Shift file up, shift file down, FIFO load, and FIFO unload are the common file shift instructions encountered. The shift file up and down instructions are used to construct synchronous word shift registers normally from 1 to 999 words long. In these output instructions, on a false-true transition of a rung decision, the data from the input word will be shifted into file, and the data in the last/first word of the file will be shifted up/down into the output word.
The shift file up instruction is used as a synchronous word shift register. When the instruction rung goes true, the data from a specified input word is shifted into the first word of the file, the data in the file is shifted up one word, and the information of the last word in the file is shifted into specified output word.
This instruction works the same as the up instruction that used as a synchronous word shift register. When the instruction rung goes true, the information from a specified input word is shifted into the last word of the file, the data in the file is shifted down one word, and the information of the first word in the file is shifted into the specified output word.
These two instructions are used together to make an asynchronous word shift register. The FIFO load instruction transfers information from a specified input address into the file, on a false-true transition of a rung decision. The unload FIFO instruction transfers data from a file into a specified output address on a false-true transition of a rung decision.
Load and unload pointers keep track of the load and unload addresses in the FIFO file so that words are taken from the file in the entered, hence the acronym FIFO (first in, first out). The load and unload pointers can be load and unload data at any point in the file. The body of the file will float between these boundaries. The user should not expect to find any particular data entry at any specific data table location in the FIFO file. Only the FIFO input and output addresses are pertinent to the FIFO operation, and they are the only words that should be manipulated by, or examined by, the user.