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SYSTEM/360 STATUS BITS
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3 bits in the PSW are used to con-
trol the System/360 mode or state.
The ASCII mode bit (PSW bit 12)
determines if decimal operations
will be done in EBCDIC mode (0) or
ASCII mode
(1).
The Wait State bit (PSW bit
14)
determines if the System/360 is in
the Running (0) or Wait
(1)
state.
An external or I/O interrupt causes
the System/360 to go from the Wait
state to the Running state.
The Problem State bit (PSW bit 15)
determines if the System/360 is in
the Problem
(1)
or Supervisor (0)
State.
Privileged instructions are only
allowed in the Supervisor state.
A
Program interrupt will occur in the
Problem state, if execution of a
privileged instruction is attempted.
63
r--------T---~-----------------------,
I
I
1
1
l ______
.L--_J. _______________________
J
I
t
I
1
r--J
l-_,
1
1
I
1
I----~
1
A M W P
I
l - -_______ J
12
15
ASCII MODE BIT:
Of bits 12-15, you are
already familiar with bit 13.
It is the
machine check mask bit.
Bit 12 is the
ASCII mode bit.
ASCII is an information
interchange code adopted by the American
Standards Association to
be
used for
data communication.
The ASCII mode bit
determines the mode in which decimal
operations will be done; EBCDIC or ASCII
mode.
A key difference is in the rep-
resentation of sign values.
If bit 12
of the PSW contains a one, the ASCII
sign codes will be internally generated
rather than the extended BCD codes for
signs.
For example:
The number 1 in EBCDIC looks like this:
1 t
1 1
000 1
Zone
Numerics
(bits)
1-64
The number 1 in ASCII looks like this:
o
1 0 1
0 0 0 1
Zone
(bits)
Numerics
When processing data with the
instructions of the decimal feature, the
following are the standard signs gener-
ated:
110
=
Plus
EBCDIC
1101
=
Minus
If bit
12
of the PSW contains a one,
the signs that will be generated when
using the decimal feature are:
1010
=
Plus
ASCII
1011
=
Minus
When a packed field is converted back
to the unpacked format by the ·Unpack·
instruction, the zero bits that are
inserted will depend on the ASCII mode
bit in the PSW.
For instance +101 in
EBCDIC mode is unpacked as follows:
Packed
J
1
1
t
r----T----~---T----'
100011000010111111001
l ____
J. ___
-.1.~~_-.1.
____
J
r---~----T----T----T--------,
Unpacked 111111000111111100001110001111
l ____
J. ____
J.-___ J.-___
J. ________
J
t
t
Zones inserted if PSW bit
12
is
o
(EBCDIC Mode)
The remainder of the packed fields
used by the decimal feature are the
same.
For instance, a +107 would look like
this:
If
PSW
bit 12
is 0
if PSW
bit
12
is
1
D
D
D
S
r----T----~---~---,
100011000010111110101 EBCDIC
l_~
__ J. ____ J. ____ J. ____
J
r-~---l----T---'
100011000010111110101 ACCII
l - _ J . _ _ .L--__ J. ____
J
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