Initial Program Load

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The Initial Program Load (commonly called IPL, or the boot sequence) is a set of instructions that the console performs every time it starts. There are multiple stages to this process, referred to as: IPL1, IPL2 and IPL3. When a 64DD is in use, there is also an IPL4 stage. Upon power-on, NMI, or "soft" reset, the program counter is set by hardware to 0xBFC00000, which is the beginning of the PIF ROM address space. This ROM is baked into the PIF-NUS chip.

When reading disassemblies, always remember that branch instructions have delay slots (which some branches may discard under certain circumstances). The operations performed in delay slots, may not be related to the branch itself.

Each line is formatted like so: [address][opcode][assembly instruction] # comments.

IPL1

This stage spans only the first 0xD4 bytes (0xBFC00000 - 0xBFC000D3) as executing instructions directly out of the PIF is relatively slow. Only what is absolutely necessary, is executed here. IPL1 resets the console to a consistent reset state, and moves the remaining bytes of the PIF ROM, into the RSP's IMEM (0xA4001000).

### IPL1 ### 0xBFC00000 - 0xBFC000D3 (0xD4 bytes long) ###

# SEGMENT 1 # Initialize CP0 Status & Config registers
[0xBFC00000][0x3C093400][LUI t1, 0x3400]        # t1 = 0x34000000
[0xBFC00004][0x40896000][MTC0 t1, SR]           # SR = t1 (enables CP0, CP1, and FPU registers)
[0xBFC00008][0x3C090006][LUI t1, 0x0006]        # t1 = 0x00060000
[0xBFC0000C][0x3529E463][ORI t1, t1, 0xE463]    # t1 = 0x0006E463
[0xBFC00010][0x40898000][MTC0 t1, Config]       # Config = t1 (sets SysAD port writeback pattern to "D", sets Big-Endian mode, and sets KSEG0 as a cached region)

# SEGMENT 2 #
# 2a: Wait for RSP halt
[0xBFC00014][0x3C08A404][LUI t0, 0xA404]        # t0 = 0xA4040000
[0xBFC00018][0x8D080010][LW t0, t0, 0x0010]     # t0 = value stored at 0xA4040010 (RSP_STATUS register)
[0xBFC0001C][0x31080001][ANDI t0, t0, 0x0001]   # t0 = t0 & 0x0001 (isolates the 'halt' bit)
[0xBFC00020][0x5100FFFD][BEQL t0, zr, 0xFFFD]   # if t0 == 0, branch to 0xBFC00018 (this is a spin loop, waiting for the RSP to halt)
[0xBFC00024][0x3C08A404][LUI t0, 0xA404]        # t0 = 0xA4040000

[0xBFC00028][0x2408000A][ADDIU t0, zr, 0x000A]  # t0 = 0x0000000A
[0xBFC0002C][0x3C01A404][LUI at, 0xA404]        # at = 0xA4040000
[0xBFC00030][0xAC280010][SW t0, at, 0x0010]     # write t0 (0x0000000A) into 0xA4040010 (RSP_STATUS register: sets 'halt' and clears 'rsp interrupt' bits)

[0xBFC00034][0x3C08A404][LUI t0, 0xA404]        # t0 = 0xA4040000
[0xBFC00038][0x8D080018][LW t0, t0, 0x0018]     # t0 = value stored at 0xA4040018 (RSP_DMA_BUSY register)
[0xBFC0003C][0x31080001][ANDI t0, t0, 0x0001]   # t0 = t0 & 0x0001 (isolates the 'halt' bit)
[0xBFC00040][0x5500FFFD][BNEL t0, zr, 0xFFFD]   # if t0 != 0, branch to 0xBFC00038 (this is a spin loop, waiting for the RSP to not be halted)
[0xBFC00044][0x3C08A404][LUI t0, 0xA404]        # t0 = 0xA4040000

# 2b: Reset PI
[0xBFC00048][0x24080003][ADDIU t0, zr, 0x0003]  # t0 = 0x00000003
[0xBFC0004C][0x3C01A460][LUI at, 0xA460]        # at = 0xA4600000    
[0xBFC00050][0xAC280010][SW t0, at, 0x0010]     # write t0 (0x00000003) into 0xA4600010 (PI_STATUS register: clears PI interrupt and resets PI controller)

# 2c: Clear video output
[0xBFC00054][0x240803FF][ADDIU t0, zr, 0x03FF]  # t0 = 0x000003FF
[0xBFC00058][0x3C01A440][LUI at, 0xA440]        # at = 0xA4400000
[0xBFC0005C][0xAC28000C][SW t0, at, 0x000C]     # write t0 (0x000003FF) into 0xA440000C (VI_V_INTR register: sets vertical interrupt trigger to half-line 0x3FF)
[0xBFC00060][0x3C01A440][LUI at, 0xA440]        # at = 0xA4400000
[0xBFC00064][0xAC200024][SW zr, at, 0x0024]     # write zr (0x00000000) into 0xA4400024 (VI_H_VIDEO register: sets the start and end of active video image to zero)
[0xBFC00068][0x3C01A440][LUI at, 0xA440]        # at = 0xA4400000
[0xBFC0006C][0xAC200010][SW zr, at, 0x0010]     # write zr (0x00000000) into 0xA4400010 (VI_V_CURRENT register: clears the VI interrupt)

