Originally posted by Pandoering
Seems like Apple admitted that the lack of L3 may cause concern among members of the video editing community because video encoding to MPEG-2 made use of it. There not the only ones.Thinksecret
Putting in the 7457 with L3 would have made this one irresistible. It seems to me that the only plausible reason for this
(pro-user insulting decision making) is that Apple after they concluded the 7457 wouldn't be ready in time, just decided to put in the 7447 and up the L2. There can be no other.
Well I'm going to respond anyway. I have a Bachelor's Degree in Computer Engineering (currently working on a Master's in Computer Science), which makes me somewhat qualified to comment on the differences between various cache levels.
While your speculation that the 7457 was not used because it was not yet available is certainly possible, I rather doubt that the 7457 or the 7447 would be available significantly before the other. There is no major difference between either chip that would cause manufacturing delays.
I speculate that instead Apple decided to forgo L3 cache for the notebook series for two reasons, L3 cache provides little benefit for *most* users, it is mostly used to compensate for slow memory bus speeds. The second reason is cost. Performance per dollar, L3 cache doesn't usually make a lot of sense, except in very specific instances.
Processor designers have been studying the optimum amounts and associativity of cache levels for many years. L3 cache is rarely used in modern systems, because doubling the size of L2 cache provides better performance (in the general case) than adding an L3 cache that is 4 times as large as L2. Since Motorola doubled the size of the L2 cache in the 74*7 models, it makes sense to drop the L3 cache even though the size of the cache *looks* smaller, it will provide equal or likely *better* performance. This is somewhat counter intuitive.
There are specific application types (those with high memory bandwith requirement) that may benefit from the tradeoff between more L2 cache and more L3 cache (obviously more of both would be best, but prohibitively expensive). This is what Apple is acknowledging with their statement. However, it needs to be added that they made the choice that benefits the *most* users, which when designing a general computing system is the right decision.
If you would like any more clarification on any of the points I raised, please feel free to ask
PS I'd like to add that it really isn't entirely up to Apple to increase the L2 cache or not, the L2 cache is on the processor die which is manufactured by Motorola. It is fixed in the design process, and can thus not be added later. L3 Cache is off-chip (on the motherboard or processor card or whatever) and can be changed in size by modifying the board.
Also, in the case of a notebook, it does make sense to not even have an L3 cache controller if not using L3 cache. In most designs, the L3 cache controller occupies a portion of the die, and the SRAM is off-chip. So if not making use of an L3 cache, it is preferable to not even have the controller (reducing the size of the die = smaller, less-heat and cheaper). Hence, Motorola is manufacturing both the 7447 and the 7457.