Difference between revisions of "Gameboy Bootstrap ROM"
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== References ==
== References ==
Revision as of 22:26, 5 March 2008
The DMG bootstrap
On July 17, 2003, neviksti published that he had been able to extract the contents of the Gameboy boot ROM from a DMG-01 on the Cherryroms.com forums. The boot ROM is a bootstrap program which is a 256 bytes big piece of code which checks the cartridge header is correct, scrolls the Nintendo bootup graphics and plays the "po-ling" sound. <ref>Mirror of the original Cherryroms thread on archive.org
When the Gameboy is turned on, the bootstrap ROM is situated in a memory page at positions $0-$FF (0-255). The CPU enters at $0 at startup, and the last two instructions of the code writes to a special register which disables the internal ROM page, thus making the lower 256 bytes of the cartridge ROM readable. The last instruction is situated at position $FE and is two bytes big, which means that right after that instruction has finished, the CPU executes the instruction at $100, which is the entry point code on a cartridge.
Neviksti managed to read out this memory area by opening the CPU of a Gameboy he got from Duo, and looking at it with a microscope. That way he managed to read the code bit by bit.
The CGB bootstrap
Neviksti has also tried to extract the bootstrap from a Gameboy Color (CGB-01) CPU. However, because that CPU uses NAND ROM and is laid out in a different way, he had no success in extracting that ROM. Up to this date, the exact design of the CGB bootstrap ROM is still unknown.
As a result of the process, neviksti also published pictures of the rest of the chip. All material published in conjunction with the hack can be found here: 
Contents of the ROM
Below is the disassembled code of the bootstrap ROM, together with Neviksti's comments. A binary file of the 256 byte area can be downloaded here: . The disassembled ROM file can also be found here: .
LD SP,$fffe ; $0000 Setup Stack XOR A ; $0003 Zero the memory from $8000-$9FFF (VRAM) LD HL,$9fff ; $0004 Addr_0007: LD (HL-),A ; $0007 BIT 7,H ; $0008 JR NZ, Addr_0007 ; $000a LD HL,$ff26 ; $000c Setup Audio LD C,$11 ; $000f LD A,$80 ; $0011 LD (HL-),A ; $0013 LD ($FF00+C),A ; $0014 INC C ; $0015 LD A,$f3 ; $0016 LD ($FF00+C),A ; $0018 LD (HL-),A ; $0019 LD A,$77 ; $001a LD (HL),A ; $001c LD A,$fc ; $001d Setup BG palette LD ($FF00+$47),A ; $001f LD DE,$0104 ; $0021 Convert and load logo data from cart into Video RAM LD HL,$8010 ; $0024 Addr_0027: LD A,(DE) ; $0027 CALL $0095 ; $0028 CALL $0096 ; $002b INC DE ; $002e LD A,E ; $002f CP $34 ; $0030 JR NZ, Addr_0027 ; $0032 LD DE,$00d8 ; $0034 Load 8 additional bytes into Video RAM LD B,$08 ; $0037 Addr_0039: LD A,(DE) ; $0039 INC DE ; $003a LD (HL+),A ; $003b INC HL ; $003c DEC B ; $003d JR NZ, Addr_0039 ; $003e LD A,$19 ; $0040 Setup background tilemap LD ($9910),A ; $0042 LD HL,$992f ; $0045 Addr_0048: LD C,$0c ; $0048 Addr_004A: DEC A ; $004a JR Z, Addr_0055 ; $004b LD (HL-),A ; $004d DEC C ; $004e JR NZ, Addr_004A ; $004f LD L,$0f ; $0051 JR Addr_0048 ; $0053 ; === Scroll logo on screen, and play logo sound=== Addr_0055: LD H,A ; $0055 Initialize scroll count, H=0 LD A,$64 ; $0056 LD D,A ; $0058 set loop count, D=$64 LD ($FF00+$42),A ; $0059 Set vertical scroll register LD A,$91 ; $005b LD ($FF00+$40),A ; $005d Turn on LCD, showing Background INC B ; $005f Set B=1 Addr_0060: LD E,$02 ; $0060 Addr_0062: LD C,$0c ; $0062 Addr_0064: LD A,($FF00+$44) ; $0064 wait for screen frame CP $90 ; $0066 JR NZ, Addr_0064 ; $0068 DEC C ; $006a JR NZ, Addr_0064 ; $006b DEC E ; $006d JR NZ, Addr_0062 ; $006e LD C,$13 ; $0070 INC H ; $0072 increment scroll count LD A,H ; $0073 LD E,$83 ; $0074 CP $62 ; $0076 $62 counts in, play sound #1 JR Z, Addr_0080 ; $0078 LD E,$c1 ; $007a CP $64 ; $007c JR NZ, Addr_0086 ; $007e $64 counts in, play sound #2 Addr_0080: LD A,E ; $0080 play sound LD ($FF00+C),A ; $0081 INC C ; $0082 LD A,$87 ; $0083 LD ($FF00+C),A ; $0085 Addr_0086: LD A,($FF00+$42) ; $0086 SUB B ; $0088 LD ($FF00+$42),A ; $0089 scroll logo up if B=1 DEC D ; $008b JR NZ, Addr_0060 ; $008c DEC B ; $008e set B=0 first time JR NZ, Addr_00E0 ; $008f ... next time, cause jump to "Nintendo Logo check" LD D,$20 ; $0091 use scrolling loop to pause JR Addr_0060 ; $0093 ; ==== Graphic routine ==== LD C,A ; $0095 "Double up" all the bits of the graphics data LD B,$04 ; $0096 and store in Video RAM Addr_0098: PUSH BC ; $0098 RL C ; $0099 RLA ; $009b POP BC ; $009c RL C ; $009d RLA ; $009f DEC B ; $00a0 JR NZ, Addr_0098 ; $00a1 LD (HL+),A ; $00a3 INC HL ; $00a4 LD (HL+),A ; $00a5 INC HL ; $00a6 RET ; $00a7 Addr_00A8: ;Nintendo Logo .DB $CE,$ED,$66,$66,$CC,$0D,$00,$0B,$03,$73,$00,$83,$00,$0C,$00,$0D .DB $00,$08,$11,$1F,$88,$89,$00,$0E,$DC,$CC,$6E,$E6,$DD,$DD,$D9,$99 .DB $BB,$BB,$67,$63,$6E,$0E,$EC,$CC,$DD,$DC,$99,$9F,$BB,$B9,$33,$3E Addr_00D8: ;More video data .DB $3C,$42,$B9,$A5,$B9,$A5,$42,$3C ; ===== Nintendo logo comparison routine ===== Addr_00E0: LD HL,$0104 ; $00e0 ; point HL to Nintendo logo in cart LD DE,$00a8 ; $00e3 ; point DE to Nintendo logo in DMG rom Addr_00E6: LD A,(DE) ; $00e6 INC DE ; $00e7 CP (HL) ; $00e8 ;compare logo data in cart to DMG rom JR NZ,$fe ; $00e9 ;if not a match, lock up here INC HL ; $00eb LD A,L ; $00ec CP $34 ; $00ed ;do this for $30 bytes JR NZ, Addr_00E6 ; $00ef LD B,$19 ; $00f1 LD A,B ; $00f3 Addr_00F4: ADD (HL) ; $00f4 INC HL ; $00f5 DEC B ; $00f6 JR NZ, Addr_00F4 ; $00f7 ADD (HL) ; $00f9 JR NZ,$fe ; $00fa ; if $19 + bytes from $0134-$014D don't add to $00 ; ... lock up LD A,$01 ; $00fc LD ($FF00+$50),A ; $00fe ;turn off DMG rom