PPU.6502

;-
; Everything related to interfacing with the PPU goes here
;-

	.rsset $0010
ppu_command_count  .rs 1 ; 11
ppu_commands_hi    .rs 20 ; 42
ppu_commands_lo    .rs 20 ; 72
ppu_commands_value .rs 20 ; 102
ppu_temp_command_hi   .rs 1 ; 43
ppu_temp_command_lo   .rs 1 ; 44
ppu_temp_command_value .rs 1 ; 45

; Wait for the VBlamk
ppu_VBlankWait:
	BIT PPU_STATUS_REG
	BPL ppu_VBlankWait
	RTS

; A = value, X = hi, Y = lo
ppu_AddCommand:
	STX ppu_temp_command_hi
	STY ppu_temp_command_lo
	STA ppu_temp_command_value

	LDX ppu_command_count
	LDA ppu_temp_command_hi
	STA ppu_commands_hi, X
	LDA ppu_temp_command_lo
	STA ppu_commands_lo, X
	LDA ppu_temp_command_value
	STA ppu_commands_value, X

	INX
	STX ppu_command_count

	RTS

ppu_ProcessCommands:
	LDX ppu_command_count
	CPX #$00
	BEZ COMMANDS_CONSUMED

	LDX #$00
	CONSUME_COMMANDS:
		LDA PPU_STATUS_REG
		LDA	ppu_commands_hi, X
		STA PPU_VIDEO_RAM_ADDRESS_REG_2
		LDA ppu_commands_lo, X
		STA PPU_VIDEO_RAM_ADDRESS_REG_2
		LDA ppu_commands_value, X
		STA PPU_VIDEO_RAM_IO_REG

		INX
		CPX ppu_command_count
		BNZ CONSUME_COMMANDS

	LDA #$00
	STA ppu_command_count

	COMMANDS_CONSUMED:
	RTS

ppu_SetPPUState:
	; This is the PPU clean up section, so rendering the next frame starts properly.
	LDA #%10001000   ; enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
	STA PPU_CONTROL_REG_1
	LDA #%00011110   ; enable sprites, enable background, no clipping on left side
	STA PPU_CONTROL_REG_2
	LDA #$00        ;;tell the ppu there is no background scrolling
	STA PPU_VIDEO_RAM_ADDRESS_REG_2
	STA PPU_VIDEO_RAM_ADDRESS_REG_2
	STA PPU_VIDEO_RAM_ADDRESS_REG_1
	STA PPU_VIDEO_RAM_ADDRESS_REG_1
	RTS

ppu_Init:
	JSR ppu_SetPPUState
	RTS
	
; NMI interupt
NMI:
	; Save all of our register states, we'll need them when we return from the interupt
	PHA
	TXA
	PHA
	TYA
	PHA

	PPU_DMA:
		LDA #SPRITE_DMA_REGION_LO
		STA PPU_STATUS_REG  ; set the low byte (00) of the RAM address
		LDA #SPRITE_DMA_REGION_HI
		STA PPU_SPRITE_DMA  ; set the high byte (02) of the RAM address, start the transfer

	JSR ppu_ProcessCommands
	JSR ppu_SetPPUState
		
	PLA
	TAY
	PLA
	TAX
	PLA
	
	RTI