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0 pointsjeffbee3y ago0 comments

The memory in an M1 is not "on die" it is plain old DRAM that they buy from the Koreans and solder to the board just like Intel and AMD and everyone else. DRAM is made on a fundamentally different semiconductor process and there will never, ever be a CPU with on-die DRAM. DRAM that can be made on a CPU logic process is called eDRAM. A huge eDRAM is a few tens of megabytes, while a huge DRAM is gigabytes. The bit cell density of eDRAM is slightly better than SRAM and 1000x lower than DRAM.

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babypuncher3y ago

I think the person you are responding too used the wrong vocabulary. Apple mounts the SoC and DRAM together in a system-in-a-package design, which is pretty different from how thin & light x86 manufacturers solder DRAM chips to the mainboard. The proximity between the SoC and DRAM is part of what makes the M1s bandwidth possible.

erik3y ago

The M1's bandwidth is possible because Apple uses high end LPDDR ram and a memory controller with a lot of channels. They aren't doing anything exotic.

Consumer PCs don't match this bandwidth because DDR DIMMs generally aren't as fast as LPDDR. Plus AMD & Intel limit their mainstream consumer CPUs to two memory channels, both for cost savings and to segment the market.

vitus3y ago

> Consumer PCs don't match this bandwidth because DDR DIMMs generally aren't as fast as LPDDR.

What about LPDDR (low-power DDR) allows it to be faster? And, by faster, do you mean lower latency? higher clock rates -> higher throughput? This is unintuitive to me.

My impression is that lower power means that you can't sustain higher clocks as readily (in fact, when overclocking RAM, it's common to increase voltage in the interest of stability).

I can't find anything about CAS latencies for LPDDR DIMMs.

edit: to clarify: when overclocking RAM, your two options are either increase voltage or increase timings, as if you want to sustain higher speeds, you need to either charge your capacitors faster, or wait more cycles for them to be charged.

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jeffbeeOP3y ago

The DRAMs in an M1 system are not really any closer to the CPU than they are in competing ARM and x86 systems.

https://cdn.arstechnica.net/wp-content/uploads/2020/09/tiger...

WithinReason3y ago

I should have said "on package", not "on chip".

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