POWER8 architecture 'flat' virtual address space

Question

As mentioned here, can anyone explain what is meant by a 'flat' 32 bit space?

Text for the lazy:

Another interesting feature of the architecture is a virtual address system which maps all addresses into a 52-bit space. In this way applications can share memory in a "flat" 32-bit space, and all of the programs can have different blocks of 32 bits each.

Have you read the Power ISA documentation from http://www.power.org/? I'm sure the answer to your question is in there somewhere. — Bradley T. Hughes, Jan 24 '13 at 17:55

score 4 · Accepted Answer · answered Jan 24 '13 at 21:58

A "flat" address space means that the virtualized 32-bit space used for each program is independently and directly accessible in consecutive offsets starting at byte 0 all the way through xFFFFFFFF.

Contrast this with what are typically termed "segmented" address spaces where an address is composed of a segment number or descriptor, and then a smaller number of bits as an offset into that segment/descriptor.

The memory management system performs the sleight of hand to translate a 32-bit address reference into a specific location within the actual 52-bit address space provided by the underlying architecture.

I think this is basically correct. A *flat* address space is one without holes. As opposed to *segmented*, I would think *unmapped* might be the contrast. Since this is for the Power8, I would say that each process could have 4GB worth of swap for itself for about 1 million processes. — artless noise, Jan 25 '13 at 01:54

POWER8 architecture 'flat' virtual address space

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