ATMEGA103 datasheet(21/141 Pages) ATMEL

21 / 141 page

ATmega103(L)

0945I–AVR–02/07

ied into T by the BST instruction and a bit in T can be copied into a bit in a register in the

• Bit 5 – H: Half-carry Flag

The Half-carry Flag H indicates a Half-carry in some arithmetic operations. See the

instruction set description on page 135 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 on page 135 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 on page 135 for detailed information.

• Bit 2 – N: Negative Flag

The Negative Flag N indicates a negative result from an arithmetical or logical operation.

See the Instruction set description on page 135 for detailed information.

• Bit 1 – Z: Zero Flag

The Zero Flag Z indicates a zero result from an arithmetical or logical operation. See the

instruction set description on page 135 for detailed information.

• Bit 0 – C: Carry Flag

The Carry Flag C indicates a carry in an arithmetical or logical operation. See the

instruction set description on page 135 for detailed information.

Note that the Status Register is not automatically stored when entering an interrupt rou-

tine or restored when returning from an interrupt routine. This must be handled by

software.

Stack Pointer – SP

The general AVR 16-bit Stack Pointer is effectively built up of two 8-bit registers in the

I/O space locations $3E ($5E) and $3D ($5D). As the ATmega103(L) supports up to

64K bytes memory, all 16 bits are used.

The Stack Pointer points to the Data SRAM Stack area where the Subroutine and Inter-

rupt Stacks are located. This Stack space in the Data SRAM must be defined by the

program before any subroutine calls are executed or interrupts are enabled. The Stack

Pointer must be set to point above $60. The Stack Pointer is decremented by one when

data is pushed onto the Stack with the PUSH instruction and it is decremented by 2

when an address is pushed onto the Stack with subroutine calls and interrupts. The

Stack Pointer is incremented by 1 when data is popped from the Stack with the POP

Bit

151413

1211

$3E ($5E)

SP15

SP14

SP13

SP12

SP11

SP10

SP9

SP8

SPH

$3D ($5D)

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

SPL

765

432

Read/Write

R/W

Initial Value

000

ATMEGA103 Datasheet(PDF) 21 Page - ATMEL Corporation

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