How to convert synchsafe integer to integer?

[Igor]

Synchsafe integers are integers that keep its highest bit (bit 7) zeroed, making seven bits out of eight available. Thus a 32 bit synchsafe integer can store 28 bits of information.

255 (00000000 11111111) encoded as a synchsafe integer is 383 (00000001 01111111).

Can someone help me convert this "01111111 01111111 01111111 01111111" to this "00001111 11111111 11111111 11111111"?

[jj2007]

Try this:

Code Select Expand

mov ecx, 01111111011111110111111101111111b
bswap ecx
movzx eax, cl ; eax=01111111b
REPEAT 3
shl eax, 7 ; move 7 bits left
shr ecx, 8
or al, cl
ENDM


[dedndave]

Code Select Expand

;EAX = synchsafe integer

mov ecx,eax
shl al,1
shl ax,1
and ecx,7F000000h
shl eax,1
and eax,0FFFFF8h
or  eax,ecx
shr eax,3

;EAX = binary integer

[Igor]

Both solutions work fine, thank you guys.  :U


Forgot to mention that this "synchsafe integer" is from ID3v2.4 for tags in mp3 files, in case someone else tries to search for it.

[drizz]

it sounds familiar...

http://www.asmcommunity.net/board/index.php?topic=29696.msg209703#msg209703

[Igor]

Wou, nice thread and very good code, thanks.