When the SNES is powered on or reset, the 65c816 will jump to the RESET CPU vector (in bank 0) in 65C02 emulation mode.
At minimum, the Reset Handler should do the following:
- Preform a long jump into a FastROM bank (if the ROM speed is FastROM)
- Disable the emulation flag (switch to 65816 native mode)
- Setup the status flags. Specifically the register size flags, decimal mode flag (cleared) and irq disable flag (set).
- Setup the Stack Pointer
- Setup the Direct Page and Data Bank registers
- Disable interrupts and rendering
Afterwords, it is recommended you:
- Clear Work-RAM using multiple fixed address DMA transfers to WMDATA. The code should be inside the Reset Handler, not inside a subroutine and before any subroutines are called as the DMA transfers would override the stack. Ensure the DMA transfers do not override any memory you would like to survive a reset (ie, high-scores).
- Reset the PPU registers to a known good state
- Clear VRAM with a fixed address DMA transfer
- Clear CGRAM with a fixed address DMA transfer
- Populate OAM with values that will ensure all sprites are off-screen
- Jump to Main
ResetHandler: jml _FastRomReset ; Change Program Bank to a FastROM bank _FastRomReset: clc xce ; Switch to native mode ; Reset Status Flags ; n (negative flag) clear ; v (overflow flag) clear ; m (memory size) set - 8 bit Accumulator ; x (index size) clear - 16 bit Index registers ; d (decimal mode) clear - decimal mode disabled ; i (irq disable) set - IRQ interrupts disabled ; z (zero flag) clear ; c (carry flag) clear rep #$ff sep #$24 ; 8 bit accumulator ; 16 bit index ldx #STACK_BOTTOM txs ; Set up Stack Pointer pea 0 pld ; Reset Direct Page register to 0 phk plb ; Set Data Bank to Program Bank ; addr addressing mode can now access PPU/MMIO/DMA registers if PB is $00 - $3F or $80 - $BF stz NMITIMEN ; Disable interrupts stz HDMAEN ; Disable HDMA lda #$8f sta INIDISP ; Disable screen ; Fill Work-RAM with zeros using two 64KiB fixed address DMA transfers to WMDATA stz WMADDL stz WMADDM stz WMADDH ; Set VMDATA address lda #$08 sta DMAP0 ; Fixed address transfer to a byte register lda #WMDATA & 0xff sta BBAD0 ; DMA transfer to WMDATA ldx #WorkRamResetByte stx A1T0 lda #WorkRamResetByte >> 16 sta A1Bn ; Set DMA source to `WorkRamResetByte` ldx #0 stx DAS0 ; Transfer size = 64KiB lda #1 sta MDMAEN ; First DMA Transfer ; x = 0 stx DAS0 ; Transfer size = 64KiB ; a = 1 sta MDMAEN ; Second DMA Transfer ; Reset PPU jsr ResetRegisters jsr ClearVRAM jsr ClearCGRAM jsr ResetOAM jml Main ; Jump to Main ; Byte value to use when clearing Work-RAM ; (can be moved to any ROM bank) WorkRamResetByte: db 0
Reset Registers Routine
As the console lacks a boot ROM and most of the PPU registers start in an unknown state, the programmer will need to create a Reset Registers routine. This routine should be separate from the Reset Handler code to allow the programmer to reset the PPU registers to a known good state at the start of every level or scene.
The first registers to reset should be the NMITIMEN and HDMAEN registers. This will prevent Interrupts or HDMA from erroneously changing the PPU registers in the middle of the routine.
The following is a recommended list of registers to reset along with the values to reset them to.
Must reset first NMITIMEN $4200 = 0 (disable Interrupts) HDMAEN $420C = 0 (disable HDMA) INIDISP $2100 = $8F (enable Force Blank, full brightness) CPU registers MEMSEL $420D = 0 (set to 1 if FastROM) WRIO $4201 = $FF Objects OBSEL $2101 = 0 OAMADDL $2102 = 0 OAMADDH $2103 = 0 (disable OAM priority rotation) Backgrounds BGMODE $2105 = 0 MOSAIC $2106 = 0 BG1SC $2107 = 0 BG2SC $2108 = 0 BG3SC $2109 = 0 BG4SC $210A = 0 BG12NBA $210B = 0 BG34NBA $210C = 0 Scroll Registers BG1HOFS $210D = 0 BG1HOFS $210D = 0 (set horizontal offset to 0) BG1VOFS $210E = $FF BG1VOFS $210E = $FF (set vertical offset to -1) BG2HOFS $210F = 0 BG2HOFS $210F = 0 BG2VOFS $2110 = $FF BG2VOFS $2110 = $FF BG3HOFS $2111 = 0 BG3HOFS $2111 = 0 BG3VOFS $2112 = $FF BG3VOFS $2112 = $FF BG4HOFS $2113 = 0 BG4HOFS $2113 = 0 BG4VOFS $2114 = $FF BG4VOFS $2114 = $FF VRAM Registers VMAIN $2115 = $80 (VRAM word access, increment by 1, no remapping) Mode 7 M7SEL $211A = 0 (no flipping or screen repeat) M7A $211B = 0 M7A $211B = $01 M7B $211C = 0 M7B $211C = 0 M7C $211D = 0 M7C $211D = 0 M7D $211E = 0 M7D $211E = $01 M7X $211F = 0 M7X $211F = 0 M7Y $2120 = 0 M7Y $2120 = 0 Windows W12SEL $2123 = 0 W34SEL $2124 = 0 WOBJSEL $2125 = 0 WH0 $2126 = 0 WH1 $2127 = 0 WH2 $2128 = 0 WH3 $2129 = 0 WBGLOG $212A = 0 WOBJLOG $212B = 0 Layer Enable TM $212C = 0 TS $212D = 0 TMW $212E = 0 TSW $212F = 0 Color Math CGWSEL $2130 = $30 (Color math disable region = everywhere) CGADSUB $2131 = 0 COLDATA $2132 = $E0 (set Fixed color data to black) Misc SETINI $2133 = 0
- SNES Development Manual Book 1, section 2-26-1: Register Clear (Initial Settings)