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⌂ ∞ Elite on the BBC Micro and NES

Stardust: nWq

[NES version, Bank 0]

       Name: nWq                                                     [Show more]
       Type: Subroutine
   Category: Stardust
    Summary: Create a random cloud of stardust

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DEATH calls nWq




 Create a random cloud of stardust containing the correct number of dust
 particles, i.e. NOSTM of them, which is 3 in witchspace and 20 (#NOST) in
 normal space. Also hides ships from the screen.

 This is called by the DEATH routine when it displays our untimely demise.



.nWq

 LDA nmiCounter         ; Set the random number seeds to a fairly random state
 CLC                    ; that's based on the NMI counter (which increments
 ADC RAND               ; every VBlank, so will be pretty random)
 STA RAND
 LDA nmiCounter
 STA RAND+1

 LDY NOSTM              ; Set Y to the current number of stardust particles, so
                        ; we can use it as a counter through all the stardust

.SAL4

 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

 JSR DORND              ; Set A and X to random numbers

 ORA #8                 ; Set A so that it's at least 8

 STA SZ,Y               ; Store A in the Y-th particle's z_hi coordinate at
                        ; SZ+Y, so the particle appears in front of us

 STA ZZ                 ; Set ZZ to the particle's z_hi coordinate

 JSR DORND              ; Set A and X to random numbers

 ORA #16                ; Set A so that it's at least 16

 AND #%11111000         ; Zero bits 0 to 2 of A so that it's a multiple of 8

 STA SX,Y               ; Store A in the Y-th particle's x_hi coordinate at
                        ; SX+Y, so the particle appears in front of us

 JSR DORND              ; Set A and X to random numbers

 STA SY,Y               ; Store A in the Y-th particle's y_hi coordinate at
                        ; SY+Y, so the particle appears in front of us

 STA SXL,Y              ; Store A in the low bytes of the Y-th particle's three
 STA SYL,Y              ; coordinates
 STA SZL,Y

 DEY                    ; Decrement the counter to point to the next particle of
                        ; stardust

 BNE SAL4               ; Loop back to SAL4 until we have randomised all the
                        ; stardust particles

                        ; Fall through into WPSHPS to hide ships from the screen