SCG9910 datasheet(4/27 Pages) ETC1

4 / 27 page

SCG9910

8-BIT MICROCONTROLLER

Score Concept Semiconductor Ltd.

Sept 2000

Stack Register (SP)

A 7-bit Stack Register (SP) stores the address for stack operation. After CPU resets, the

value is $00. The SP has to be initialized at $FF by software. This means the stack

frame starts from the highest address memory location.

Program Status (PS)

This is an 8-bit Program Status Register. However only 4-bit is used for controlling

ALU operations and instruction execution sequences.

Program Status

Bit

7654

321

Figure 3. 8-bit Program Status

Carry Flag (C)

Whenever there is a carry or borrow occurs after an arithmetic operation, carry

flag is set to 1. Otherwise, it is cleared to 0.

Besides, the “Rotate” instructions can also change the carry flag which value is a

bit shifted out of the specified source operand.

Executing single instruction of “SETC” or “CLRC” can also alter this flag.

Upon returning from an interrupt service routine, this flag will be restored.

Zero Flag (Z)

For arithmetic and logical operations, Zero flag will be set to 1 if the result is

zero.

Besides, for operation that involves moving source operand to accumulator, zero

flag will also be set to 1 if the content of the source operand is zero.

Upon returning from an interrupt service routine, this flag will be restored.

Sign Flag (N)

Sign flag stores the most significant bit of a result after the following operations: -

Arithmetic

Logic

Move from source operand to accumulator

This flag will also be restored upon returning from an interrupt service routine.

Reserved

Sign

Zero Carry

Interrupt

Mask

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