Main loop: Main flight loop (Part 1 of 16)

       Name: Main flight loop (Part 1 of 16)                         [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: Seed the random number generator
  Deep dive: Program flow of the main game loop
             Generating random numbers
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main game loop (Part 2 of 6) calls via M%


 The main flight loop covers most of the flight-specific aspects of Elite. This
 section covers the following:

   * Seed the random number generator


 Other entry points:

   M%                   The entry point for the main flight loop


.M%

 LDA QQ11               ; If this is not the space view, jump to main1 to skip
 BNE main1              ; the following, as we only need to flip the drawing
                        ; bitplane for animating the space view

 JSR FlipDrawingPlane   ; Flip the drawing bitplane so we draw into the bitplane
                        ; that isn't visible on-screen

.main1

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA K%                 ; We want to seed the random number generator with a
                        ; pretty random number, so fetch the contents of K%,
                        ; which is the x_lo coordinate of the planet. This value
                        ; will be fairly unpredictable, so it's a pretty good
                        ; candidate

 EOR nmiTimerLo         ; EOR the value of K% with the low byte of the NMI
                        ; timer, which gets updated by the NMI interrupt
                        ; routine, so this will be fairly unpredictable too

 STA RAND               ; Store the seed in the first byte of the four-byte
                        ; random number seed that's stored in RAND