• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual inter-
rupt enable control is then performed in separate control registers. If the Global Interrupt Enable
Register is cleared, 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. The I-bit can also be set and cleared in
software with the SEI and CLI instructions, as described in the instruction set reference.
• Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or desti-
nation 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. Half carry is useful in
BCD arithmetic. 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 complement over-
flow 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 in an arithmetic or logic operation. See the
“Instruction Set Description” for detailed information.
• Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. 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.
General Purpose
Register File
The Register file is optimized for the AVR Enhanced RISC instruction set. In order to achieve the
required performance and flexibility, the following input/output schemes are supported by the
Register file:
•
•
•
•
One 8-bit output operand and one 8-bit result input
Two 8-bit output operands and one 8-bit result input
Two 8-bit output operands and one 16-bit result input
One 16-bit output operand and one 16-bit result input
shows the structure of the 32 general purpose working registers in the
CPU.
12
ATmega128
2467R–AVR–06/08
ATMEL [ ATMEL CORPORATION ]
