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SINGLE COMPONENT SYSTEM
I
A"
I
A,O
I Ag I As I
A7
I
A61
A51
A41
Aal
A21
A,
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AO
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Conventional Program Counter
•
Counts
OOOH
to
7FFH
• Overflows 7FFH
to
OOOH
PROGRAM COUNTER
SAVED IN STACK
I
I CY I AC I
FO
I
BS
I 1
MSB
CY CARRY
STACK POINTER
I
AC
AUXILLARY
CARRY
FO
FLAG
0
BS
REGISTER
BANK
SELECT
PROGRAM STATUS WORD (PSW)
POINTER
111
110
101
100
011
010
001
000
·
·
·
·
·
·
·
•
·
•
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·
:
•
PSW
pc
s
_"
PC4-7
PCo;J
LSB
R23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
R8
MSB
LSB
PROGRAM COUNTER STACK
2-6
2.1.7 Program Status Word
An
8-bit status word which can
be
loaded to
and from the
accumulator exists called the
Program
Status Word (PSW). The accom-
panying figure shows the information available
in the word. The Program Status Word is
actually a collection of flip-flops throughout
the machine which can
be
read or written as a
whole. The ability to write
to
PSW allows for
easy restoration of machine status after a
power down sequence.
The upper four bits of
PSW are stored
in
the
Program Counter
Stack with every call
to
subroutine or interrupt vector
and
are
optionally
restored
upon
return
with
the
RETR
instruction.
The RET return instruction does not update
PSw.
The PSW bit definitions are as follows:
Bits a -
2:
Stack Pointer bits
(So,
s"
S2)
Bit
3:
Not used ("1" level when
read)
Bit 4: Working Register Bank
Switch
Bit
(BS)
a = Bank a
1
= Bank 1
Bit 5:
Flag a bit
(Fa)
user controlled
flag
which can be comple-
mented or cleared,
and
tested
with the
conditional jump in-
struction JFa.
Bit
6:
Auxiliary Carry (AC) carry bit
generated
by
an
ADD instruc-
tion
and
used
by
the decimal
adjust instruction DA
A.
Bit
7:
Carry (CY) carry flag which
indicates that the previous
operation has
resulted
in
over-
flow
of the accumulator.
2.1.8 Conditional Branch Logic
The
conditional branch logic within the pro-
cessor enables several conditions internal
and
external
to
the processor to
be
tested by
the users program.
By
using the conditional
jump instruction the following conditions can
effect a change in the sequence
of
the
program execution.