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1.2
Registers
1.2.1
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 (R0 to R7), or the high (R0H to R7H) and
low (R0L to R7L) bytes can be accessed separately as 8-bit registers. The register length is
determined by the instruction.
R7 also functions as the stack pointer, used implicitly by hardware in processing interrupts and
subroutine calls. In assembly language, the letters SP can be coded as a synonym for R7. As
indicated in figure 1-4, R7 (SP) points to the top of the stack.
SP (R7)
1.2.2
Control Registers
The CPU has 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. Instructions are fetched by 16-bit (word) access, so the least significant bit
of the PC is ignored (always regarded as 0).
(2) Condition Code Register (CCR): This 8-bit register indicates the internal status of the CPU
with an interrupt mask (I) bit and five flag bits: half-carry (H), negative (N), zero (Z),
overflow (V), and carry (C) flags. The two unused bits are available to the user. The bit
configuration of the condition code register is shown below.
Figure 1-4. Stack Pointer
Unused area
Stack area
7
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