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7721 Group User’s Manual

2.1 Central processing unit

CENTRAL PROCESSING UNIT (CPU)

2–5

2.1.5 Program counter (PC)

The program counter is a 16-bit counter that indicates the low-order 16 bits of the address (24 bits) at

which an instruction to be executed next (in other words, an instruction to be read out from an instruction

queue buffer next) is stored. The contents of the high-order program counter (PCH) become “FF16,” and the

low-order program counter (PCL) becomes “FE16” at reset. The contents of the program counter becomes

the contents of the reset’s vector address (addresses FFFE16, FFFF16) immediately after reset.

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 access space is divided in units of 64 Kbytes. This unit is called “bank.” (Refer to section “2.3 Access

space.”)

The program bank register is an 8-bit register. This register indicates the high-order 8 bits (bank) of the

address (24 bits) at which an instruction to be executed next (in other words, an instruction to be read out

from an instruction queue buffer next) is stored. These 8 bits are called bank.

When a carry occurs after adding the contents of the program counter or adding the offset value to the

contents of the program counter in the branch instruction and others, the contents of the program bank

program counter, the contents of the program bank register is automatically decremented by 1. Accordingly,

there is no need to consider bank boundaries in programming, usually.

This register is cleared to “0016” at reset.

2.1.7 Data bank register (DT)

The data bank register is an 8-bit register. In the following addressing modes using the data bank register,

the contents of this register is used as the high-order 8 bits (bank) of a 24-bit address to be accessed.

Use the LDT instruction to set a value to this register.

This register is cleared to “0016” at reset.

qAddressing modes using data bank register

•Direct indirect

•Direct indexed X indirect

•Direct indirect indexed Y

•Absolute

•Absolute bit

•Absolute indexed X

•Absolute indexed Y

•Absolute bit relative

•Stack pointer relative indirect indexed Y

PCH

PCL

b0 b15

b8 b7

(b16)

(b23)

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