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2.4.3
Control Registers
The control registers are the 24-bit program counter (PC) and the 8-bit condition code register
(CCR).
Program Counter (PC): This 24-bit counter indicates the address of the next instruction the CPU
will execute. The length of all CPU instructions is 2 bytes (one word), so the least significant PC
bit is ignored. When an instruction is fetched, the least significant PC bit is regarded as 0.
Condition Code Register (CCR): This 8-bit register contains internal CPU status information,
including the interrupt mask bit (I) and half-carry (H), negative (N), zero (Z), overflow (V), and
carry (C) flags.
Bit 7—Interrupt Mask Bit (I): Masks interrupts other than NMI when set to 1. NMI is accepted
regardless of the I bit setting. The I bit is set to 1 at the start of an exception-handling sequence.
Bit 6—User Bit or Interrupt Mask Bit (UI): Can be written and read by software using the
LDC, STC, ANDC, ORC, and XORC instructions. This bit can also be used as an interrupt mask
bit. For details see section 5, Interrupt Controller.
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.
32
SP (ER7)
Figure 2.5 Stack
Free area
Stack area
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