The ARM9 bios does a number of things on boot, before entering a wait on ARM7 loop.

Here is pseudocode for the process up until the first signaling operation:

 

  (*(vuint32*)0x040001A4) = 0;
  (*(vuint32*)0x040001A1) = 0x80; // not a mistake, it isn't word aligned

  CPSR = 0xDF;  // system mode, inhibit IRQ, FIQ

  DelayLoop(0x2000);
  WAIT_CR = 0x2000;
  DelayLoop(0x2000);

  vuint16 temp = (*(vuint16*)0x027FFFFE);
  (*(vuint16*)0x027FFFFE) = temp;
  (*(vuint16*)0x027FFFFE) = temp;

  (*(vuint16*)0x027FFFFE) = 0xFFDF;
  (*(vuint16*)0x027FFFFE) = 0xE732;

  temp = (*(vuint16*)0x027E57FE);

  WAIT_CR = 0x6000;

  WriteCP15c1(0x00002078);
  DTCM_baseAddr = 0x00800000; // size = 16 KB (c9,c1=0x0080000A)

  vuint32 ignored = CP15.c1;
  cp15.c1 = 0x00012078;

  IME = 0;

  SP_svc = 0x803FC0;
  SPSR_svc = 0;
  SP_irq = 0x803FA0;
  SPSR_irq = 0;
  SP = 0x00803EC0;

  ZeroMemory(DTCM_BaseAddress+0x3E00 .. DTCM_BaseAddress+0x3FFF);

Doesn't really belong here, but there is no great place for it at the moment:

// Low ROM constants

// Related to firmware
0x000010A0:16
0x000010A2:16
0x000010A4:16

0x00000024:32 = uint32 CONSTANT_FOR_1B4;
0x00000028:8 = uint8 CONSTANT_FOR_1BA;
0x00000029:8 = uint8 CONSTANT_FOR_1B0;

// 0x1078..0x1087 (16 bytes, used as a table)
0x00001088:32 = CONST_1088;
0x0000108C:32 = CONST_108C;
0x00001090:32 = CONST_1090;
0x00001094:32 = CONST_1094;
0x00001098:32 = CONST_1098;
0x0000109C:32 = CONST_109C;

//////////////////////////////////////////////////////////////////////
Memory addresses:
//////////////////////////////////////////////////////////////////////

0x037F8000:       rtcAfterInit (control, year, month, day, week, hour, minute, second)
0x037F8000:32
 0x037F8000:16
 0x037F8002:16

0x037F8004:32
 0x037F8004:16
 0x037F8006:16

0x037F800A:16 = uint16 streamState; // 0..7 ish, see SerialStreamReadByte
0x037F8014 = ? (3rd arg of ARM10EC)


0x037F802C:32 = f(rtcAfterInit)
0x037F8030:32 = g(rtcAfterInit)
0x037F8038:32 = h(rtcAfterInit)
0x037F8034:32 = i(rtcAfterInit)

0x037F8040:16 = headerChecksOK

0x037F8058:32 = int sizeToRead
0x037F805C:16 = int16 cardIRQstate
0x037F805E:16 = int16 cirqBitIndex  // set to zero somewhere I'll assume
0x037F8060:32 = uint8 * cirqDataAddress;
0x037F8064:16 = int16 arm9binStart;
0x037F8066:16 = int16 cirqUnknown;
0x037F8068:16 = int16 cirqOffset;
0x037F806E:16 = int16 cirqUnknown2


0x037F806C:32 = timer3_params[0];
0x037F8070:32 = timer3_params[1];
0x037F8074:32 = timer3_params[2];
0x037F8078:32 = timer3_params[3];

//////////////////////////////////////////////////////////////////////

0x0380FC00:32 = fcSourceROM;
0x0380FC04:32 = fcDestination;
0x0380FC08:32 = dataLeftForMCIRQ
0x0380FC0C:32 = fcLength

StartCardTransfer(0, 0x0380FC04:32 + r1 + r2, r5, params)

