ST90158 - DEVICE ARCHITECTURE
SYSTEM REGISTERS (Cont’d)
2.3.2 Flag Register
decw),
Test (tm, tmw, tcm, tcmw, btset).
The Flag Register contains 8 flags which indicate
the CPU status. During an interrupt, the flag regis-
ter is automatically stored in the system stack area
and recalled at the end of the interrupt service rou-
tine, thus returning the CPU to its original status.
Inmostcases, theZeroflag issetwhenthe contents
of the register being used as an accumulator be-
come zero, following one of the above operations.
This occurs for all interrupts and, when operating
in nested mode, up to seven versions of the flag
register may be stored.
Bit 5 = S: Sign Flag.
The Sign flag is affected by the same instructions
as the Zero flag.
FLAG REGISTER (FLAGR)
R231- Read/Write
Register Group: E (System)
Reset value: 0000 0000 (00h)
The Sign flag is set when bit 7 (for a byte opera-
tion) or bit 15 (for a word operation) of the register
used as an accumulator is one.
7
0
Bit 4 = V: Overflow Flag.
The Overflow flag is affected by the same instruc-
tions as the Zero and Sign flags.
C
Z
S
V
DA
H
-
DP
When set, the Overflow flag indicates that a two's-
complement number, in a result register, is in er-
ror, since it has exceeded the largest (or is less
than the smallest), number that can be represent-
ed in two’s-complement notation.
Bit 7 = C: Carry Flag.
The carry flag is affected by:
Addition (add, addw, adc, adcw),
Subtraction (sub, subw, sbc, sbcw),
Compare (cp, cpw),
Shift Right Arithmetic (sra, sraw),
Shift Left Arithmetic (sla, slaw),
Swap Nibbles (swap),
Rotate (rrc, rrcw, rlc, rlcw, ror,
rol),
Decimal Adjust (da),
Multiply and Divide (mul, div, divws).
Bit 3 = DA: Decimal Adjust Flag.
The DA flag is used for BCD arithmetic. Since the
algorithm for correcting BCD operations is differ-
ent for addition and subtraction, this flag is used to
specify which type of instruction was executed
last, so that the subsequent Decimal Adjust (da)
operation can perform its function correctly. The
DA flag cannot normally be used as a test condi-
tion by the programmer.
When set, it generally indicates a carry out of the
most significant bit position of the register being
used as an accumulator (bit 7 for byte operations
and bit 15 for word operations).
Bit 2 = H: Half Carry Flag.
The carry flag can be set by the Set Carry Flag
(scf) instruction, cleared by the Reset Carry Flag
(rcf) instruction, and complemented by the Com-
plement Carry Flag (ccf) instruction.
The H flag indicates a carry out of (or a borrow in-
to) bit 3, as the result of adding or subtracting two
8-bit bytes, each representing two BCD digits. The
H flag is used by the Decimal Adjust (da) instruc-
tion to convert the binary result of a previous addi-
tion or subtraction into the correct BCD result. Like
the DA flag, this flag is not normally accessed by
the user.
Bit 6 = Z: Zero Flag. The Zero flag is affected by:
Addition (add, addw, adc, adcw),
Subtraction (sub, subw, sbc, sbcw),
Compare (cp, cpw),
Shift Right Arithmetic (sra, sraw),
Shift Left Arithmetic (sla, slaw),
Swap Nibbles (swap),
Bit 1 = Reserved bit (must be 0).
Rotate (rrc, rrcw, rlc, rlcw, ror,
rol),
Bit 0 = DP: Data/Program Memory Flag.
This bit indicates the memory area addressed. Its
value is affected by the Set Data Memory (sdm)
and Set Program Memory (spm) instructions. Re-
fer to the Memory Management Unit for further de-
tails.
Decimal Adjust (da),
Multiply and Divide (mul, div, divws),
Logical (and, andw, or, orw, xor,
xorw, cpl),
Increment and Decrement (inc, incw, dec,
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