mirror of
https://github.com/sehugg/8bitworkshop.git
synced 2024-12-01 13:50:30 +00:00
103 lines
1.9 KiB
NASM
103 lines
1.9 KiB
NASM
|
|
;;;;; CONSTANTS
|
|
|
|
PPU_CTRL equ $2000
|
|
PPU_MASK equ $2001
|
|
PPU_STATUS equ $2002
|
|
PPU_SCROLL equ $2005
|
|
PPU_ADDR equ $2006
|
|
PPU_DATA equ $2007
|
|
DMC_FREQ equ $4010
|
|
|
|
;;;;; CARTRIDGE FILE HEADER
|
|
|
|
processor 6502
|
|
seg Header
|
|
org $7FF0
|
|
|
|
NES_MAPPER equ 0 ;mapper number
|
|
NES_PRG_BANKS equ 2 ;number of 16K PRG banks, change to 2 for NROM256
|
|
NES_CHR_BANKS equ 1 ;number of 8K CHR banks (0 = RAM)
|
|
NES_MIRRORING equ 1 ;0 horizontal, 1 vertical, 8 four screen
|
|
|
|
.byte $4e,$45,$53,$1a ; header
|
|
.byte NES_PRG_BANKS
|
|
.byte NES_CHR_BANKS
|
|
.byte NES_MIRRORING|(NES_MAPPER<<4)
|
|
.byte NES_MAPPER&$f0
|
|
|
|
;;;;; CODE
|
|
|
|
seg Code
|
|
org $8000
|
|
start:
|
|
_exit:
|
|
sei ;disable IRQs
|
|
cld ;decimal mode not supported
|
|
ldx #$ff
|
|
txs ;set up stack pointer
|
|
inx ;increment X to 0
|
|
stx PPU_MASK ;disable rendering
|
|
stx DMC_FREQ ;disable DMC interrupts
|
|
stx PPU_CTRL ;disable NMI interrupts
|
|
jsr WaitSyncSafe ;wait for VSYNC
|
|
; clear RAM -- not a subroutine because we clear the stack too
|
|
lda #0
|
|
tax
|
|
.clearRAM
|
|
sta $0,x
|
|
sta $100,x
|
|
; skip $200-$2FF, used for OAM display list
|
|
sta $300,x
|
|
sta $400,x
|
|
sta $500,x
|
|
sta $600,x
|
|
sta $700,x
|
|
inx
|
|
bne .clearRAM
|
|
; wait for PPU warmup
|
|
jsr WaitSync
|
|
; set palette background
|
|
ldy #$0
|
|
lda #$3f
|
|
sta PPU_ADDR
|
|
sty PPU_ADDR
|
|
lda #$1c
|
|
sta PPU_DATA
|
|
; enable PPU rendering
|
|
lda #0
|
|
sta PPU_ADDR
|
|
sta PPU_ADDR ;PPU addr = 0
|
|
sta PPU_SCROLL
|
|
sta PPU_SCROLL ;scroll = 0
|
|
lda #$90
|
|
sta PPU_CTRL ;enable NMI
|
|
lda #$1e
|
|
sta PPU_MASK ;enable rendering
|
|
.endless
|
|
jmp .endless ;endless loop
|
|
|
|
;;;;; SUBROUTINES
|
|
|
|
; wait for VSYNC to start
|
|
WaitSyncSafe: subroutine
|
|
bit PPU_STATUS
|
|
WaitSync:
|
|
bit PPU_STATUS
|
|
bpl WaitSync
|
|
rts
|
|
|
|
;;;;; INTERRUPT HANDLERS
|
|
|
|
nmi:
|
|
irq:
|
|
rti
|
|
|
|
;;;;; CPU VECTORS
|
|
|
|
org $fffa
|
|
.word nmi ;$fffa vblank nmi
|
|
.word start ;$fffc reset
|
|
.word irq ;$fffe irq / brk
|
|
|