NASM - The Netwide Assembler
NASM Forum => Example Code => Topic started by: mik3ca on February 14, 2021, 06:42:13 PM
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Since I'm dealing with data of an opposite endian, I found it beneficial to flip entire strings of input so I can process 16-bit data structures without flipping bits.
This is the code I came up with to flip a string so it comes out in reverse. Note that I'm working in a real DOS environment so I have to use segments and offsets.
Anyways, This code uses ES:SI as the pointer to the string, and CX as the size.
Size is rounded up to the next even number so flip works properly.
I'm just wondering if there is a solution that can give me better speed.
flipit:
cmp CX,0
je nflip
push DI
push SI
mov DI,SI
rcr CX,1
shl CX,1
jnc flst
add CX,2
flst:
add DI,CX
flst2:
dec DI
mov AL,[ES:DI]
mov AH,[ES:SI]
mov [ES:DI],AH
mov [ES:SI],AL
inc SI
loop flst2
pop SI
pop DI
nflip:
ret
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Not so simple...
Even if the pair ES:SI is normalized (SI is lower as possible), what will happen if SI or DI overflows (or underflows)?
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We can use the reversed() function to get the reverse of the string in a list and then use the . join() function to concatenate all the characters to our original string. Time Complexity of this approach is O(N) where N is the length of the string to be reversed.
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I really fail to see why NOT to use BSWAP and ROL (or ROR) instructions to change thee endianess of DWORDs and WORDs...
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Ahhhh, just a reminder: Since Nehalem microarchitecture (first generation of Core i5, released in 2008), processors have the MOVBE instruction (MOV Big Endian):
movbe reg32,mem32
movbe mem32, reg32
movbe reg16,mem16
movbe mem16,reg16