0x0380FC10:32 = global params[0]
0x0380FC14:32 = global params[1]
0x0380FC18:32 = global params[2]
0x0380FC1C:32 = global params[3]

0x0380FC20:32 = CustomMCIRQVector

CardIRQVector = 0x0380FC40:32
TimerIRQVector = 0x0380FC44:32
VBlankIRQVector = 0x0380FC48:32

0x0380FFF8:32 = IF_BUFFER

//////////////////////////////////////////////////////////////////////

0x027FF800:32 = int CurrentCardID
0x027FF804:32 = int Command10CardID
0x027FF808:16 = uint16 computedHeaderCRC;
0x027FF80C:16 = uint16 arm9copiedCRC
0x027FF810:16 = int16 state147A
0x027FF812:16 = desiredBit
0x027FF814:16 = int16 firmwareAuthenticated (1=yes)
0x027FF816:16 = int16 rtcInitFailure; // RTC initialization failed if non-zero

0x027FF820:32 = arm9 firmware execute address
0x027FF824:32 = arm7 firmware execute address
0x027FF82C:16 = err, perhaps firmware loaded successfully (we get a ZeroRAM64() if non-zero, and both arm7/arm9 hang if zero)



0x027FF840:16 = arm7biosCRC;

0x027FF848:32 = source of arm9 firmware exec addr (copied down to 820)
0x027FF84C:32 = source of arm7 firmware exec addr (copied down to 824)

//////////////////////////////////////////////////////////////////////

0x027FFE00..0x027FFF70   NDSHeader

//0x027FFE00..0B      // NDSHeader.GameTitle
//0x027FFE0C..FFE11   // NDSHeader.First 6 characters of game code
0x027FFE12:16     // NDSHeader.GameCodeLast2
//0x027FFE14:32     // NDSHeader.cartSize
//0x027FFE18..1E     // unknown
//0x027FFE1F:8       // Auto-boot flags
0x027FFE20:32     // NDSHeader.arm9romSource
0x027FFE24:32     // NDSHeader.arm9executeAddress (the arm9 bios looks at it)
0x027FFE28:32     // NDSHeader.arm9destination
0x027FFE2C:32     // NDSHeader.arm9binarySize

0x027FFE30:32     // NDSHeader.arm7romSource
0x027FFE34:32     // NDSHeader.arm7executeAddress
0x027FFE38:32     // NDSHeader.arm7destination
0x027FFE3C:32     // NDSHeader.arm7binarySize

//FE40..FE4F      // filename table / FAT
//FE50..FE5F      // overlays, not going to be seen
0x027FFE60:32     // NDSHeader.unknown2a
0x027FFE64:32;    // NDSheader.unknown2b
//0x027FFE68:32   // NDSheader.IconOffset (not going to be seen)
//0x027FFE6C:16   // NDSheader.SecureCRC16
0x027FFE6E:16     // NDSHeader.unknown2d (0x051E typically)
//0x027FFE70:32     // NDSHeader.unknownRAM1
//0x027FFE74:32     // NDSHeader.unknownRAM2
0x027FFE78:32     // NDSHeader.unknown3c. first half
0x027FFE7C:32     // NDSHeader.unknown3c. second half
//0x027FFE80:32     // NDSHeader.ROMsize
0x027FFE84:32     // NDSHeader.HeaderSize  -- fixme, no, its not!
//FE88..FFBF        // zeros
//FEC0..FF5B      // NDSHeader.GBAlogo
0x027FFF5C:16     // is NDSHeader.LogoCRC16
0x027FFF5E:16     // is NDSHeader.HeaderCRC16
0x027FFF60:32     // NDSHeader.wordAt0x160
0x027FFF64:32     // NDSHeader.wordAt0x164
0x027FFF68:32     // NDSHeader.wordAt0x168
//0x027FFF6C:32

//////////////////////////////////////////////////////////////////////

0x027FFFFC:16 = 1 when header or logo crc-16 check failed, 0 if passed or not attempted
0x027FFFEC:16 = 1 when header or logo crc-16 check failed, 0 if passed or not attempted