Register Model
e200z3 Power Architecture Core Reference Manual, Rev. 2
2-70
Freescale Semiconductor
2.17.1
Context Control Register (CTXCR)
The future versions of the e200z3 core may include optional hardware support for fast context switching
to provide real-time capabilities for embedded systems. The initial version of e200z3 does not implement
additional register contexts. A new privileged 32-bit special-purpose register (SPR) called the context
control register (CTXCR) is defined in the core CPU. The CTXCR controls the context registers that are
mapped to the current context and holds current, alternate, and saved context information. Supervisor
software reads the CTXCR to determine whether multiple contexts are supported in hardware, and if so,
the number implemented. When multiple register contexts are present (CTXCR[NUMCTX] is non-zero),
CTXCR is also writable. Otherwise, writes are ignored, and the register reads as all zeros. CTXCR is
shown in
.
2.18
SPR Register Access
SPRs are accessed with the mfspr and mtspr instructions. The following sections outline additional access
requirements.
2.18.1
Invalid SPR References
System behavior when an invalid SPR is referenced depends on the apparent privilege level of the register,
which is determined by bit 5 in the SPR address. If the invalid SPR is accessible in user mode, an illegal
exception is generated. If the invalid SPR is accessible only in supervisor mode and the CPU core is in
supervisor mode (MSR[PR] = 0), an illegal exception is generated. If the invalid SPR address is accessible
only in supervisor mode and the CPU is not in supervisor mode (MSR[PR] = 1), a privilege exception is
generated.
2.18.2
Synchronization Requirements for SPRs
Except for the following registers, no synchronization is required for accessing SPRs beyond those stated
in Book E. EREF completely describes synchronization requirements. Software requirements for
32
55
56
63
Field
—
Reset
All zeros
R/W
R/W (Writes are ignored because no additional contexts are implemented.)
SPR
SPR 560
Figure 2-54. Context Control Register (CTXCR)
Table 2-38. System Response to Invalid SPR Reference
SPR Address Bit 5
Mode
MSR[PR]
Response
0
—
—
Illegal exception
1
Supervisor
0
Illegal exception
1
User
1
Privilege exception
Содержание e200z3
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