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7480 Group and 7481 Group User's Manual

1-16

HARDWARE

States of the CPU internal registers immediately after system is released from reset are as follows:

• The interrupt disable flag (I) of the processor status register (PS) is set to ‘1’.

• The high-order 8 bits (PCH) of the program counter contain the contents of address ‘FFFF16’, and the low-

order 8 bits (PCL) contain the contents of address ‘FFFE16’.

Since the contents of the CPU internal registers not mentioned above are undefined immediately after

system is released from reset, it is necessary to initialize these registers by software.

1.7.1 Accumulator (A)

The accumulator is an 8-bit register. Data manipulations, such as arithmetic or logical operation and

transfers, are performed using this register.

1.7.2 Index Register X (X)

Index register X is an 8-bit register that performs addressing in the index addressing mode.

1.7.3 Index Register Y (Y)

Index register Y is an 8-bit register that performs addressing for certain instructions in the index addressing

mode.

1.7.4 Stack Pointer (S)

The stack pointer is an 8-bit register. It indicates the start address of the stack area where the contents

of registers pushed at subroutine call or interrupt are stored.

The low-order 8 bits in the stack are addressed by the stack pointer, and the high-order 8 bits are

addressed by the content of the stack page selection bit. When this bit is ‘0’, the high-order 8 bits indicate

‘0016’, and when ‘1’, they indicate ‘0116’.

For the 7480 Group and 7481 Group, the stack page selection bit is assigned to bit 2 of the CPU mode

Note: In the 7480 Group and 7481 Group, however, the product with RAM whose memory size is 192 bytes

or less does not have RAM on 1 page. Therefore, clear this bit to ‘0’.

Figure 1.7.2 shows the operation for pushing onto and pulling from the stack. Push the contents of

necessary registers other than those described here onto stack by software.

Table 1.7.1 lists the push and pull instructions for the accumulator and the processor status register.

Initialize the stack pointer by software because it is undefined immediately after system is released from

reset.

Table 1.7.1 Push and Pull Instructions for Accumulator and Processor Status Register

1.7 Central Processing Unit (CPU)

Accumulator

Processor Status Register

Push Instructions

PHA

PHP

Pull Instructions

PLA

PLP

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