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code had a setting of 00.
If the condi-
tion code were 01 and the mask field
were 0010, a branch would not occur.
Sometimes the four possible settings
of the condition code are referred to as
decimal digits.
Condition Code
Referred To As
00
01
10
11
o
1
2
3
The bits of the branch on condition
mask field correspond to the condition
code settings in a left to right
fashion.
8--------11
r---·----------,
J1
1
1
11
Mask Field
L _____________ J
0,
1,
2,
3
Condition Code
To test for a specific condition code
setting, the corresponding bit of the
mask field must contain a
1.
If the mask field contains 0000 none
of the possible condition code settings
will be tested.
Therefore, a branch
never occurs if the mask field contains
0000.
If the mask field contains
1111,
all
or any of the possible condition code
settings is tested.
Because the condi-
tion code always contains one of its
four possible settings, a mask field of
1111
always results in a branch.
SUMMARY:
With its
4
bit mask field, the branch on
condition instruction has many uses:
a.
It can be used as a no-op instruc-
tion, by having a mask field of
0000.
b.
It can test for a specific result
(such as an equal compare) by set-
ting one of the bits of the mask
field-r1000) •
c.
It can test for a multiple result
(such as an equal or low compare)
1-48
by setting tWG or more bits of the
mask field (1100).
d.
It can be used as an unconditional
branch by having a mask field of
1111.
REVI.EW QUESTIONS ON INSTRUCTION
SEQUENCING AND BRANCHING
1 •
PSW is short for
2.
The PSW is
bits long.
3.
The address of the next instruction
to be fetched is contained in bits
to
of the PSW.
4.
After an instruction has been
fetched (read out) the instruction
address portion of the PSW is usu-
ally incremented by ____ ,
or
•
5.
The amount to increase the instruc-
tion address in the PSW is deter-
mined by bits
, _____ of the
instructions
6.
The PSW which is being used to
fetch instructions is sometimes
referred to as the •
• PSW.
7.
This PSW is located in (main
storage/ a general register/some
type of internal register or stor-
age area) •
8.
Branching is accomplished by
replacing the
in the
PSW with the ·Branch to· ___
a
9.
The condition code is in bits ___ ,
of the PSW.
10.
The condition code has _ _ _ possi-
ble settings •.
11.
The condition code in the PSW is
changed by (all/some) instructions
that are executed.
12.
Indicate the condition code setting
after the following results:
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