Why does my CPU only support 32GB RAM when it has 39 address bits?

Question

The following is the output of cat /proc/cpuinfo

user@localhost:~$ cat /proc/cpuinfo 
processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 60
model name  : Intel(R) Core(TM) i3-4130 CPU @ 3.40GHz
stepping    : 3
microcode   : 0x12
cpu MHz     : 800.000
cache size  : 3072 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2
apicid      : 0
initial apicid  : 0
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 fma cx16 xtpr pdcm pcid sse4_1 sse4_2 movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
bogomips    : 6784.48
clflush size    : 64
cache_alignment : 64
address sizes   : 39 bits physical, 48 bits virtual
power management:

Now, 39 bits should get me 512GiB addressable physical memory. However, the Intel(R) Core(TM) i3-4130 CPU @ 3.40GHz only supports 32GiB. Why is this? How can I reconcile this calculation?

I am not asking about the number of slots in the motherboard, maximum ram / DIMM cards etc. I am looking for how the physical bits, address lines, and the maximum amount of physical RAM fit.

Also, 48 bits should account for 256TiB of virtual address space. Is this true??

Answer 1

It only supports 32GB of RAM because of the memory controller. It was designed with this limitation. Similar, it is limited to 25.6 GB/s bandwidth.

Answer 2

Because the number of physical address bits that the chipset supports is a superset of the address bits that the DRAM controller supports.

If you max out the DRAM capacity on the system, there still needs to be room for other to things to be mapped into physical address space: PCIe configuration space, MMIO spaces for graphics card and other controllers. The DRAM controller is only mapped to a small portion of the physical space. The maximum size of DRAM that the DRAM system supports must be less than the size of the physical address space of the system.

By DRAM system I mean the address size the DRAM controller supports, the singular DDR generation the controller's physical pin interface is built for, the number of channels, the external wiring (chip selects might be physically disconnected, limiting the number of ranks per channel – DDR4 supports 4 ranks per channel, be it 4 single rank DIMMs (yes DIMM not SIMM), 2 dual rank modules, or 1 quad rank module), the DIMM type that the DRAM controller pin interface is built for (if it's LRDIMM then it supports logical ranks giving up to 32 total ranks), the number of banks and bank groups supported (should be 4 banks and 4 bank groups but it could be less and may only have one bank group wire instead of 2), the number of row select pins (the controller could only have 16 instead of 17).

Answer 3

All of the Intel LGA 1150 socket cpu's are limited to 32GB.