PIF-NUS

The PIF-NUS (or PIF, or PIF(P)-NUS on PAL) manages multiple critical functions of the N64 console. It is a physical microchip found on the console's motherboard, which is based on the Sharp SM5 Microcontroller. It is not clear whether SGI or Nintendo intended this to stand for "Peripheral InterFace" or not. While the naming is unintuitive, the Peripheral (or Parallel) Interface is used to read/write to the game ROM and devices like the 64DD; whereas, the PIF handles the following:

• Console startup and piracy protections
  • Stores the first 2 stages of the Initial Program Load (IPL) that is executed by the VR4300 CPU
• Console reset button to avoid corrupting save game data
• Controller and EEPROM read/write via JoyBus protocol

Pinout

PIF Decapped with Pins numbered

PIF Pinout (28 Pin SOP Package)

	N64 Function	SM5 Function	Pin		Pin	SM5 Fuction	N64 Function	Direction
	PIF Clock		Pin 1		Pin 28	VDD	VDD	Power
	RC Cold		Pin 2		Pin 27		Reset Button	Input
	CIC D Out		Pin 3		Pin 26		N/C (No Connect)
	RC Rand		Pin 4		Pin 25		INT 2 VR4300 CPU	Output
	CIC D In		Pin 5		Pin 24		EEPROM Data	I/O
	/Cold		Pin 6		Pin 23		EEPROM Data	I/O
	NMI VR4300 CPU		Pin 7		Pin 22		Player 4 Controller	I/O
	Power Good		Pin 8		Pin 21		Player 4 Enable	Output
	PIF CLK Input from RSP		Pin 9		Pin 20		Player 3 Controller	I/O
Input	Test 0	??	Pin 10		Pin 19		Player 3 Enable	Output
	PIF ADR from RSP		Pin 11		Pin 18		Player 2 Controller	I/O
Input	Test 1	??	Pin 12		Pin 17		Player 2 Enable	Output
	PIF DATA from RSP		Pin 13		Pin 16		Player 1 Controller	I/O
Power	GND	GND	Pin 14		Pin 15		Player 1 Enable	Output

Internal ROMs and RAM

Since PIF is based on the Sharp SM5 which is a programmable microcontroller, its logic is executed by a program that is burnt into an internal ROM, called PIF-SM5-ROM. This program is written for the SM5 4-bit core, and has been dumped via chip decapping. The jago85/UltraPIF_MCUproject on GitHub is a compatible implementation based on the STM32 architecture that can be inspected for further studying what PIF does in details. Everything described in this page is implemented by the means of this internal program.

Moreover, PIF contains a second internal ROM (1984 bytes) and a small RAM (64 bytes). These memories are usually referred to as PIF-ROM and PIF-RAM, but it is important not confuse this PIF-ROM with the previous PIF-SM5-ROM. Both PIF-ROM and PIF-RAM are memory mapped to the VR4300 address space via the SI interface in RCP (so each access actually requires a serial bus transmission and is thus quite slow).

The PIF-ROM contains the first two stages of code for the VR4300 boot process (IPL1 and IPL2) and is only memory mapped to VR4300 during the boot. After the boot process is finished, before jumping into the game code, the PIF locks the PIF-ROM for security reason, so that it cannot be accessed by VR4300 anymore. The PIF-ROM is slightly different between PAL and NTSC console: it actually hardcodes the region and communicate it to VR4300 during the boot via the PIF-RAM.

Dumping the PIF-ROM can be done via software thanks to a loophole: it is in fact possible to boot the console once, setup a hardware breakpoint at BFC0 0000 via the MIPS COP0 Watch register, and then soft-reset the console; as soon as the boot resumes, the interrupt will trigger; a registered handler for that interrupt would then be able to read the contents of PIF-ROM that are now unlocked, and dump them somewhere (eg: into SRAM). Check the hcs64/pif_rom_dumper project on GitHub for an example of implementing this technique.

The PIF-RAM is always available to be accessed by VR4300 and is used to perform communication with the PIF. Normally, it is used as part of the Joybus protocol to communicate with controllers and EEPROMs.

RAM-based communication protocol

Communication between VR4300 and PIF happens using the 64-byte PIF-RAM. The last byte of PIF-RAM (offset 0x3F) is called the "command byte" and is interpreted as a bit mask: each bit corresponds to a different command that VR4300 asks the PIF to perform. While PIF is running, it is constantly monitor PIF-RAM and soon as it sees a bit going to 1 in the command byte, it performs the requested function and then turns off the bit. It is possible for the VR4300 to set more than one bit at the same time, but in general they are not fully orthogonal with each other. The rest of the PIF-RAM is used to provide the arguments for the requested command.

