Adding L2 Cache to 7400/7410 CPUs

[evanboonie]

Has anyone ever tried adding L2 cache to an early G4 7400 or 7410 CPU? According to their datasheets, they support up to 2MB of cache. As far as I can tell, Apple and all third party upgrade manufacturers never used more than 1MB on one. I found some cache modules that have the exact same pinout (save for one additional address line, this is the part that worries me) and word size as the original, but are twice as large. The new modules are also rated for 333Mhz. I've heard that the cache is one of the limitations in overclocking a 7410 to 600Mhz; as it also clocks the original cache chips to 300Mhz when they are only rated for 250Mhz. I'm curious if I could swap them on to the original CPU card and get the full 2MB of cache that the 7400 supports. They are only about $15 each and I'd need two of them for each CPU. I'm thinking of trying this on a 533Mhz 7410 I have laying around and seeing if it boots up in my cube. It might even allow me to hit 600Mhz on a 120Mhz bus speed.

 

[ervus]

Yes, what you suggest is possible. I have also seen a few aftermarket 7410 cards with 2MB of L2 cache. The performance difference between no L2 cache and 1MB is much bigger than between 1MB and 2MB. The gcc compiler seems to like cache so a while ago I was testing different types of G4 CPUs and their cache configurations to see the differences (https://forums.macrumors.com/threads/g4-7448-and-7457-swap.2233532/post-30661142). It was getting off topic but I also tested a dual 500MHz 7400 with 2MB, 1MB, and no L2 cache. The results were (TFF builds/day):
2.36
2.24
1.09

 

[evanboonie]

Yes, what you suggest is possible. I have also seen a few aftermarket 7410 cards with 2MB of L2 cache. The performance difference between no L2 cache and 1MB is much bigger than between 1MB and 2MB. The gcc compiler seems to like cache so a while ago I was testing different types of G4 CPUs and their cache configurations to see the differences (https://forums.macrumors.com/threads/g4-7448-and-7457-swap.2233532/post-30661142). It was getting off topic but I also tested a dual 500MHz 7400 with 2MB, 1MB, and no L2 cache. The results were (TFF builds/day):
2.36
2.24
1.09

This is some great info, thanks! It sounds like more cache doesn't make much of a difference, but hopefully the higher speeds will open up some higher potential core clocks. I think I'm going to try to swap one of my 7410 cards this weekend and see how far I can manage to overclock it. The stock 7410 cards have a lot of voltage headroom (They appear to run at 1.7V but can handle 2.1V) I'd like to see just how fast I can get one to go.

 

[evanboonie]

Sadly, my cache swap didn't work. When I installed it in my Sawtooth Power Mac, it wouldn't chime and and the CPU got pretty warm pretty quickly. My best guess is that single pin difference on the new chips I found isn't compatible. I may try again if I can ever find a 100% pin compatible replacement for them at a faster clockspeed than the original. I really don't think it would be worth it though. I was not aware that the 7410 actually already uses faster cache chips than the 7400 did. They are already rated at 300Mhz from what I can tell. I also misread the VID resistor configuration and the 7410s from Apple don't have as much voltage headroom as I thought. They actually run at 2v out of the gate, so we only have 0.1v to play with. Instead, I think I'm going to try transplanting two 7410s and their cache chips to a dual CPU card intended for a Sawtooth/AGP/Gigabit Power Mac and see if I can get dual 600Mhz working on one of these older machines instead.

 

[evanboonie]

Well, that didn't work either. I very well could have damaged one of my 7410s when abusing them like this, so that isn't to say it couldn't be done. But I'm probably giving up on this for now, too much hassle for too little performance increase.

 

[ervus]

Which part didn't work? You tried swapping CPU chips on a dual card from a Gigabit Ethernet model? This mod is possible, and the lower power would be a good reason to do it because these cards can be used in the Cube. A dual 600MHz Cube is a nice upgrade, and doesn't stress the VRM much. Check out 68kMLA.

 

[evanboonie]

Yeah, my attempt was swapping the CPUs and cache chips from single 7410 cards on to a dual CPU Gigabit card. I'm sure it is possible, I'm thinking that I just damaged something along the way due to heat, ESD, or a short. When I installed the swapped card into my Gigabit Ethernet, it just wouldn't chime. I'm not too worried about it though. I have some other options that give me a lot more speed. I was really just playing around with this to see if I could pull it off.