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3.3
Memory Protection Strategy
The memory protection strategy in XC82x prevents unauthorized read out of critical data
and user IP from Flash memory by blocking all external access to the device.
This is achieved by using the Boot Mode Index (BMI) to control the boot options such
that once the BMI is programmed to enter user mode (productive), it is not allowed to
enter the other boot modes without an erase of the BMI by the user code.
Therefore, boot options that load and execute external code will be blocked and only
user code starting from address 0000
3.4
Special Function Registers
The Special Function Registers (SFRs) occupy direct internal data memory space in the
range 80
to FF
. All registers, except the program counter, reside in the SFR area. The
H
H
SFRs include pointers and registers that provide an interface between the CPU and the
on-chip peripherals. As the 128-SFR range is less than the total number of registers
required, address extension mechanisms are required to increase the number of
addressable SFRs. The address extension mechanisms include:
•
Mapping
•
Paging
3.4.1
Address Extension by Mapping
Address extension is performed at the system level by mapping. The SFR area is
extended into two portions: the standard (non-mapped) SFR area and the mapped SFR
area. Each portion supports the same address range 80
addressable SFRs to 256. The extended address range is not directly controlled by the
CPU instruction itself, but is derived from bit RMAP in the system control register
SYSCON0 at address 8F
SYSCON0 must be set. However, the SFRs in the standard area can be accessed by
clearing bit RMAP.
As long as bit RMAP is set, the mapped SFR area can be accessed. This bit is not
cleared automatically by hardware. Thus, before standard/mapped registers are
accessed, bit RMAP must be cleared/set, respectively, by software.
User's Manual
Memory Organization, V 0.1
H
. To access SFRs in the mapped area, bit RMAP in SFR
H
Figure 3-2
shows how the SFR area can be selected.
can be executed.
to FF
H
3-4
XC82x
Memory Organization
, bringing the number of
H
V1.0, 2010-02
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