When I first read rumors about Apple’s M3 Pro having 18GB and 36GB memory options, I thought it was an obvious error. But sure enough, the “Scary Fast” event confirmed the same amounts. M3 Pro is available instantly in the MacBook Pro 14 and 16 and certainly will be in more machines later on. How can this be?
The Logic Behind “The Usual” Memory Configuration
We have grown accustomed to the knowledge that the amount of memory (RAM) must be a power of 2, like 8GB, 16GB, 32GB, and so on. Let’s try to unravel the math for the base configuration of 18GB.
First, we need to understand why this has been true in the past.
A typical consumer processor (CPU) has 2 memory channels. In today’s PC world, it is either DDR4 or DDR5.
Both channels are 64 bits wide, making a total of 128 bits.
Each memory stick or chip size is again a size like 4GB, 8GB, 16GB, and so on.
If we have 2x16GB memory sticks in 2 channels, then we have 32GB of memory in total. This configuration looks typical, familiar, and understandable.
Breaking Down Apple M3 Pro’s Memory Configuration
Now, two new concepts have appeared to make this 18 GB memory a reality.
New memory chips have appeared with 1.5x the size – 3Gbit (2×1.5), 6Gbit (4×1.5), 12 Gbit (8×1.5), and so on. These can form memory modules of 12GB, 24GB, and so on. Modern Macs don’t have memory sticks, of course; the memory is soldered onto the same package as the CPU.
The M2 Pro had 4 memory channels, which is why it still had “normal” memory options like 16GB and 32GB. The M2 Max already had these 1.5x size memory chips installed, allowing for a maximum memory of 96GB (64 x 1.5).
Now, here’s a surprise: the M3 Pro has 3 memory channels, 25% fewer.
As we combine 3 memory channels with 1.5x size memory chips, we get 3 x 6GB = 18GB of memory.
There you go, mystery solved!
This brief explanation intentionally excludes topics like the effect of having 8GB of memory as a minimum, memory speed, and CPU core configurations. This M3 generation certainly shuffles the deck and is more controversial.
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