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CENTRAL PROCESSING UNIT (CPU)

7733 Group User’s Manual

2–5

2.1.5 Program counter (PC)

The program counter consists of 16 bits. This counter indicates the low-order 16 bits of a store address,

which consists of 24 bits, of an instruction to be executed next, in other words an instruction which is read

from an instruction queue buffer.

At reset, value “FF16” is set to the high-order byte (PCH) of the program counter and value “FE16” is set

to the low-order byte (PCL) of the counter. And then, immediately after reset, the contents of the reset’s

vector addresses (addresses FFFE16, FFFF16) are set to the counter.

Figure 2.1.3 shows the program counter and the program bank register.

Fig. 2.1.3 Program counter and program bank register

2.1.6 Program bank register (PG)

The program bank register consists of 8 bits. (Refer to Figure 2.1.3.) This register indicates the high-order

8 bits of a store address, which consists of 24 bits, of an instruction to be executed next, in other words

an instruction which is read from an instruction queue buffer. These 8 bits indicate “bank.” The contents

of the program bank register is automatically incremented by 1 when a carry occurs in the following cases:

•When a certain value is added to the contents of the program counter

•When the displacement is added to the program counter by executing a branch instruction and others

The contents of the program bank register is automatically decremented by 1 when a borrow occurs in the

following case:

•When a certain value is subtracted from the contents of the program counter

Therefore, when normally programming, it is not necessary to give consideration to bank boundaries.

At reset, this register is cleared to “0016.”

PCH

PCL

b0 b15

b8 b7

(b16)

(b23)

2.1 Central processing unit

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