Processor Programming (Continued)
3.4.2 Memory Address Space
3.5.1 Offset Mechanism
The processor directly addresses up to 4 GB of physical
memory even though the memory controller addresses
only 256 MB of DRAM. Memory address space is
accessed as BYTES, WORDS (16 bits) or DWORDs (32
bits). WORDS and DWORDs are stored in consecutive
memory bytes with the low-order byte located in the low-
est address. The physical address of a WORD or DWORD
is the byte address of the low-order byte.
In all operating modes, the offset mechanism computes
an offset (effective) address by adding together up to
three values: a base, an index and a displacement. The
base, if present, is the value in one of eight general regis-
ters at the time of the execution of the instruction. The
index, like the base, is a value that is contained in one of
the general registers (except the ESP register) when the
instruction is executed. The index differs from the base in
that the index is first multiplied by a scale factor of 1, 2, 4
or 8 before the summation is made. The third component
added to the memory address calculation is the displace-
ment that is a value supplied as part of the instruction.
Figure 3-3 illustrates the calculation of the offset address.
The processor allows memory to be addressed using nine
different addressing modes. These addressing modes are
used to calculate an offset address, often referred to as an
effective address. Depending on the operating mode of
the CPU, the offset is then combined, using memory man-
agement mechanisms, into a physical address that is
applied to the physical memory devices.
Nine valid combinations of the base, index, scale factor
and displacement can be used with the CPU instruction
set. These combinations are listed in Table 3-19. The
base and index both refer to contents of a register as indi-
cated by [Base] and [Index].
Memory management mechanisms consist of segmenta-
tion and paging. Segmentation allows each program to
use several independent, protected address spaces. Pag-
ing translates a logical address into a physical address
using translation lookup tables. Virtual memory is often
implemented using paging. Either or both of these mecha-
nisms can be used for management of the GXLV proces-
sor memory address space.
In real mode operation, the CPU only addresses the low-
est 1 MB of memory and the offset contains 16-bits. In
protected mode the offset contains 32 bits. Initialization
and transition to protected mode is described in Section
3.9.4 “Initialization and Transition to Protected Mode” on
page 93.
3.5 OFFSET, SEGMENT, AND PAGING
MECHANISMS
The mapping of address space into a sequence of mem-
ory locations (often cached) is performed by the offset,
segment, and paging mechanisms.
Index
Base
Displacement
In general, the offset, segment and paging mechanisms
work in tandem as shown below:
Scaling
x1, x2, x4, x8
instruction offset offset mechanism offset address
offset address
linear address
segment mechanism
paging mechanism
linear address
physical page.
+
As will be explained, the actual operations depend on sev-
eral factors such as the current operating mode and if
paging is enabled.
Offset Address
(Effective Address)
Note: The paging mechanism uses part of the linear
Figure 3-3. Offset Address Calculation
address as an offset on the physical page.
www.national.com
64
Revision 1.1
NSC [ National Semiconductor ]
