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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 2-9 shows the
stack.
Free area
SP (ER7)
Stack area
Figure 2-9 Stack
2.4.3
Control Registers
The control registers are the 24-bit program counter (PC), 8-bit extended control register (EXR),
8-bit condition-code register (CCR), and 64-bit multiply-accumulate register (MAC).
(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.
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