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ATMEGA48 Datasheet(PDF) 11 Page - ATMEL Corporation |
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ATMEGA48 Datasheet(HTML) 11 Page - ATMEL Corporation |
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11 / 377 page  11 2545T–AVR–05/11 ATmega48/88/168 7.4 Status register The Status Register contains information about the result of the most recently executed arithme- tic 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 – AVR Status Register The AVR Status Register – SREG – is defined as: • 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 by the application 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 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 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 7 65 43 21 0 0x3F (0x5F) 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 |
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