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M37640E8FP

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

Mitsumi 

Ver 1.4

1.9 CENTRAL PROCESSING UNIT

The central processing unit (CPU) has six registers:

• Accumulator (A)

• Index Register X (X)

• Index Register Y (Y)

• Stack Pointer (S)

• Processor Status Register (PS)

• Program Counter (PC)

MITSUBISHI MICROCOMPUTERS

7640 Group

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

1.9.1 Register Structure

Five of the CPU registers are 8-bit registers. These are

the Accumulator (A), Index register X (X), Index Regis-

ter Y (Y), Stack pointer (S), and the Processor Status

register (PS) as shown in Figure 1.4.

The Program counter (PC) is a 16-bit register consist-

ing of two 8-bit registers (PCH and PCL).

After a hardware reset, bit 2 (I Flag) of the PS is set

high and the values at the address FFFA16 and

FFFB16 are stored in the PC, but the values of the

other bits of the PS and other registers are undefined.

Initialization of the undefined registers may be neces-

sary for some programs.

15

PCH

Fig. 1.4. Register Structure

7

Accumulator

0

7

Index Register X

0

7

Index Register Y

0

7

Stack Pointer

0

7

0

PCL

Program Counter

7 N V T B D I Z C 0 Processor Status Register

Carry Flag (bit 0)

Zero Flag (bit 1)

Interrupt Disable Flag (bit 2)

Decimal Mode Flag (bit 3)

Break Flag (bit 4)

Index X Mode Flag (bit 5)

Overflow Flag (bit 6)

Negative Flag (bit 7)

6

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