I was looking at the Intel i9 12900k specifications stating that the memory types supported are DDR5 4800 MT/s and DDR4 at 3200 MT/s.
What is that translated to MHz? And will a CPU with max memory type of DDR5 4800 MT/s support modules at 6000 MHz?
32 Answers
Using Megahertz to denote data transfer rate comes from early days which used Single Data Rate memory (SDR). At the time, your DRAM frequency was equal to your memory speed because data was transmitted only once every clock cycle.
However, things changed with the advent of Double Data Rate (DDR) memory. Modern DDR memory modules transmit data twice per clock cycle: Once at the rising edge of the square wave and once at the falling edge.
So, what you often see advertised as 3600 MHz DDR4 Memory is a module operating at an 1800 MHz clock rate with an effective transfer rate of 3600 MT/s (Megatransfers/second).
Megatransfer is a bandwidth-agnostic term usually used for the rate of signal on I/O buses. It tells you how many transfers are taking place per second but is independent of the memory bus width (pipe thickness in bits).
Take for example a DDR4-3200 memory module: As DDR4 has a bus width of 64 bits (8 bytes), and as DDR4-3200 transmits data at a maximum rate of "3200 MT/s*8 Bytes", this means an effective transfer rate 25,600 MB/s.
This value is called "Peak transfer rate" in JEDEC specification, and is a much better way to measure memory speed.
For a discussion in depth, see the articleMT/s vs MHz (Datarate vs Frequency) in RAM Modules.
Theory:
Hz is simply defined as 1/s. By saying something has a clock of 1Hz you actually mean something cycles by a rate of 1/s.
Transfers per second (T/s) is a measurement of transfers over a bus per second.
So instead of saying 1MT/s you could also say 1MTHz.
Practice:
Memory speed is commonly expressed in effective frequency, which means data transfer cycles performed each second. For example, DDR5 6000 refers to 6000 million (or 6 billion, or 10^9) transfers per second. This means the advertised clock of RAM (e.g. 6000MHz) is not the memory chips clock speed, but the rate at which data is transferred to and from the RAM.
So in practice the advertised theoretical speed in MHz is 1/1 equal to the theoretical transfer rate in MT/s.
