VHDL Programming

324 Chapter Thirteen

Shift

The next device to be described is the shiftentity. The shiftentity is

used to perform shifting and rotation operations within the CPU. The

shiftentity has a 16-bit input bus, a 16-bit output bus, and a selinput

that determines which shift operation to perform. This is shown by the

symbol in Figure 13-9.

The types of shift operations that can be performed by the shiftentity

are shown in Figures 13-10 and 13-11.

As can be seen by the figures, the shiftentity can perform a shift left,

shift right, rotate left, and rotate right operation. One operation that is

not shown by the figures is a pass through operation in which all input

bits are passed through to the output unchanged. Following is an entity

that performs these operations:

library IEEE;

use IEEE.std_logic_1164.all;

use work.cpu_lib.all;

entity shift is

port ( a : in bit16;

port( sel : in t_shift;

port( y : out bit16);

end shift;

architecture rtl of shift is

begin

shftproc: process(a, sel)

begin

case sel is

when shftpass =>

y <= a after 1 ns;

when shl =>

y <= a(14 downto 0) & ‘ 0 ’ after 1 ns;

when shr =>

y <= ‘ 0 ’ & a(15 downto 1) after 1 ns;

y

a

sel

Shift

Figure 13-9
Shift Symbol.