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11
8266D-MCU Wireless-06/12
ATmega128RFA1
7.4 Status Register
The Status Register contains information about the result of the most recently executed
arithmetic instruction. This information can be used for altering program flow in order to
perform conditional operations. Note that the Status Register is updated after all ALU
operations, as specified in the Instruction Set Reference. This will in many cases
remove the need for using the dedicated compare instructions, resulting in faster and
more compact code. The Status Register is not automatically stored when entering an
interrupt routine and restored when returning from an interrupt. This must be handled by
software.
7.4.1 SREG – Status Register
Bit
7
6
5
4
3
2
1
0
$3F ($5F)
I
T
H
S
V
N
Z
C
SREG
Read/Write
RW
Initial Value
0
Bit 7 – I - Global Interrupt Enable
The global interrupt enable bit must be set (one) for the interrupts to be enabled. The
individual interrupt enable control is then performed in separate control registers. If the
global interrupt enable bit is cleared (zero), none of the interrupts are enabled
independent of the individual interrupt enable settings. The I-bit is cleared by hardware
after an interrupt has occurred, and is set by the RETI instruction to enable subsequent
interrupts.
Bit 6 – T - Bit Copy Storage
The bit copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T bit as source
and destination for the operated bit. A bit from a register in the register file can be
copied into T by the BST instruction, and a bit in T can be copied into a bit in a register
in the register file by the BLD instruction.
Bit 5 – H - Half Carry Flag
The half carry flag H indicates a half carry in some arithmetic operations. See the
Instruction Set Description for detailed information.
Bit 4 – S - Sign Bit
The S-bit is always an exclusive or between the negative flag N and the two's
complement overflow flag V. See the Instruction Set Description for detailed
information.
Bit 3 – V - Two's Complement Overflow Flag
The two's complement overflow flag V supports two's complement arithmetics. See the
Instruction Set Description for detailed information.
Bit 2 – N - Negative Flag
The negative flag N indicates a negative result after the different arithmetic and logic
operations. See the Instruction Set Description for detailed information.
Bit 1 – Z - Zero Flag
The zero flag Z indicates a zero result after the different arithmetic and logic operations.
See the Instruction Set Description for detailed information.
Bit 0 – C - Carry Flag
The carry flag C indicates a carry in an arithmetic or logic operation. See the Instruction
Set Description for detailed information. Note that the status register is not automatically