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SP (ER7)
Stack area
Figure 2-6 Stack
2.4.3
Control Registers
The control registers are the 24-bit program counter (PC), 8-bit extended control register (EXR),
and 8-bit condition-code register (CCR).
(1) 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).
(2) Extended Control Register (EXR): This 8-bit register contains the trace bit (T) and three
interrupt mask bits (I2 to I0).
Bit 7—Trace Bit (T): Selects trace mode. When this bit is cleared to 0, instructions are executed
in sequence. When this bit is set to 1, a trace exception is generated each time an instruction is
executed.
Bits 6 to 3—Reserved: These bits are reserved. They are always read as 1.
Bits 2 to 0—Interrupt Mask Bits (I2 to I0): These bits designate the interrupt mask level (0 to
7). For details, refer to section 5, Interrupt Controller.
Operations can be performed on the EXR bits by the LDC, STC, ANDC, ORC, and XORC
instructions. All interrupts, including NMI, are disabled for three states after one of these
instructions is executed, except for STC.
(3) Condition-Code Register (CCR): This 8-bit register contains internal CPU status
information, including an interrupt mask bit (I) and half-carry (H), negative (N), zero (Z),
overflow (V), and carry (C) flags.
Rev. 5.00, 12/03, page 34 of 1088
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