Chapter 2. Registers
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2-5
User Programming Model
2.2.3 User Stack Pointer (A7)
The CF4e architecture supports two unique stack pointer (A7) registers—the supervisor
stack pointer (SSP) and the user stack pointer (USP). This support provides the required
isolation between operating modes as dictated by the virtual memory management scheme
provided by the memory management unit (MMU). The SSP is described in Section 2.3.3,
“Supervisor/User Stack Pointers (A7 and OTHER_A7).”
2.2.4 Program Counter (PC)
The PC holds the address of the executing instruction. For sequential instructions, the
processor automatically increments PC. When program flow changes, the PC is updated
with the target instruction. For some instructions, the PC specifies the base address for
PC-relative operand addressing modes. If two 16-bit instructions are dispatched together,
the PC is advanced by 4 bytes, so that it points to the next instruction after this pair.
2.2.5 Condition Code Register (CCR)
The CCR occupies SR[7–0], as shown in Figure 2-3. CCR[4–0] are indicator flags based
on results generated by arithmetic operations.
CCR fields are described in Table 2-1.
7
5
4
3
2
1
0
Field
—
X
N
Z
V
C
Reset
000
Undefined
R/W
R
R/W
R/W
R/W
R/W
R/W
Figure 2-3. Condition Code Register (CCR)
Table 2-1. CCR Field Descriptions
Bits
Name
Description
7–5
—
Reserved. These bits are read as 0; writes have no effect.
4
X
Extend condition code bit. Assigned the value of the carry bit for arithmetic operations; otherwise not
affected or set to a specified result. Also used as an input operand for multiple-precision arithmetic.
3
N
Negative condition code bit. Set if the msb of the result is set; otherwise cleared.
2
Z
Zero condition code bit. Set if the result equals zero; otherwise cleared.
1
V
Overflow condition code bit. Set if an arithmetic overflow occurs, implying that the result cannot be
represented in the operand size; otherwise cleared.
0
C
Carry condition code bit. Set if a carry-out of the data operand msb occurs for an addition or if a
borrow occurs in a subtraction; otherwise cleared.
F
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