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XC2200 Derivatives
System Units (Vol. 1 of 2)
Central Processing Unit (CPU)
User’s Manual
4-30
V2.1, 2008-08
CPUSV2_X, V2.2
Bitfield BANK in register PSW selects which of the three physical register banks is
activated. The selected bank can be changed explicitly by any instruction which writes
to the PSW, or implicitly by a RETI instruction, an interrupt or hardware trap. In case of
an interrupt, the selection of the register bank is configured via registers BNKSELx in the
Interrupt Controller ITC. Hardware traps always use the global register bank.
The local register banks are built of dedicated physical registers, while the global register
bank represents a cache. The banks of the memory-mapped GPRs (global bank) are
located in the internal DPRAM. One bank uses a block of 16 consecutive words. A
Context Pointer (CP) register determines the base address of the current selected bank.
To provide the required access speed, the GPRs located in the DPRAM are cached in
the 5-port register file (only one memory-mapped GPR bank can be cached at the time).
If the global register bank is activated, the cache will be validated before further
instructions are executed. After validation, all further accesses to the GPRs are
redirected to the global register bank.
Figure 4-6
Register Bank Selection via Register CP
MCA04921
R15
R13
R12
R11
R10
R9
R8
R7
R6
R5
R4
R3
R2
R1
R0
16-Bit Context Pointer
15
0
Internal DPRAM
R15
R0
Global
local
Register File
(CP) + 30
R14
(CP) + 28
(CP) + 2
(CP)