2023-11-01 20:52:49
When the MacBook Pro M3 was announced, some people were surprised by the values announced by Apple for RAM: 18, 24 or 36 GB are in fact not classic values. But if the quantity of RAM is generally a power of 2 for the capacity of the strips, this is not an obligation and the LPDDR5 memory used by Apple has brought some subtleties.
It is not an error.
The first point to take into account: Apple does not use “bars” in the classic sense of the term. The company places LPDDR5X memory chips as close as possible to the processor, to simplify the design of motherboards. This is also a constraint of this type of memory: it has long been necessarily soldered to the motherboard.
From a practical standpoint, the components you can see alongside Apple’s SoCs are made up of multiple stacked memory chips, featuring a 16-bit bus. The organization depends partly on the system on chip, but each component can manage a 64-bit bus (M1, M2 and probably M3) or 128 bits (M1 Pro, M1 Max, M2 Max and probably M3 Max). The density can vary: the M1 Pro chip was coupled to two chips interfaced on 128 bits (256 bits in total), the M2 Pro chip went through four chips on 64 bits, for the same bandwidth, as iFixit shows it.
On the left, two components for the M1 Pro. On the right, four components for the M2 Pro. But the same bandwidth.
The number of components therefore depends on the memory bus, which is 128 bits (M1, M2, M3), 192 bits (M3 Pro), 256 bits (M1 Pro, M2 Pro), 384 bits (M3 Max) or 512 bits ( M1 Max, M2 Max, M3 Max). It even reaches 1,024 bits on the M1 Ultra and M2 Ultra variants.
Non-binary memory
For decades, the increase in RAM capacity was binary: a 16 Gb chip followed an 8 Gb chip, which replaced a 4 Gb chip, etc. With DDR5 memory, this point has been modified: non-binary capabilities are possible. This is the case at Apple but also in the bars: there is 24 GB DDR5 sticksFor example.
A 24 GB strip.
In Apple’s case, the M2 and M3 chips can therefore receive up to 24 GB of RAM, in the form of eight 24 Gb (gigabit) chips stacked in two groups. It’s the same thing for the M3 Pro and M3 Max: a model equipped with 18 GB of RAM actually integrates twelve 12 GB modules, grouped by four (i.e. a 192-bit bus). In the M3 Max chips, Apple also uses 24 Gb modules to reach 48 GB in total (on a 512-bit bus) but 32 Gb modules (a first) to reach 128 GB of RAM.
This expensive option uses very modern chips.
Finally, remember that even if the strips have values that double with each generation jump before DDR5, the total can be a value which is not a power of 2. In fact, it is not obligatory to couple the strips in pairs (you can install a 4 GB stick and an 8 GB stick, for a total of 12 GB) and in case the computer has more than two slots, various combinations are possible (two 16 GB sticks and two 4 GB sticks for example). Likewise, Apple has long integrated RAM into certain models (certain Power Macintoshes, the first iBooks, etc.) with the possibility of adding one or more modules. An iBook G3, for example, has 32 or 64 MB of RAM with the option of reaching 544 or 576 MB total.
The choice to go to 18 GB may therefore seem atypical, but it is logical in the case of the M3 Pro chips: with a 192-bit bus, it is not possible to offer 16 GB in total.
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