros and m · ones concatenated with another and stops in m · ones and n · zeros, so the whole thing reversed in principle? And in between you want to see all possible values you can create using these bits but it has to be in that specific sequence?
Re: Bit Shifting
Posted: Tue Feb 07, 2023 6:18 pm
by NicTheQuick
I think what you are looking for is a function which gives you the next lexicographic permutation of a number.
Here is an explanation:
https://youtu.be/ZRNO-ewsNcQ?t=1004
And here a code:
https://www.geeksforgeeks.org/lexicogra ... on-in-cpp/
Re: Bit Shifting
Posted: Tue Feb 07, 2023 6:31 pm
by interfind
NicTheQuick wrote: Tue Feb 07, 2023 5:53 pm
Does the sequence always start with n · zeros and m · ones concatenated with another and stops in m · ones and n · zeros, so the whole thing reversed in principle? And in between you want to see all possible values you can create using these bits but it has to be in that specific sequence?
Yes , this should be what i'm trying. in exactly that sequence in Assembler.
Re: Bit Shifting
Posted: Tue Feb 07, 2023 6:35 pm
by interfind
Yes, but not in lexicographic rather in the sequence i was showing in my Post's.
Re: Bit Shifting
Posted: Tue Feb 07, 2023 6:45 pm
by NicTheQuick
interfind wrote: Tue Feb 07, 2023 6:31 pm
NicTheQuick wrote: Tue Feb 07, 2023 5:53 pm
Does the sequence always start with n · zeros and m · ones concatenated with another and stops in m · ones and n · zeros, so the whole thing reversed in principle? And in between you want to see all possible values you can create using these bits but it has to be in that specific sequence?
Yes , this should be what i'm trying. in exactly that sequence in Assembler.
Wait. Why in Assembler? Do you not trust compiler optimizations? This seems unnecessary to me.
Re: Bit Shifting
Posted: Tue Feb 07, 2023 6:49 pm
by interfind
NicTheQuick wrote: Tue Feb 07, 2023 6:45 pm
interfind wrote: Tue Feb 07, 2023 6:31 pm
NicTheQuick wrote: Tue Feb 07, 2023 5:53 pm
Does the sequence always start with n · zeros and m · ones concatenated with another and stops in m · ones and n · zeros, so the whole thing reversed in principle? And in between you want to see all possible values you can create using these bits but it has to be in that specific sequence?
Yes , this should be what i'm trying. in exactly that sequence in Assembler.
Wait. Why in Assembler? Do you not trust compiler optimizations? This seems unnecessary to me.
For Speeed. When the sequence goes over 64Bit , this can take more days to finish.
Re: Bit Shifting
Posted: Tue Feb 07, 2023 7:12 pm
by NicTheQuick
Up to how many bits should it work?
Re: Bit Shifting
Posted: Tue Feb 07, 2023 7:17 pm
by interfind
NicTheQuick wrote: Tue Feb 07, 2023 7:12 pm
Up to how many bits should it work?
Up to the 50's Bit Position and up to N (1-26) Bit's set to 1
Re: Bit Shifting
Posted: Tue Feb 07, 2023 8:32 pm
by mk-soft
Where is bit 0 for you?
Bit 0 is always on the far right.