The SetPPUTablesTo0 (Bank 7) subroutine - Elite on the BBC Micro and NES

       Name: SetPPUTablesTo0                                         [Show more]
       Type: Subroutine
   Category: PPU
    Summary: Set nametable 0 and pattern table 0 for drawing the icon bar
  Deep dive: The split-screen mode in NES Elite
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.SetPPUTablesTo0

 LDA #0                 ; Clear bit 7 of setupPPUForIconBar, so this routine
 STA setupPPUForIconBar ; doesn't get called again until the next NMI interrupt
                        ; at the next VBlank (as the SETUP_PPU_FOR_ICON_BAR
                        ; macro and SetupPPUForIconBar routine only update the
                        ; PPU when bit 7 is set)

 LDA ppuCtrlCopy        ; Set A to the current value of PPU_CTRL

 AND #%11101110         ; Clear bits 0 and 4, which will set the base nametable
                        ; address to $2000 (for nametable 0) and the pattern
                        ; table address to $0000 (for pattern table 0)

 STA PPU_CTRL           ; Update PPU_CTRL to set nametable 0 and pattern table 0

 STA ppuCtrlCopy        ; Store the new value of PPU_CTRL in ppuCtrlCopy so we
                        ; can check its value without having to access the PPU

 CLC                    ; Clear the C flag

 RTS                    ; Return from the subroutine

[X]

Configuration variable PPU_CTRL = $2000

The PPU control register, which allows us to choose which nametable and pattern table is being displayed, and to toggle the NMI interrupt at VBlank

[X]

Variable ppuCtrlCopy in workspace ZP

Contains a copy of PPU_CTRL, so we can check the PPU configuration without having to access the PPU

[X]

Variable setupPPUForIconBar in workspace ZP

Controls whether we force the nametable and pattern table to 0 when the PPU starts drawing the icon bar

PPU: SetPPUTablesTo0

[NES version, Bank 7]