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- ; ******************************************************************
- ; Sega Megadrive ROM header
- ; ******************************************************************
- dc.l 0x00FFE000 ; Initial stack pointer value
- dc.l EntryPoint ; Start of program
- dc.l Exception ; Bus error
- dc.l Exception ; Address error
- dc.l Exception ; Illegal instruction
- dc.l Exception ; Division by zero
- dc.l Exception ; CHK exception
- dc.l Exception ; TRAPV exception
- dc.l Exception ; Privilege violation
- dc.l Exception ; TRACE exception
- dc.l Exception ; Line-A emulator
- dc.l Exception ; Line-F emulator
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Spurious exception
- dc.l Exception ; IRQ level 1
- dc.l Exception ; IRQ level 2
- dc.l Exception ; IRQ level 3
- dc.l HBlankInterrupt ; IRQ level 4 (horizontal retrace interrupt)
- dc.l Exception ; IRQ level 5
- dc.l VBlankInterrupt ; IRQ level 6 (vertical retrace interrupt)
- dc.l Exception ; IRQ level 7
- dc.l Exception ; TRAP #00 exception
- dc.l Exception ; TRAP #01 exception
- dc.l Exception ; TRAP #02 exception
- dc.l Exception ; TRAP #03 exception
- dc.l Exception ; TRAP #04 exception
- dc.l Exception ; TRAP #05 exception
- dc.l Exception ; TRAP #06 exception
- dc.l Exception ; TRAP #07 exception
- dc.l Exception ; TRAP #08 exception
- dc.l Exception ; TRAP #09 exception
- dc.l Exception ; TRAP #10 exception
- dc.l Exception ; TRAP #11 exception
- dc.l Exception ; TRAP #12 exception
- dc.l Exception ; TRAP #13 exception
- dc.l Exception ; TRAP #14 exception
- dc.l Exception ; TRAP #15 exception
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.l Exception ; Unused (reserved)
- dc.b "SEGA GENESIS " ; Console name
- dc.b "(C)SEGA 1992.SEP" ; Copyrght holder and release date
- dc.b "YOUR GAME HERE " ; Domestic name
- dc.b "YOUR GAME HERE " ; International name
- dc.b "GM XXXXXXXX-XX" ; Version number
- dc.w 0x0000 ; Checksum
- dc.b "J " ; I/O support
- dc.l 0x00000000 ; Start address of ROM
- dc.l __end ; End address of ROM
- dc.l 0x00FF0000 ; Start address of RAM
- dc.l 0x00FFFFFF ; End address of RAM
- dc.l 0x00000000 ; SRAM enabled
- dc.l 0x00000000 ; Unused
- dc.l 0x00000000 ; Start address of SRAM
- dc.l 0x00000000 ; End address of SRAM
- dc.l 0x00000000 ; Unused
- dc.l 0x00000000 ; Unused
- dc.b " " ; Notes (unused)
- dc.b "JUE " ; Country codes
- EntryPoint: ; Entry point address set in ROM header
- tst.w 0x00A10008 ; Test mystery reset (expansion port reset?)
- bne Main ; Branch if Not Equal (to zero) - to Main
- tst.w 0x00A1000C ; Test reset button
- bne Main ; Branch if Not Equal (to zero) - to Main
- HBlankInterrupt:
- VBlankInterrupt:
- rte ; Return from Exception
- Exception:
- rte ; Return from Exception
- Loop:
- move.l #0xF, d0 ; Move 15 into register d0
- move.l d0, d1 ; Move contents of register d0 into d1
- jmp Loop ; Jump back up to 'Loop'
- move.l #0x00000000, d0 ; Place a 0 into d0, ready to copy to each longword of RAM
- move.l #0x00000000, a0 ; Starting from address 0x0, clearing backwards
- move.l #0x00003FFF, d1 ; Clearing 64k's worth of longwords (minus 1, for the loop to be correct)
- @Clear:
- move.l d0, -(a0) ; Decrement the address by 1 longword, before moving the zero from d0 to it
- dbra d1, @Clear ; Decrement d0, repeat until depleted
- ;TMSS
- move.b 0x00A10001, d0 ; Move Megadrive hardware version to d0
- andi.b #0x0F, d0 ; The version is stored in last four bits, so mask it with 0F
- beq @Skip ; If version is equal to 0, skip TMSS signature
- move.l #'SEGA', 0x00A14000 ; Move the string "SEGA" to 0xA14000
- @Skip:
- ;Z80_Init()
