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General register ER7 has the function of stack pointer (SP) in addition to its general-register
function, and is used implicitly in exception handling and subroutine calls. Figure 2-5 shows the
stack.
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) or a multiple of 2 bytes, 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.
SP (ER7)
Figure 2-5 Stack
Free area
Stack area
21
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