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I was recently quite surprised to find out that modern CPUs only have around 2 billion transistors and even GPUs like the GTX 1080 Ti have only 12 billion. A quick calculation on the other hand shows that a 256 GB micro SD card has roughly 1 trillion transistors (assuming 2 bits per transistor with MLC NAND) occupying the same area as a multi core CPU (the i7-8700K has a die area of 149mm^2, an SD card is roughly 165mm^2).

This seems very off to me but I can't see what I am missing, I know there is probably an error with my calculations as even the 11GB of VRAM of the GTX 1080 Ti should have more than 12 billion transistors but like I said I don't see where I am going wrong.

I can't understand how we can put orders of magnitude more transistors on the same space in relatively cheap electronic parts but have a far greater difficulty doing this with high end CPUs etc which cost much more too. Also given that both processors and flash memory are built on a similar process size surely the transistor density should be roughly the same for both of them. Can anyone clear up this confusion for me?

EDIT: I have made the question more specific as requested.

Hadi Khan
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1 Answers1

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Transistor count in processors varies widely. The original 6502 microprocessor had somewhere between 3.5 and 4.2 thousand transistors depending on the exact part number, and it's not unusual to find 'modern' microcontrollers with that few. Minimalistic ARM chips were around 22 thousand, and the counts are pretty similar for other equivalent RISC cores, though it's not unusual for a single core to end up in the hundreds of thousands if it's got lots of features. The original 80386 was around 275 thousand, and similar power CISC cores are generally about the same. On the far other end of the spectrum, an 8-core Ryzen 7 1700 is roughly 4.8 billion, which is a bit high, but still not out of the ordinary for 'modern' CPU's (and correlates well with the likely case of that sample CPU with 2 billion transistors having four cores).

The quote of 12 billion for the GTX 1080 Ti actually sounds about right to me for just the GPU chip itself, excluding the VRAM and any accessory chips. It's got way more 'cores' than a modern CPU, but each of those 'cores' is exponentially simpler to implement because they are far more specialized. The VRAM is dynamic RAM, which means you're looking at one transistor per bit (so 11GB translates to 11 trillion transistors minimum). The secondary chips are harder to quantify, since they vary from OEM to OEM, but usually are not too high in transistor count.

Your estimate of 1 trillion for a 256GB MLC NAND SD card doesn't sound too far off to me either, but you have to keep in mind:

  • SD cards aren't just the flash memory, they also have controller chips, which are about as complex as a 6502, or at the outside worst an ARM core. THis will increase your estimate, but not by much.
  • MLC NAND is actually not just 2-bit (some people use it that way, others don't). If instead the SD card is using higher bit counts per cell (4-bit cells do exist), then that decreases your count considerably.
  • An SD card has different operating voltages, signaling frequencies, power consumption, and head dissipation from a typical 'modern' CPU or GPU. THis has a big impact on how the die can be laid out, which in turn has a big impact on die size.
Austin Hemmelgarn
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  • Very nice write-up. Especially the last point is VERY important. CPU's and GPU are internally more complicated and have to deal with much higher power-levels so the die must have ways to get the excess heat to the heatsink(s). This limits the layout and transistor density on the die. Flash memory is much simpler in structure and has far less power run through it which allows for higher densities of transistors. – Tonny Aug 16 '18 at 11:26