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MG69M220 Datasheet

Version 1.09

5

Function Description

5.1

Registers

A

Y

X

P

PCH

PCL

1

S

5.2

Accumulator

The accumulator is a general-purpose 8-bit register, which stores the results of most arithmetic and logic

operations. In addition, the accumulator usually contains one of two data words used in these operations.

5.3

Index Register(X,Y)

There are two 8-bit index registers (X and Y), which may be used to count program steps or to provide an index

value to be used in generating an effective address. When executing an instruction, which specifies indexed

addressing, the CPU fetches the OP Code and the base address, and modifies the address by adding the index

register to it prior to performing the desired operation. Pre- or post-index of index address is possible.

5.4

Processor Status Register

The 8-bit processor status register contains seven status flags. Some of the flags are controlled by the program,

others may be controlled both the program and the CPU.

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

N

V

1

B

D

I

Z

C

N: Signed flag, 1 = negative, 0 = positive

V: Overflow flag, 1 = true, 0 = false

B: BRK interrupt command, 1 = BRK, 0 = IRQB

D: Decimal mode, 1 = true, 0 = false

I: IRQB disable flag, 1 = disable, 0 = enable

Z: Zero flag, 1 = true, 0 = false

C: Carry flag, 1 = true, 0 = false

5.5

Program Counter(PC)

The 16-bit program counter register provides the addresses, which step the micro-controller through sequential

program instructions. Each time the micro-controller fetches an instruction from program memory, the lower byte of

the program counter (PCL) is placed on the low-order 8 bits of the address bus and the higher byte of the program

counter (PCH) is placed on the high-order 8 bits. The counter is incremented each time an instruction or data is

fetched from program memory.

5.6

Stack Point(S)

The stack pointer is an 8-bit register, which is used to control the addressing of the variable-length stack. The stack

pointer is automatically incremented and decremented under control of the micro-controller to perform stack

manipulations under direction of either the program or interrupts (/NMI or /IRQ). The stack allows simple

implementation of nested subroutines and multiple level interrupts. The stack pointer is initialized by the user’s

firmware.