虚拟 memory 中的地址转换

Question

I'm watching these series and I don't understand what is the 12 bits page offset (I only raise 2^12-1 and got 4Kb and I though it is the size of the page itself but I'm not sure it's correct). Also I don't understand what is the virtual page number . I've watched all the series 1-6 from the guy, and did understand what he presented. I also enjoy the explanation with the houses and streets but I still don't get it.

This is the picture he explains:

Answer 1

OK - the diagram you posted is a very good description of "virtual memory".

With regard to your most recent question:

Q: What does the page offset?

1. When your program wants to access memory, it uses a "Virtual Address".

2. "Virtual addresses" map to "Physical addresses".

   The mapping involves a combination of "page #" (what "page" does the data we're interested in reside in?) and an "offset".

3. The "offset" is simply the location within a page where our data resides.

   Offset "0" is the first byte in the page.

   Offset "9" is the 10th byte in the page.

   Etc. Etc.

Perhaps part of your confusion is:

Q: Why do we need "virtual memory" (and pages and address mappings) in the first place???

Consider an operating system like DOS. There is no virtual addressing, there are no "pages". Everything is just "physical memory".

In DOS, a 16-bit address IS the physical address. It points directly to a specific location - a specific byte - in physical memory. The "address" IS the "offset", from physical memory location "0". (Strictly speaking, a DOS "address" is actually the 16-bit offset into the current "segment", which is in the Intel 8086 "segment register". But we digress...)

One of the problems with a simple, non-paged system like DOS is RAM limits . Another problem is that any DOS application can access - and potentially corrupt - the memory used by any other active DOS application (including DOS TSRs).

Virtual Memory solves these - and many other - problems. Even though "paging" introduces extra complexity, systems that support virtual memory can be much more powerful, robust and secure.

Q: Does that help?