Question on NOT

[xoreaxeax]

Are the the following lines equivalent?

and eax, NOT 31

and eax,-32

I found the first line in some code that converts a bitmap image to a file and was just wondering why it was written that way.

I also found the formula in C and it looks like they do it the same exact way too:

int bytes=(((24*bitmap_dx+31)&(~31)/8)*bitmap_dy;
                                                  ^^^^

[qWord]

Are the the following lines equivalent?

yes.
See also Two's complement

[dedndave]

they are the same, in terms of math
but - slightly different, if you were to want to understand or modify the code, later
i prefer the -32 form   :P

as for NOT 31, why not just say 0FFFFFFE0h

i am pretty sure these instructions all code as 3-byte opcodes, no matter how you write it

the end result is....
the length of each line (row) in bitmap image data is always evenly divisable by 4
i.e., if the image data for a line requires 15 bytes, the line data must be 16 bytes in length
it's one of the "rules" for bitmap image data

[jj2007]

Look at the binary representations:

  PrintLine Bin$(-32)
  PrintLine Bin$(not 31, f)

Result:
  11111111111111111111111111100000
  11111111111111111111111111100000
  10987654321098765432109876543210

By the way, and al, not 31 and and al, -32 also do the job (but they might be slower).

[dedndave]

yes - i use AND AL,-4, and it seems to work well

Code Select Expand

int bytes=(((24*bitmap_dx+31)&(~31)/8)*bitmap_dy

seems like the equation is a little over-complicated, to begin with
for a 24-bit image....

Code Select Expand

(((3 * bitmap_dx) +3) AND -4) * bitmap_dy
you could also use

Code Select Expand

((3 * (bitmap_dx +1)) AND -4) * bitmap_dy