Z86D990 Datasheet, PDF (64/102 Pages) Zilog, Inc. – Low-Voltage Micro controllers with ADC

Z86D990/Z86D991 OTP and Z86L99X ROM
Low-Voltage Microcontrollers with ADC
57
Program Control Flag Register (Flags)
The Program Control Flag register (see Table 15) reflects the current status of the
Z8 as shown in Table 15. The FLAGS register contains six bits of status informa-
tion that are set or cleared by CPU operations. Four of the bits (C, V, Z, and S) can
be tested for use with conditional jump instructions. Two flags (H and D) cannot be
tested and are used for BCD arithmetic. The two remaining flags in the register
(F1 and F2) are available to the user, but they must be set or cleared by instruc-
tions and are not usable with conditional jumps.
Table 15. FLAGS Register [Group/Bank F0h, Register C (R252)]
Bit
7
6
5
4
Bit/Field C
Z
S
V
R/W
R/W R/W R/W R/W
Reset
X
X
X
X
R = Read, W = Write, X = Indeterminate
Bit
Position
Bit/Field
R/W Value
7_______ Carry Flag (C) R/W 1
_6______ Zero Flag (Z) R/W 1
__5_____ Sign Flag (S) R/W X
___4____ Overflow
Flag (V)
R/W 1
3
2
1
0
D
H
F2
F1
R/W R/W R/W R/W
X
X
X
X
Description
Indicates the “carry out” of bit 7
position of a register being used as
an accumulator; on Rotate and Shift
instructions this bit contains the most
recent value shifted out of the
specified register
Indicates that the contents of an
accumulator register is zero following
an arithmetic or logical operation
Stores the value of the most
significant bit of a result following an
arithmetic, logical, Rotate, or Shift
operation; in arithmetic operations on
signed numbers, a positive number is
identified by a 0, and a negative
number is identified by a 1
For signed arithmetic, Rotate, and
Shift operations, the flag is set to 1
when the result is greater than the
maximum possible number (>127) or
less than the minimum possible
number (<-128) that can be
represented in two’s complement
form; following logical operations, this
flag is set to 0
PS003807-1002
PRELIMINARY