Skip to navigation

⌂ ∞ Elite on the BBC Micro and NES

Drawing the screen: DrawScreenInNMI

[NES version, Bank 0]

       Name: DrawScreenInNMI                                         [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Configure the NMI handler to draw the screen
  Deep dive: Views and view types in NES Elite

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BuyAndSellCargo calls DrawScreenInNMI
             * DrawScreenInNMI_b0 calls DrawScreenInNMI
             * eq calls DrawScreenInNMI
             * EQSHP calls DrawScreenInNMI
             * HighlightLaserView calls DrawScreenInNMI
             * HME2 calls DrawScreenInNMI
             * qv calls DrawScreenInNMI
             * TT102 calls DrawScreenInNMI
             * TT147 calls DrawScreenInNMI
             * UpdateEquipment calls DrawScreenInNMI




.DrawScreenInNMI

 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 #0                 ; Tell the NMI handler to send nametable entries from
 STA firstNameTile      ; tile 0 onwards

 LDA #100               ; Tell the NMI handler to only clear nametable entries
 STA maxNameTileToClear ; up to tile 100 * 8 = 800 (i.e. up to the end of tile
                        ; row 24)

 LDA #37                ; Tell the NMI handler to send pattern entries from
 STA firstPattern       ; pattern 37 in the buffer

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

 JSR DrawBoxEdges       ; Draw the left and right edges of the box along the
                        ; sides of the screen, drawing into the nametable buffer
                        ; for the drawing bitplane

 JSR CopyNameBuffer0To1 ; Copy the contents of nametable buffer 0 to nametable
                        ; buffer

 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

 LDA firstFreePattern   ; Tell the NMI handler to send pattern entries from the
 STA firstPattern       ; first free pattern onwards, so we don't waste time
                        ; resending the static patterns we have already sent

 RTS                    ; Return from the subroutine