SCG9910
8-BIT MICROCONTROLLER
Score Concept Semiconductor Ltd.
4
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
0
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: -
a.
Arithmetic
b.
Logic
c.
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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