Hello. How to right or left shift a sequence of bytes by specified N bits effectively? Suggestions ?

Have a look at shld and shrd. You can chain them together to make multiple {q|d}word shifts. If the shift amount is more than 32 bits (or 64 bits for 64-bit code) then shift by whole units and add the sh{l|r}d to the last few bits.

Yes, it might work. Thank you.

The FASMG source code has a routine to do multi-byte shift.

See EXPRESSIONS.INC, shift_term_value_left: and shift_term_value_right:

These routines are sub-optimal for larger inputs, since they shift just a single byte per loop iteration. But I preferred to keep them simple, as optimizing them would have little effect on the overall performance of the assembler. For longer inputs SHLD/SHRD are probably a good recommendation. And fasm 1 uses SHLD/SHRD in its SHL/SHR operators implementation.