40 lines
1.9 KiB
PHP
Executable File
40 lines
1.9 KiB
PHP
Executable File
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Macro to initialize the NES by disabling rendering and clearing RAM values
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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.macro INIT_NES
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sei ; Disable all IRQ interrupts
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cld ; Clear decimal mode (not supported by the NES)
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ldx #$FF
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txs ; Initialize the stack pointer at address $FF
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inx ; Increment X, causing a rolloff from $FF to $00
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stx PPU_CTRL ; Disable NMI
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stx PPU_MASK ; Disable rendering (masking background and sprites)
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stx $4010 ; Disable DMC IRQs
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lda #$40
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sta $4017 ; Disable APU frame IRQ
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bit PPU_STATUS ; Read from PPU_STATUS to reset the VBlank flag
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Wait1stVBlank: ; Wait for the first VBlank from the PPU
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bit PPU_STATUS ; Perform a bit-wise check with the PPU_STATUS port
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bpl Wait1stVBlank ; Loop until bit-7 (sign bit) is 1 (inside VBlank)
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txa ; A = 0
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ClearRAM:
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sta $0000,x ; Zero RAM addresses from $0000 to $00FF
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sta $0100,x ; Zero RAM addresses from $0100 to $01FF
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sta $0200,x ; Zero RAM addresses from $0200 to $02FF
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sta $0300,x ; Zero RAM addresses from $0300 to $03FF
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sta $0400,x ; Zero RAM addresses from $0400 to $04FF
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sta $0500,x ; Zero RAM addresses from $0500 to $05FF
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sta $0600,x ; Zero RAM addresses from $0600 to $06FF
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sta $0700,x ; Zero RAM addresses from $0700 to $07FF
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inx
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bne ClearRAM
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Wait2ndVBlank: ; Wait for the second VBlank from the PPU
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bit PPU_STATUS ; Perform a bit-wise check with the PPU_STATUS port
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bpl Wait2ndVBlank ; Loop until bit-7 (sign bit) is 1 (inside VBlank)
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.endmacro
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