- move.w #0x0100, 0x00A11100 ; Request access to the Z80 bus, by writing 0x0100 into the BUSREQ port
- move.w #0x0100, 0x00A11200 ; Hold the Z80 in a reset state, by writing 0x0100 into the RESET port
- @Wait:
- btst #0x0, 0x00A11100 ; Test bit 0 of A11100 to see if the 68k has access to the Z80 bus yet
- bne @Wait ; If we don't yet have control, branch back up to Wait
- move.l #Z80Data, a0 ; Load address of data into a0
- move.l #0x00A00000, a1 ; Copy Z80 RAM address to a1
- move.l #0x29, d0 ; 42 bytes of init data (minus 1 for counter)
- @Copy:
- move.b (a0)+, (a1)+ ; Copy data, and increment the source/dest addresses
- dbra d0, @Copy
- move.w #0x0000, 0x00A11200 ; Release reset state
- move.w #0x0000, 0x00A11100 ; Release control of bus
- Z80Data:
- dc.w 0xaf01, 0xd91f
- dc.w 0x1127, 0x0021
- dc.w 0x2600, 0xf977
- dc.w 0xedb0, 0xdde1
- dc.w 0xfde1, 0xed47
- dc.w 0xed4f, 0xd1e1
- dc.w 0xf108, 0xd9c1
- dc.w 0xd1e1, 0xf1f9
- dc.w 0xf3ed, 0x5636
- dc.w 0xe9e9, 0x8104
- dc.w 0x8f01
- ;PSG_Init()
- move.l #PSGData, a0 ; Load address of PSG data into a0
- move.l #0x03, d0 ; 4 bytes of data
- @Copy:
- move.b (a0)+, 0x00C00011 ; Copy data to PSG RAM
- dbra d0, @Copy
- PSGData:
- dc.w 0x9fbf, 0xdfff
- ;VDP_Init()
- move.l #VDPRegisters, a0 ; Load address of register table into a0
- move.l #0x18, d0 ; 24 registers to write
- move.l #0x00008000, d1 ; 'Set register 0' command (and clear the rest of d1 ready)
- @Copy:
- move.b (a0)+, d1 ; Move register value to lower byte of d1
- move.w d1, 0x00C00004 ; Write command and value to VDP control port
- add.w #0x0100, d1 ; Increment register #
- dbra d0, @Copy
- VDPRegisters:
- dc.b 0x20 ; 0: Horiz. interrupt on, plus bit 2 (unknown, but docs say it needs to be on)
- dc.b 0x74 ; 1: Vert. interrupt on, display on, DMA on, V28 mode (28 cells vertically), + bit 2
- dc.b 0x30 ; 2: Pattern table for Scroll Plane A at 0xC000 (bits 3-5)
- dc.b 0x40 ; 3: Pattern table for Window Plane at 0x10000 (bits 1-5)
- dc.b 0x05 ; 4: Pattern table for Scroll Plane B at 0xA000 (bits 0-2)
- dc.b 0x70 ; 5: Sprite table at 0xE000 (bits 0-6)
- dc.b 0x00 ; 6: Unused
- dc.b 0x00 ; 7: Background colour - bits 0-3 = colour, bits 4-5 = palette
- dc.b 0x00 ; 8: Unused
- dc.b 0x00 ; 9: Unused
- dc.b 0x00 ; 10: Frequency of Horiz. interrupt in Rasters (number of lines travelled by the beam)
- dc.b 0x08 ; 11: External interrupts on, V/H scrolling on
- dc.b 0x81 ; 12: Shadows and highlights off, interlace off, H40 mode (40 cells horizontally)
- dc.b 0x34 ; 13: Horiz. scroll table at 0xD000 (bits 0-5)
- dc.b 0x00 ; 14: Unused
- dc.b 0x00 ; 15: Autoincrement off
- dc.b 0x01 ; 16: Vert. scroll 32, Horiz. scroll 64
- dc.b 0x00 ; 17: Window Plane X pos 0 left (pos in bits 0-4, left/right in bit 7)
- dc.b 0x00 ; 18: Window Plane Y pos 0 up (pos in bits 0-4, up/down in bit 7)
- dc.b 0x00 ; 19: DMA length lo byte
- dc.b 0x00 ; 20: DMA length hi byte
- dc.b 0x00 ; 21: DMA source address lo byte
- dc.b 0x00 ; 22: DMA source address mid byte
- dc.b 0x00 ; 23: DMA source address hi byte, memory-to-VRAM mode (bits 6-7)
- ; Set IN I/O direction, interrupts off, on all ports
- move.b #0x00, 0x000A10009 ; Controller port 1 CTRL
- move.b #0x00, 0x000A1000B ; Controller port 2 CTRL
- move.b #0x00, 0x000A1000D ; EXP port CTRL
- move.l #0x00000000, a0 ; Move 0x0 to a0
- movem.l (a0), d0-d7/a1-a7 ; Multiple move 0 to all registers
- ; Init status register (no trace, A7 is Interrupt Stack Pointer, no interrupts, clear condition code bits)
- move #0x2700, sr
- Main:
- ; Here goes your game code
- jmp __main ; Jump to the game code!
- __end ; Very last line, end of ROM address
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