A computer has a finite amount of RAM available, but each
HP-UX process has a 4GB virtual address space apportioned in four
one-gigabyte quadrants, termed virtual memory.
Virtual memory is the software construct that allows each
process sufficient computational space in which to execute. It
is accomplished with hardware support.
Virtual Space in PA-RISC |
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As software is compiled and run, it generates virtual addresses
that provide programmers with memory space many times larger than
physical memory alone. The number of bits available for the space
determines the ultimate size of the virtual address space. At PA-RISC
1.x, the operating system has 32-bit physical addressing and 48-bit
virtual addressing (the latter consisting of 16-bit space and 32-bit
offset to allow for 4 GB per space); the total virtual address range
is
(2 ^ 16) * 4 GB = 262,144 GB
By comparison, Level 2 has a far greater total virtual address
range of
( 2 ^ 32 ) * 4 GB = 17,179,869,184 GB
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 | NOTE: Understand, however, that a single process has significant
limitations on the virtual address space it is allowed to access.
For example, a SHARE_MAGIC executable
text is limited to 1 GB and data is limited to 1 GB. The total amount
of shared virtual address space in the system is limited to 1.75
GB. |
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Physical Addresses |
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A physical address points to a page in memory that represents
4096 bytes of data. The physical address also contains an offset
into this page. Thus, the complete physical address is composed
of a
physical page number(PPN) and page offset. The PPN is the 20 most
significant bits of the physical address where the page is located.
These bits are concatenated with an 12-bit page offset to form
the 32-bit physical address.
To handle the translation of the virtual address to a physical
address the virtual address also needs to be looked at as a
virtual page number(VPN) and page offset. Since the page size is
4096 bytes, the low order 12 bits of the offset are assumed to be
the offset into the page. The space ID and the high order 20 bits
of the offset are the VPN.
For any given address you can determine the page number by
discarding the least significant 12 bits. What remains is the virtual
page number for a virtual address or the physical page number for
the physical address.
The next figure shows the bit layout of a virtual address
of 0x0.4873.
The virtual page number's address must be translated
to obtain the associated physical page number, with page offset
0x873.
![[Bit layout of physical address]](img/gfx3.gif)
![[Bit layout of virtual page address]](img/gfx4.gif)