Att sub-routine calls a Delay routine that contains an instruction to go to Preload when either button is pressed.
The program below shows Preload sub-routine. Preload puts "5" into file 30h and clears the 9 files for the display. Read below for the purpose of files 30h and 3Ah. The following diagram shows the board-files and the two "hidden" files:

When either button is pressed during Attract Mode,  the micro goes to Tic Tac Toe section of the program. The loop routine for Tic Tac Toe is Main.  It is accessed via Preload sub-routine.
The effect you will see on the Tic Tac Toe project is this: The display goes blank. When button "A" is pressed, a flashing orange cursor starts at the top left square. Each time button "A" is pressed, it increments across each row until it reaches the bottom right-hand square. It then disappears and starts at the top left square.
When button "B" is pressed, the flashing cursor is turned into a red "Player" piece. The micro then executes a number of sub-routines and produces a green "Computer" piece on the display.
Main is a loop routine similar to a RASTER on a TV. It takes information from 9 files (31h, 32h, 33h, 34h, 35h, 36h, 37h, 38h, and 39h) and displays it by showing the top row, middle row and bottom row. This is repeated at a rapid rate so that all rows appear to be illuminated at the same time. This is called SCANNING or MULTIPLEXING.
The cursor actually starts in file 30h. This file is not displayed and thus the cursor is "hidden." At the end of the display it is transferred to file 3A and thus it is removed from the display. The program returns the cursor to file 30h.
Main is not an "intelligent" routine. It's a simple "display" routine. It simply shows whatever is contained in the 9 display files.
A file can contain one of four values.
If it contains the value 01, bit0 is set and this represents the player. When bit0 is set, the red element of a LED will be illuminated, via Main sub-routine. We will see in a moment, how this is done.
If it contains the value 04, bit2 is set and this represents the computer. When bit2 is set, the green element of a LED will be illuminated, via Main sub-routine.
If it contains the value 05, bits 0and 2 are set and this represents the cursor. The LED will display "orange."
If it contains the value 08, bit3 is set and this represents the "hidden cursor."
On page 2 of this project, the activation of the display is explained. The six elements of the three top LEDs are taken to 6 lines of port B and when the corresponding "sinking" transistor is turned on, the LEDs will illuminate.
A table for the 64 combinations is also included and this shows how the different patterns are produced.
The display routine in Main uses bits 0 and 2 of the first three files to turn on the red and green emitters in top 3 LEDs. The sinking transistor is turned on and the top row is displayed. This is repeated with the middle and bottom rows.
To make the cursor flash, the program needs to know where it is located. When the cursor is visible on the display, it has the value 05. This represents the red and green crystals in a LED. The program turns on the top, middle and bottom rows for 20h loops (thirty-two loops)
After 20h loops, the micro goes to Main 19 where it executes ten loops, looking for 05, by testing bits 0 and 2. When 05 is found, it puts 08 into the file and returns to Main. If 05 is not found, the micro looks for 08. When 08 is found, it puts 05 in the file and returns to Main. This produces a flashing cursor with an equal on/off ratio. The cursor value is also changed, even when it is hidden in file 30h.