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Z80 & 68k Assembly =>
z80 & ez80 Assembly
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DarkerLine
ceci n'est pas une |
Super Elite (Last Title)
Joined: 04 Nov 2003
Posts: 8328
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 Posted: 15 Jan 2009 10:35:07 am Post subject: |
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A checksum is just any type of sum calculated from an arbitrarily long data file -- so that if a byte gets changed somehow, the checksum becomes different and you know that something got changed. Adding all the bytes together and ignoring overflow is one way.
Last edited by Guest on 15 Jan 2009 10:35:18 am; edited 1 time in total |
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FloppusMaximus
Advanced Member
Joined: 22 Aug 2008
Posts: 472
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| Posted: 15 Jan 2009 06:08:54 pm Post subject: |
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darkstone knight wrote: isnt the checksum the amount of set bits in a number?
10010000 becomes 2..
What you're thinking of is known as population count or Hamming weight. Fastest way to compute it? LUT, of course. |
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Galandros
Active Member
Joined: 29 Aug 2008
Posts: 565
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| Posted: 29 Jan 2009 01:23:27 pm Post subject: |
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FloppusMaximus wrote: For a simple 8-bit checksum I don't see any way to improve on
Code: xor a
loop:
add a,(hl)
inc hl
djnz loop
(unless of course you know that L + B < 257, in which case use "inc l" instead of "inc hl".)
Why not the sp trick?
Code:
xor a
di
ld (saveSP),sp
ld sp,Data
ldb,number_of_bytes
loop2:
pop hl
add a,h
add a,l
djnz loop2
ld sp,(saveSP)
ei
ret
saveSP:
.dw 0
And if you unroll more like this:
Code:
xor a
di
ld (saveSP),sp
ld sp,Data
ldb,number_of_bytes
loop2:
pop hl
pop de
add a,h
add a,l
add a,d
add a,e
djnz loop2
ld sp,(saveSP)
ei
ret
saveSP:
.dw 0
Better. |
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FloppusMaximus
Advanced Member
Joined: 22 Aug 2008
Posts: 472
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| Posted: 29 Jan 2009 11:47:06 pm Post subject: |
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Good point. I don't tend to think of using the stack pointer in unusual ways, which is perhaps a flaw in my coding style. (Although, to be honest, the only time I would consider doing so is in extremely speed-optimized code, which I don't write very often.)
In any case, this thread is supposed to be about obfuscation! Let's see some more bizarre-looking code! Has anyone ever written a (real, useful) program that uses the CPD instruction? I don't think I have. |
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Galandros
Active Member
Joined: 29 Aug 2008
Posts: 565
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| Posted: 30 Jan 2009 07:43:37 am Post subject: |
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FloppusMaximus wrote: Good point. I don't tend to think of using the stack pointer in unusual ways, which is perhaps a flaw in my coding style. (Although, to be honest, the only time I would consider doing so is in extremely speed-optimized code, which I don't write very often.)
In any case, this thread is supposed to be about obfuscation! Let's see some more bizarre-looking code! Has anyone ever written a (real, useful) program that uses the CPD instruction? I don't think I have. Stack and shadow registers combined use can be used to speed lightning code. While sp retrieves data from memory, shadow registers compensate the pushes and pops and/or you feel like having more registers to play with. The only down is you must have interrupts disabled, and so no gray...
And in most ASM tasks, you don't need desperate speed increase. It is better to optimise for size. It is a thing I appreciate very much in programs.
I will look to the CPD. Also, I have ideas for obfuscating code. Not forgotten... |
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