# 2d: Stop audio
[0xBFC00070][0x3C01A450][LUI at, 0xA450]        # at = 0xA4500000
[0xBFC00074][0xAC200000][SW zr, at, 0x0000]     # write zr (0x00000000) into 0xA4500000 (AI_DRAM_ADDR register: sets DMA RDRAM address to zero)
[0xBFC00078][0x3C01A450][LUI at, 0xA450]        # at = 0xA4500000
[0xBFC0007C][0xAC200004][SW zr, at, 0x0004]     # write zr (0x00000000) into 0xA4500004 (AI_LENGTH register: sets transfer length to zero)

# 2e: Wait for any RSP DMAs to complete
[0xBFC00080][0x3C08A404][LUI t0, 0xA404]        # t0 = 0xA4040000
[0xBFC00084][0x8D080010][LW t0, t0, 0x0010]     # t0 = value stored at 0xA4040010 (RSP_STATUS register)
[0xBFC00088][0x31080004][ANDI t0, t0, 0x0004]   # t0 = t0 & 0x0004 (isolates the 'dma busy' bit)
[0xBFC0008C][0x5500FFFD][BNEL t0, zr, 0xFFFD]   # if t0 != 0, branch to 0xBFC00084 (this is a spin loop, waiting for any RSP DMAs to complete)
[0xBFC00090][0x3C08A404][LUI t0, 0xA404]        # t0 = 0xA4040000

# SEGMENT 3 # Copy IPL2 from PIF to RSP IMEM
# 3a: Initialize start/end addresses
[0xBFC00094][0x3C0BA400][LUI t3, 0xA400]        # t3 = 0xA4000000
[0xBFC00098][0x3C0CBFC0][LUI t4, 0xBFC0]        # t4 = 0xBFC00000
[0xBFC0009C][0x3C0DBFC0][LUI t5, 0xBFC0]        # t5 = 0xBFC00000

[0xBFC000A0][0x256B1000][ADDIU t3, t3, 0x1000]  # t3 = 0xA4001000 (start of RSP IMEM)
[0xBFC000A4][0x258C00D4][ADDIU t4, t4, 0x00D4]  # t4 = 0xBFC000D4 (start of IPL2 in PIF ROM)
[0xBFC000A8][0x25AD071C][ADDIU t5, t5, 0x071C]  # t5 = 0xBFC0071C (end   of IPL2 in PIF ROM)

# 3b: Load/Store 1 word (4 bytes) at a time, moving instruction data from [0xBFC000D4 - 0xBFC0071B], into [0xA4001000 - 0xA4000647]
[0xBFC000AC][0x8D890000][LW t1, t4, 0x0000]     # t1 = value stored at address t4
[0xBFC000B0][0x258C0004][ADDIU t4, t4, 0x0004]  # increment t4 (+4)
[0xBFC000B4][0x256B0004][ADDIU t3, t3, 0x0004]  # increment t3 (+4)
[0xBFC000B8][0x158DFFFC][BNE t4, t5, 0xFFFC]    # if t4 != t5, branch to 0xBFC000AC (repeat the loop)
[0xBFC000BC][0xAD69FFFC][SW t1, t3, 0xFFFC]     # write t1 into address (t3 - 4)

# SEGMENT 4 # Jump to IPL2 in IMEM
[0xBFC000C0][0x3C0BA400][LUI t3, 0xA400]        # t3 = 0xA4000000
[0xBFC000C4][0x3C1DA400][LUI sp, 0xA400]        # sp = 0xA4000000
[0xBFC000C8][0x256B1000][ADDIU t3, t3, 0x1000]  # t3 = 0xA4001000 (beginning of IMEM and IPL2)
[0xBFC000CC][0x01600008][JR t3]                 # jump to IMEM/IPL2 (and executes delay slot)
[0xBFC000D0][0x37BD1FF0][ORI sp, sp, 0x1FF0]    # sp = 0xA4001FF0 (this prepares sp for use in IPL2)

IPL2

After IPL1 finishes moving the instructions of this stage to RSP IMEM, it jumps to 0x04001000 to start this second stage.

Commentated disassembly to be added here...

IPL3

This stage is executed out of RSP DMEM, which was loaded with the first 0x1000 bytes of the game cartridge (or alternatively, whatever may be inserted in the cartridge or expansion slot on the bottom of the console). IPL2 will jump to 0x04000040 (0x40 bytes after the start of DMEM, to account for the ROM header).

There are six known variants of the IPL3 stage. However, a seventh variant can be seen when analyzing the dumps of the GameBooster 64, Action Replay Pro 64, and GameShark Pro (v3.3): where the 0x0FC0 bytes after the header, are all zeros, and are dynamically loaded by the cartridge.

For each of the six variants, there is a matching CIC chip found in each cartridge. There are 10 official CIC chips:

NTSC PAL % of ROMs (Qty)
CIC-NUS-6102 CIC-NUS-7101 87.6% (826)
CIC-NUS-6103 CIC-NUS-7103 6.5% (61)
CIC-NUS-6105 CIC-NUS-7105 4.5% (43)
CIC-NUS-6106 CIC-NUS-7106 0.09% (8)
CIC-NUS-6101 CIC-NUS-7102 0.05% (5)

Each of these NTSC/PAL pairs share an IPL3 variant, except 6101 and 7102.

Details to be added here...

Details to be added here...

Details to be added here...

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Details to be added here...

IPL4

TODO