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2.1 Memory Organization and Segmentation
The physical memory of an 80386 system is organized as a sequence of 8-bit
bytes. Each byte is assigned a unique address that ranges from zero to a
maximum of 2^(32) -1 (4 gigabytes).
80386 programs, however, are independent of the physical address space.
This means that programs can be written without knowledge of how much
physical memory is available and without knowledge of exactly where in
physical memory the instructions and data are located.
The model of memory organization seen by applications programmers is
determined by systems-software designers. The architecture of the 80386
gives designers the freedom to choose a model for each task. The model of
memory organization can range between the following extremes:
- A "flat" address space consisting of a single array of up to 4
gigabytes.
- A segmented address space consisting of a collection of up to 16,383
linear address spaces of up to 4 gigabytes each.
Both models can provide memory protection. Different tasks may employ
different models of memory organization. The criteria that designers use to
determine a memory organization model and the means that systems programmers
use to implement that model are covered in Part -- Programming.
2.1.1 The "Flat" Model
In a "flat" model of memory organization, the applications programmer sees
a single array of up to 2^(32) bytes (4 gigabytes). While the physical
memory can contain up to 4 gigabytes, it is usually much smaller; the
processor maps the 4 gigabyte flat space onto the physical address space by
the address translation mechanisms described in
Chapter 5
. Applications
programmers do not need to know the details of the mapping.
A pointer into this flat address space is a 32-bit ordinal number that may
range from 0 to 2^(32) -1. Relocation of separately-compiled modules in this
space must be performed by systems software (e.g., linkers, locators,
binders, loaders).
2.1.2 The Segmented Model
In a segmented model of memory organization, the address space as viewed by
an applications program (called the logical address space) is a much larger
space of up to 2^(46) bytes (64 terabytes). The processor maps the 64
terabyte logical address space onto the physical address space (up to 4
gigabytes ) by the address translation mechanisms described in
Chapter 5
.
Applications programmers do not need to know the details of this mapping.
Applications programmers view the logical address space of the 80386 as a
collection of up to 16,383 one-dimensional subspaces, each with a specified
length. Each of these linear subspaces is called a segment. A segment is a
unit of contiguous address space. Segment sizes may range from one byte up
to a maximum of 2^(32) bytes (4 gigabytes).
A complete pointer in this address space consists of two parts (see
Figure 2-1
):
- A segment selector, which is a 16-bit field that identifies a
segment.
- An offset, which is a 32-bit ordinal that addresses to the byte level
within a segment.
During execution of a program, the processor associates with a segment
selector the physical address of the beginning of the segment. Separately
compiled modules can be relocated at run time by changing the base address
of their segments. The size of a segment is variable; therefore, a segment
can be exactly the size of the module it contains.
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