1. Shift Register Instructions • FFL FIFO Load • FFU FIFO Unload • LFL LIFO Load • LFU LIFO Unload • The FIFO is a First In First Out Register used to buffer data. • The LIFO is a Last In First Out Register used as a stack

2. FIFO Load (FFL) • On a false-to-true rung transition, the FFL instruction loads words into a user-created file called a FIFO stack. This instruction’s counterpart, FIFO unload (FFU), is paired with a given FFL instruction to remove elements from the FIFO stack.

3. FIFO Unload (FFU) • On a false-to-true rung transition, the FFU instruction unloads words from a user-created file called a FIFO stack. The data is unloaded using first-in, first-out order. After the unload completes, the data in the stack is shifted one element toward the top of the stack and the last element is zeroed out.

4. For asynchronous type lines, such as pallet (assembly) lines in which individual pallet are stopped and released in zones, the FIFO instructions are a better choice for a data tracking system than the LIFO instructions. The designers must be careful to never let the FIFO get out of sync because it’s not self-clearing and will cause data to be skewed forever until the FIFO is clear

6. LIFO Load (LFL) • On a false-to-true rung transition, the LFL instruction loads words into a user-created file called a LIFO stack. This instruction’s counterpart, LIFO unload (LFU), is paired with a given LFL instruction to remove elements from the LIFO stack.

7. LIFO Unload (LFU) • On a false-to-true rung transition, the LFU instruction unloads words or long words from a user-created file called a LIFO stack. The data is unloaded using last-in, first-out order.

8. One of the main uses of LIFO instruction is for the temporary storage of data words between functions or process when tracking item on a stack. One program can save a block of data in an array and then allow a second program or process to retrieve the data when control is passed to the other program