VHDL Programming

Shift Operators

VHDL87 did not contain operators to allow shifting or rotating. Most of

these functions were built by VHDL standard package creators. Without

the built-in operators however, overloaded shift and rotate operators were

not possible. VHDL93 includes built-in shift and rotate operators:sll

(shift left logical),srl (shift right logical),sla (shift left arithmetic),sra

(shift right arithmetic),rol (rotate left), and ror (rotate right). These op-

erators allow shifting and rotating operations for any one-dimensional ar-

ray type. These operators work as follows:

SLL — shift left logical

q <= a SLL b;

qequals ashifted left by bbits and filled on the right with the value

a’left. If bis negative then ais shifted right.

SRL — shift right logical

q <= a SRL b;

qequals ashifted right by bbits and filled on the left with the value

a’left. If bis negative then ais shifted left.

SLA — shift left arithmetic

q <= a SLA b;

qequals ashifted left by bbits and filled on the right with a(a’right).

If bis negative then ais shifted right.

SRA — shift right arithmetic

q <= a SRA b;

qequals ashifted right by bbits and filled on the left with a(a’left).

If bis negative then ais shifted left.

Appendix D: VHDL93 Updates 463