Description of PIF commands

Bit	Command	Description	Arguments	Results
0x01	Run Joybus protocol	This is the most used command during normal game run. It is used to poll the controllers and accessory PAKs. See below for more information.	A sequence of joybus commands must be provided in PIF-RAM starting at 0.	The results are written in PIF-RAM, overwriting each Joybus command.
0x02	Challenge / response for protection (CIC-NUS-6105)	The CIC-NUS-6105 implements a challenge/response security protocol that was used as anti-piracy measure. The VR4300 can execute this protection protocol any time it wants to verify that an authentic CIC-NUS-6105 is present in the cartridge: a random challenge string is provided by VR4300, sent to CIC, and the response is sent back.	15 challenge bytes at offset 0x30 in PIF-RAM.	15 response bytes at offset 0x30 in PIF-RAM.
0x04	Unknown
0x08	Terminate boot process	This command is sent by VR4300 when the boot process is done. PIF expects this command before 5 seconds from boot, otherwise it freezes itself and the whole console.	None?	None?
0x10	ROM lockout	This command asks the PIF to lock the PIF-ROM. It is part of the sequence to terminate the boot. After this command is received, PIF makes sure that the PIF-ROM is not exposed anymore via the serial bus (and thus accessible by VR4300) for security purposes.	None	None
0x20	IPL3 Checksum verification	This commands tells PIF to verify whether the provided checksum matches the expected one for IPL3. PIF reads the expected checksum from CIC and compares it: if it matches, it sets bit 0x80 in command byte (this is the only command that uses the command byte itself as a result).	8 byte checksum at offset 0x32 in PIF-RAM	Bit 0x80 of command byte is set if the checksum is correct.
0x40	Clear PIF RAM	This command simply fills the whole PIF RAM with 0.	None	PIF-RAM filled with 0.
0x80	Unused	This bit is used as response bit for command 0x20, and is not handled by PIF as a command.	None	None

Console startup

1. PIF holds all of the console components in reset mode
2. PIF starts communicating with the CIC inside the cartridge
   1. CIC sends 4 bits (nibble) including region encoding
   2. CIC sends 4 bit encrypted seed value
   3. CIC sends 4 bit checksum
3. PIF checks that these are the expected values
   1. If the values don't match, the PIF stops the boot (same scenario if no cartridge is inserted)
      1. The user would generally reset using the button or power off
      2. Remove/Re-insert cartridge (or change games)
      3. Power on (go to step #1)
4. PIF writes the encrypted seed value to the PIF-RAM at offset 0x24 (mapped at 0xBFC0 07E4)
5. PIF releases the reset pin for the whole console
   1. The PIF (console) and CIC (cartridge) communication continues as long as the console is powered on
   2. If there is ever a failure in the data exchange the console will be reset.
6. VR4300 starts running code from address 0xBFC0 0000 which is mapped to PIF ROM via SI interface
7. IPL1 is now executed directly fetching opcodes from PIF-ROM.
   1. These instructions are executed in this very slow manner.
   2. Thankfully IPL1 is only 52 instructions + some looping.
   3. It performs some really basic hardware initialization.
   4. It then copy the rest of the PIF-ROM (IPL2) to the RSP IMEM. Notice that at this point RDRAM is not initialized yet, so it cannot be used. RSP IMEM is instead available without any initialization and is much faster than PIF ROM thanks to the parallel bus.
   5. Jump to RSP IMEM to execute IPL2
8. IPL2 is executed by the VR4300 reading the instructions from RSP IMEM
   1. More general hardware initialization
   2. If it's determined to be a 64DD disk, it will jump to 0xA600 0000
   3. Load IPL3 from the cartridge ROM into the RSP DMEM
   4. Calculate the checksum of IPL3, using the checksum seed provided by CIC at the beginning.
   5. VR4300 asks PIF to verify that the checksum is correct (PIF command 0x20).
   6. VR4300 tells the PIF that the PIF-ROM can now be locked (PIF command 0x10).
   7. Jump to RSP DMEM to execute IPL3
9. IPL3 is executed by the VR4300 reading the instructions from the RSP DMEM.
   1. Initialize RDRAM
   2. Depending on reset type
      1. Power On: Invalidate VR4300 ICache & DCache
      2. Reset : Writeback VR4300 ICache & DCache
   3. Move IPL3 execution from DMEM to RDRAM
   4. DMA 1 MB of Game code to RDRAM
   5. Authenticate 1 MB of Game Program
   6. Reset RSP
   7. Clear Interrupts
   8. Clear IPL3 from DMEM
   9. Clear IPL2 from IMEM
   10. Jump to Game code in RDRAM

Console Reset

The reset process is driven by the PIF, which is connected to the physical reset button. The actual reset is done via a NMI to VR4300 which resets it by starting again the full boot process, but it is important to notice that RCP is not reset in any way. The boot code expects the RCP to be idle when the boot is initiated and is not guaranteed to work if the RCP is active in any way (DMAs in progress, RDP drawing triangles, RSP executing code, etc.), which means that it is up to the VR300 to stop issuing commands to the RCP and putting it in idle state before the reset is executed. To do so, VR4300 is given a forewarn that a reset is incoming via an interrupt (aptly called "pre-NMI") and is given grace time of 500ms before the actual NMI arrives.

This is the full sequence:

1. User presses Console Reset button
2. PIF reads the button state
3. PIF toggles VR4300 Interrupt 2 (INT2) also known as "pre-NMI".
   1. This is the time and opportunity for the game to finish saving game data and stop issuing commands to RCP to avoid graphics/audio corruption and/or a hard freeze.
4. PIF sends the RESET command to CIC (command 0b11)
5. CIC waits for 500ms (grace time)
6. CIC acknowledges the RESET command to PIF by writing a 0 bit.
7. PIF toggles VR4300 Non-Maskable Interrupt (NMI) which resets it.
   1. PIF also unlocks the internal PIF ROM so that the boot process can start executing IPL1.

Controller and EEPROM communication

This is the main task of PIF while the game is running. All communication with connected hardware like controllers and accessory PAKs is made through the PIF, using the Joybus protocol.

The VR4300 writes a Joybus packet into PIF-RAM and then triggers a handshake (by modifying the last byte of PIF-RAM which acts as "command" byte). PIF will execute the commands in the packet and provide the results into PIF-RAM. VR4300 would then read there results from there. (TODO: better document this process).