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The maximum random access memory (RAM) installed in any computer system is limited by hardware, software and economic factors. The hardware may have a limited number of address bus bits, limited by the processor package or design of the system. Some of the address space may be shared between RAM, peripherals, and read-only memory.
The DDR4 standard allows for DIMMs of up to 64 GB in capacity, compared to DDR3's maximum of 16 GB per DIMM. [ 1 ] [ 8 ] [ failed verification ] Unlike previous generations of DDR memory, prefetch has not been increased above the 8n used in DDR3; [ 9 ] : 16 the basic burst size is eight 64-bit words, and higher bandwidths are achieved by ...
The IBM System/360 of the 1960s was an early 32-bit computer; it had 32-bit integer registers, although it only used the low order 24 bits of a word for addresses, resulting in a 16 MiB (16 × 1024 2 bytes) address space. 32-bit superminicomputers, such as the DEC VAX, became common in the 1970s, and 32-bit microprocessors, such as the Motorola ...
Many 32-bit computers have 32 physical address bits and are thus limited to 4 GiB (2 32 words) of memory. [3] [4] x86 processors prior to the Pentium Pro have 32 or fewer physical address bits; however, most x86 processors since the Pentium Pro, which was first sold in 1995, have the Physical Address Extension (PAE) mechanism, [5]: 445 which allows addressing up to 64 GiB (2 36 words) of memory.
Data is accessed in bursts of either 16 or 32 transfers (256 or 512 bits, 32 or 64 bytes, 8 or 16 cycles DDR). Bursts must begin on 64-bit boundaries. Since the clock frequency is higher and the minimum burst length longer than earlier standards, control signals can be more highly multiplexed without the command/address bus becoming a bottleneck.
Mac OS X Tiger through Mac OS X Snow Leopard support PAE and the NX bit on IA-32 processors; Snow Leopard was the last version to support IA-32 processors. On x86-64 processors, all versions of macOS use 4-level paging (IA-32e paging rather than PAE) to address memory above 4GB. Mac Pro and Xserve systems can use up to 64 GB of RAM. [23]
DDR5 octuples the maximum DIMM capacity from 64 GB to 512 GB. [8] [3] DDR5 also has higher frequencies than DDR4, up to 8GT/s which translates into 64 GB/s (8 gigatransfers/second × 64-bits/module / 8 bits/byte = 64 GB/s) of bandwidth per DIMM. Rambus announced a working DDR5 dual in-line memory module (DIMM) in September 2017.
Physical Address Extension or PAE was first added in the Intel Pentium Pro, and later by AMD in the Athlon processors, [47] to allow up to 64 GB of RAM to be addressed. Without PAE, physical RAM in 32-bit protected mode is usually limited to 4 GB. PAE defines a different page table structure with wider page table entries and a third level of ...
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