Rainbow Electronics AT90S1200 - Manual (Page 11)

11
AT90S1200
0838H–AVR–03/02
Status Register – SREG The AVR status register (SREG) at I/O space location $3F is defined as:
• 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 indepen-
dent 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, S = N
⊕V
The S-bit is always an exclusive or between the negative flag N and the two’s comple-
ment 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 stored when entering an interrupt rou-
tine and restored when returning from an interrupt routine. This must be handled by
software.
Bit 7 6 5 4 3 2 1 0
$3F I T H S V N Z C SREG
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0