The Jumper Pak is a piece of hardware for the Nintendo 64 games console made by Nintendo. One of them is included with every console bought, although most people will not know it.

On the top of the console there is an expansion port. The Expansion Pak was the only thing ever manufactured to go in there; it provided extra Random Access Memory to double the console's capability. However, Nintendo realised that they would need something to fill the gap in the console before the Expansion Pak was released. They came up with the jumper pak - a plastic object about 2" by 1" by 1.5" which fitted into the expansion slot. As far as I know there is no real circuitry on board the jumper pak (because otherwise is would be stupid to remove it), but it does have an edge connector in common with the Expansion Pak so that it can slot into the same space. Presumably there is a check carried out when you turn on the system to see if there is either a Jumper Pak or and Expansion Pak plugged in, but I never checked what happens if you leave the slot empty, and it's hard to get the Expansion Pak out. I don't suppose anyone out there has had a look at what happens if you run a pakless machine? All N64s come with a Jumper Pak in, and you are only supposed to take it out when you buy an expansion pak. To make sure you remember this there is a sticker saying DO NOT REMOVE THIS STICKER. In the manual for the Expansion Pak, the first thing you are told to do is "Remove the Sticker". I'll never understand Nintendo.

Servo5678 says "Your theory is correct. The only thing a Jumper Pak does tech-wise is to tell your N64 "This N64 Has No Expansion Pak". As for the "Do not Remove" sticker, if someone pre-Expansion Pak removed the sticker they'd be tempted to remove the Jumper Pak, resulting in errors or damage to the system by removing the Jumper Pak without that nifty plastic tool that comes with the Expansion Pak. That sticker is supposed to scare people away until it's time."

Triften says "The N64 uses RDRAM and the architecture requires the use of terminators in unused memory slots. The jumper pak acts as such until you get an Expansion Pak."

UPDATE!! Servo5678 has finally laid to rest the whole "Pak" issue with his writeup in Game Pak.

ANOTHER UPDATE!! I think it's just worth pointing out that Jumper Paks, despite what you read on eBay or anywhere else, are not "rare" or "like gold dust*". You get one with every new N64, and since to my knowledge none of them were ever sold with an expansion pak preinstalled, that means there must be an equal number of them to the number of N64s. However, some people who enjoy conning others have started selling them on ebay (both the UK and US sites, as Servo5678 and I discovered to our dispair). The only advice I can offer is, if you have a working N64, there is no possible reason to buy one, because you either already have one, or you have an Expansion Pak which is superior anyway. Unless you're a crazy collector, don't buy one, despite what people might tell you.

* - They actually are like gold dust, except for the fact that they are neither gold or dust.