Drawing the screen: DrawMessageInNMI

       Name: DrawMessageInNMI                                        [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Configure the NMI to send the portion of the screen that contains
             the in-flight message to the PPU (i.e. tile rows 22 to 24)
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ChangeCmdrName calls DrawMessageInNMI
             * ChangeLetter calls DrawMessageInNMI
             * FlightLoop4To16 calls DrawMessageInNMI
             * InputName calls DrawMessageInNMI
             * PrintFlightMessage calls DrawMessageInNMI
             * SetSelectedSystem calls DrawMessageInNMI
             * YESNO calls DrawMessageInNMI


.DrawMessageInNMI

 JSR WaitForPPUToFinish ; Wait until both bitplanes of the screen have been
                        ; sent to the PPU, so the screen is fully updated and
                        ; there is no more data waiting to be sent to the PPU

 LDA firstFreePattern   ; Tell the NMI handler to send pattern entries up to the
 STA lastPattern        ; first free pattern, for both bitplanes
 STA lastPattern+1

 LDA #88                ; Tell the NMI handler to send nametable entries from
 STA firstNameTile      ; tile 88 * 8 = 704 onwards (i.e. from the start of tile
                        ; row 22)

 LDA #100               ; Tell the NMI handler to send nametable entries up to
 STA lastNameTile       ; tile 100 * 8 = 800 (i.e. up to the end of tile row 24)
 STA lastNameTile+1     ; in both bitplanes

 LDA #%11000100         ; Set both bitplane flags as follows:
 STA bitplaneFlags      ;
 STA bitplaneFlags+1    ;   * Bit 2 set   = send tiles up to end of the buffer
                        ;   * Bit 3 clear = don't clear buffers after sending
                        ;   * Bit 4 clear = we've not started sending data yet
                        ;   * Bit 5 clear = we have not yet sent all the data
                        ;   * Bit 6 set   = send both pattern and nametable data
                        ;   * Bit 7 set   = send data to the PPU
                        ;
                        ; Bits 0 and 1 are ignored and are always clear
                        ;
                        ; The NMI handler will now start sending data to the PPU
                        ; according to the above configuration, splitting the
                        ; process across multiple VBlanks if necessary

 JMP WaitForPPUToFinish ; Wait until both bitplanes of the screen have been
                        ; sent to the PPU, so the screen is fully updated and
                        ; there is no more data waiting to be sent to the PPU,
                        ; and return from the subroutine using a tail call