Gameboy Bootstrap ROM
Contents
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</ref>
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 $100, which is the entry point code on a cartridge.
Neviksti managed to read out this memory area by opening the CPU of a Gameboy he got from Duo, and looking at it with a microscope. That way he managed to read the code bit by bit.
The CGB bootstrap
Neviksti has also tried to extract the bootstrap from a Gameboy Color (CGB-01) CPU. However, because that CPU uses NAND ROM and is laid out in a different way, he had no success in extracting that ROM. Up to this date, the exact design of the CGB bootstrap ROM is still unknown.
Impact
Apart from amazement, the discovery led to the inclusion of a feature to emulate the bootstrap ROM in the emulators KiGB and BGB.
Other findings
As a result of the process, neviksti also published pictures of the rest of the chip. All material published in conjunction with the hack can be found here: [1]
Contents of the ROM
Below is the disassembled code of the bootstrap ROM, together with Neviksti's comments. A binary file of the 256 byte area can be downloaded here: [2]. The disassembled ROM file can also be found here: [3].
LD SP,$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
References
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