50
AT94KAL Series FPSLIC
Rev. 1138G–FPSLI–11/03
Status Register – SREG
The AVR status register
– SREG – at I/O space location $3F ($5F) is defined as:
Note:
1. Note that the status register is not automatically stored when entering an interrupt routine
and restored when returning from an interrupt routine. This must be handled by software.
• Bit 7 - I: Global Interrupt Enable
The global interrupt enable bit must be set (one) for the interrupts to be enabled. The individ-
ual interrupt enable control is then performed in separate control registers. If the global
interrupt enable register is cleared (zero), none of the interrupts are enabled independent of
the individual interrupt enable settings. The I-bit is cleared by the 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 des-
tination 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.
• 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.
• Bit 3 - V: Two’s Complement Overflow Flag
The two’s complement overflow flag V supports two’s complement arithmetics.
• Bit 2 - N: Negative Flag
The negative flag N indicates a negative result from an arithmetical or logical operation.
• Bit 1 - Z: Zero Flag
The zero flag Z indicates a zero result from an arithmetical or logical operation.
• Bit 0 - C: Carry Flag
The carry flag C indicates a carry in an arithmetical or logical operation.
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). Future versions of FPSLIC may support up to 64K
Bytes of memory; therefore, all 16 bits are used.
Bit
7
6
5
4
3
2
1
0
$3F ($5F)
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
Bit
15
14
13
12
11
10
9
8
$3E ($5E)
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
SPH
$3D ($5D)
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
SPL
7
6
5
4
3
2
1
0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
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
0
0
0
0
0
0
0
0