Elan eSL Series, User Manual

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Chapter 2
eSL/eSLS Series (+ eSLZ000) User’s Manual Architecture
••••
27
2.4.5 Status Register (SR)
The Status Register contains the following system status bits :
Bit15 10 9 8 4 3 2 1 Bit0
GIE
X *
x
*
x
*
x
*
SME S6R F/I x
*
x
*
x
*
T N Z V C
*
x =
Don’t care. Reserved for future enhancements
Where:
Carry (C) Flag:
C is set when a carry or borrow occurs during an arithmetic operation. The
Carry Flag bit is set or reset, depending on the operation that is performed.
For
ADD
instructions C = 1
:
Carry occurs
C = 0 : No carry occurs
For
SUBTRACT
instructions
:
C = 1: No borrow occurs
C = 0: Borrow occurs
For COMPARE instructions : Same as SUBTRACT instruction
For ROTATION instructions : The Carry flag is used as a link between the
least significant bit (LSB) and most significant
bit (MSB).
Overflow (V) Flag:
V is set when a two complement overflows occurs as a result of an operation.
V = 1 : Overflow occurred
V = 0 : No overflow occurred
Zero (Z) flag:
The Z bit is set when all the resulting bits are 0s.
Z = 1: The result equals zero after operation
(R1:R0 = 0 when under MAC operation)
Negative (N) Flag
The negative flag stores the state of the most significant bit of the output result.
N = 1 : The result of the operation is negative.
N = 0 : The result of the operation is not negative.
Test (T) Flag
T is used by Bit test operation instruction (BTEST)
T = 1: The tested bit is 1
T = 0: The tested bit is 0