I'm sure someone has asked something along these lines, but no matter how I word it, I can't seem to find a definitive answer
Does RAM get some kind of indication as to how much data the CPU requires or is there a set amount of data and if the CPU needs more, it needs to call again (and what is that set amount of data).
During a cache miss, the CPU will require at least a single cache line (32, 64 or 128 bytes). Does the CPU need to query RAM multiple times to get a single cache line (assuming the set amount is 8 bytes) or does the RAM return more data per call or does RAM have some way of being notified as to how much data is required?
I'm aware the CPU is most likely going to call more than just a single cache line to avoid the lengthy request from RAM if more data is needed in future from the same block of memory.
If you do know and have more information, the following extras would also interest me:
- If this is different depending on CPU make (generational differences can also be interesting but if you do add information about that, please skip similar generations)
- If this is different depending on RAM type
- How the CPU can request a different sized cache line or block size (only applies if RAM returns a fixed size that's the size of a block or cache line)
- Any other useful relevant information or resources for further reading
I seem to have found a clue to the answer when looking at the transfer rate of RAM, specifically where it indicates how to calculate the transfer rate from the specifications and from "DDR SDRAM prefetch architecture" which indicates how many 64 bit "blocks" can be requested per call to RAM (e.g. 8 for DDR3 and DDR4, 4 for DDR2 and 2 for DDR1)