Curious about Unified Memory and M4 models

These seem to be the choices for M4-family Macs released to date:

Plain M4

• 16, 24, or 32 GB – 24" iMac, Mac mini. (On M4 iMacs, 32 GB is only available for Macs with four ports and a 512 GB or larger SSD.)

M4 Pro

• 24 or 48 GB – 14" or 16" MacBook Pro, Mac mini.
• 64 GB – Mac mini. (On MacBook Pros, you must order a M4 Max chip with a 40-core GPU in order to select 64 GB of RAM.)

M4 Max

• 36 GB – 14" or 16" MacBook Pro (32-core GPU)
• 64 or 128 GB – 14" or 16" MacBook Pro (40-core GPU)

Not 100% certain, but I have been going on the working assumption that the "unusual" memory sizes allow certain address spaces to be reserved to the efficiency, performance, neural and GPU cores and then "shared" for fast DMA between the cores, while also preserving the usual memory paging architecture.

The M-series SOC has some very unique features like massively deep speculative pipelining, so I expect that some of the memory architecture also supports that with pipelines holding massive stacks in RAM that might need to be flushed without touching the usual page tables.

Memory has always been installable in various combinations.

This is probably just marketing/psychology. People balk at a double-size upgrade, sometimes with a more-than-double price, but might go for a smaller one.

Gordon Rankin wrote:

Anyway I am curious as I am planning to update from my MacBook Pro 15 i9x6/32GB/1TB to a new M4 series.

In the past and as it is with the standard M4 the memory is 8 based, so 16GB or 32GB everything doubles.

Were it gets a bit weird for me is when the memory goes to 12 based so 24, 36, 48... for the M4 Pro and MAX. I know it's still divisible by 64 bit buss, but why the radical difference? Thanks Gordon

Not sure what vintage your Intel 15" MBP...(?)

RAM— does not have to be paired

The black Macbook 2006-2007 shipped with 3GB of RAM...

see if there is anything here of value—

64-bit programs not to be confused with processor architecture...

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

SoC M series Macs, like the M1, M2, M3, M4 are all considered 64-bit processor architectures, specifically using the ARM64 (AArch64) extension set, which is the 64-bit version of the ARM architecture; meaning they are fully 64-bit capable.

you can read more—

https://arstechnica.com/apple/2024/11/review-the-fastest-of-the-m4-macbook-pros-might-be-the-least-interesting-one/

macOS 15 Sequoia: The Ars Technica review

compressed memory goes back a ways:

https://arstechnica.com/gadgets/2013/10/os-x-10-9/#page-17

Unified memory the new MBP SoC—

While some operations lend themselves to the CPU, others are ideal for the GPU. An analogy would be transport via jet plane (CPU: moving a small amount fast), or super tanker (GPU: moving a huge amount with longer latency). However, many operations require sharing data between the CPU and GPU some memory transfer can be the bottleneck. In contrast, the Apple SoC chip has unified memory, allowing the GPU and CPU to share memory without penalty. ref: https://github.com/neurolabusc/AppleSiliconForNeuroimaging