2.2
Register Descriptions
2.2.1
General Registers
All the general registers can be used as both data registers and address registers. When used as
address registers, the general registers are accessed as 16-bit registers (R0 to R7). When used as
data registers, they can be accessed as 16-bit registers, or the high and low bytes can be accessed
separately as 8-bit registers (R0H to R7H and R0L to R7L).
R7 also functions as the stack pointer, used implicitly by hardware in processing interrupts and
subroutine calls. In assembly-language coding, R7 can also be denoted by the letters SP. As
indicated in figure 2.2, R7 (SP) points to the top of the stack.
Unused area
SP (R7)
Stack area
Figure 2.2 Stack Pointer
2.2.2
Control Registers
The CPU control registers include a 16-bit program counter (PC) and an 8-bit condition code
register (CCR).
(1) Program Counter (PC): This 16-bit register indicates the address of the next instruction the
CPU will execute. Each instruction is accessed in 16 bits (1 word), so the least significant bit of
the PC is ignored (always regarded as 0).
(2) Condition Code Register (CCR): This 8-bit register contains internal status information,
including carry (C), overflow (V), zero (Z), negative (N), and half-carry (H) flags and the interrupt
mask bit (I).
Bit 7—Interrupt Mask Bit (I): When this bit is set to 1, all interrupts except NMI are masked.
This bit is set to 1 automatically by a reset and at the start of interrupt handling.
Bit 6—User Bit (U): This bit can be written and read by software (using the LDC, STC, ANDC,
ORC, and XORC instructions).
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