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2-2

FUNCTIONAL DESCRIPTION

2.1 Central processing unit

7220 Group User’s Manual

2.1 Central processing unit

The CPU of the M37221M6-XXXSP/FP has six main registers.

The program counter (PC) is a 16-bit register consists of PCH and PCL, both of which are 8-bit registers.

The other five registers: the accumulator (A), index register X (X), index register Y (Y), stack pointer (S) and

processor status register (PS), all have an 8-bit configuration.

Note: The contents of registers above except the following are indeterminate after a hardware reset. Therefore,

initialize these registers by software.

• The Interrupt disable flag I of the processor status register = “1”

• The program counter = the contents of addresses FFFE16 and FFFF16

Figure 2.1.1 shows the registers configuration diagram of M37221M6-XXXSP/FP.

NV T B D I Z

PCL

Decimal Operation Mode Flag

Break Flag

X Modified Operation Mode Flag

Overflow Flag

Negative Flag

PCH

Accumulator (A)

Index Register X (X)

Index Register Y (Y)

Stack Pointer (S)

Program Counter (PC)

Processor Status Register (PS)

Carry Flag

Zero Flag

Interrupt Disable Flag

Fig. 2.1.1 Registers configuration diagram

2.1.1 Accumulator (A)

The accumulator is the central register of the microcomputer and 8-bit register.

This general-purpose register is used with considerable for arithmetic operations, data transfer, temporary

clearing, condition judgments, etc.

2.1.2 Index register X (X), index register Y (Y)

The M37221M6-XXXSP/FP has the index register X and the index register Y, both of which are 8-bit

registers.

In the addressing modes which use these index registers, the register contents are added to the specified

address and this becomes the actual address. These modes are used for referencing subroutine tables and

memory tables.

The index registers, which have increment, decrement, comparison and data transfer functions, are also

used as simple accumulators.

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