sooooooooorrry, i read byte swapping and went for swap. Rob, you are right, the fastest way to reverse the bits in a byte is a table - smile Mahmood - this is abt as fast in C as in assembler.
A discussion of various methods can be found at
http://www.semiconductors.philips.com/acrobat/a.../AN709.pdf

Albeit for the XA, all applies well to the '51


Erik