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Bit 5—Half-Carry Flag (H): When the ADD.B, ADDX.B, SUB.B, SUBX.B, CMP.B, or NEG.B
instruction is executed, this flag is set to 1 if there is a carry or borrow at bit 3, and cleared to 0
otherwise. When the ADD.W, SUB.W, CMP.W, or NEG.W instruction is executed, the H flag is
set to 1 if there is a carry or borrow at bit 11, and cleared to 0 otherwise. When the ADD.L,
SUB.L, CMP.L, or NEG.L instruction is executed, the H flag is set to 1 if there is a carry or
borrow at bit 27, and cleared to 0 otherwise.
Bit 4—User Bit (U): Can be written and read by software using the LDC, STC, ANDC, ORC, and
XORC instructions.
Bit 3—Negative Flag (N): Stores the value of the most significant bit of data, regarded as the
sign bit.
Bit 2—Zero Flag (Z): Set to 1 to indicate zero data, and cleared to 0 to indicate non-zero data.
Bit 1—Overflow Flag (V): Set to 1 when an arithmetic overflow occurs, and cleared to 0 at other
times.
Bit 0—Carry Flag (C): Set to 1 when a carry is generated by execution of an operation, and
cleared to 0 otherwise. Used by:
• Add instructions, to indicate a carry
• Subtract instructions, to indicate a borrow
• Shift and rotate instructions
The carry flag is also used as a bit accumulator by bit manipulation instructions.
Some instructions leave flag bits unchanged. Operations can be performed on CCR by the LDC,
STC, ANDC, ORC, and XORC instructions. The N, Z, V, and C flags are used by conditional
branch (Bcc) instructions.
For the action of each instruction on the flag bits, see appendix A.1, Instruction List. For the I and
UI bits, see section 5, Interrupt Controller.
2.4.4
Initial CPU Register Values
In reset exception handling, PC is initialized to a value loaded from the vector table, and the I bit
in CCR is set to 1. The other CCR bits and the general registers are not initialized. In particular,
the initial value of the stack pointer (ER7) is also undefined. The stack pointer (ER7) must
therefore be initialized by an MOV.L instruction executed immediately after a reset.
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