IBM 1620 - Page 93

alternative logic paths are provided. On e or the other
of the paths is selected, de pending upon the set ting of
the appropriate switch.
Machin e Check Indicators and Switches
Machine operation may be altered by the <.:ondition of
a machine check indicator and an associated check
sw itch ( Fi gure 10). An indicator that is turncd on
cau e th
computer to halt if the associated check
sw itch is set to STOP , or to continue in automatic mod e
if the associated
check switch is set to PROGRAl\L R e-
gardless of the check switch setting , the associated
check light provid es a visual sign of the indicator status .
Pr essing the reset key turns all check indicators and
light s off. Parit y, I/ O, and overflow check indicators
are provided.
PARITY HE K INDI CATO R
Int ernal data flow e rror s are reco rd ed by the parity
check indicator s: MBR- ' andl\IBR-o. ormally, the par -
ity
chec k switch is set to STOP.
HBR -E ( Memory Buff er R eg ist er-Even) Check
Light. This li ght and indic ator are turned on when the
digit in the even address portion of the l\IBR ha s a parit y
error.
HBR-O (M emo ry Buff er R eg i ste r-Odd ) Check U ght.
This li g ht and indicator are turned on when the digit
in the odd address portion of the l\IBR has a parit y error.
~fARS ( H emo ry Address R eg ister Storage) Check
Li g ht . This li g ht turns on when a digit in l\IARS has a
parity error. This is an unconditional machin e stop
and is not affected by the position of the parity check
switch.
Figure 10. Indicator Displays and Switch es
92
INPUT / OUTPUT (I/ O) CHEC K Il\'DICATORS
RD CHK (Read Check) Li g ht . This light and indica-
tor are turn ed on when an input character with a parity
er ror i detected prior to conversion of input data to
BCD code.
WR CHK (Wr ite Check) Li ght. This light and indi-
cator are turned on when an output character with an
eve n numb er of bits is de tect ed during conversion of
output data from BCD to output code.
OVERFLOW ARITH HK (A RITHMETI C C HE CK) Il\'DI CATOR
An overflow that occurs as a res ult of an add , subtract ,
divide,
or compar operation turns on the overflow
check indicator and light.
CO . ' SOLE PROGRA '[ SWIT HES
Th ere are four modifier switch s in this group. Th y
are l abeled PROGR AM SWITCHES on the console and ar
number d 1 through 4.
REGISTER DISPL AY Il\'DI CATORS Al\'D SWITCHE
Th e console pan el displays the cont nts of r eg isters by
means
of small incandescent li g ht s, used to re pr ese nt
th bit pr ese nt in eac h digit of a register ( Fi g l1J' e 11 ).
Each li g ht , r ep resenting a particular bit pOSition, is on
only
when its corresponding bit is pr ese nt in the digit
dis pla ye d.
H e mory Buff er Reaister ( MBR ). The two stored
digit s aff cted by a core storage address are display d
in the
l\{BR. \\ hen the core st orage location addressed
for display is an eve n-numb ered address, th e digit at
this location
is plac ed in the MBR display in the E
(ev n lin ); the 0 (o dd ) line eontains the digit in the
next highe
r-numb ered location. If th e core st orage lo-
cation
addressed for dis pla y is an odd-numbered ad-
dr ess, the digit at this location is placed in the MBR
display on the 0 line; th e E line contains the digit in
the next lowe
r-numb ered location. \Vhen the machine
is in alphabe tic mod e, the comp lete 2-digit represen-
tation
of' an alpham eric character may be viewed at
one time.
Memory Data Regist er ( 1DR ). On e line of six indi-
cator lights displays the bit configuration of each digit
in core storage as it is read out. These digits can be
seen on single cycle operation by using the SCE key (de-
sc rib ed under CONTROL SWITCHES, KEYS, A TO SIGNAL
LIGHT ). The digit displayed in the {OR display is dupli-
cated in the MBR-even or MBR-odd display, depending
on whether the digit read out is lo cated at an even or
an odd numbered core storage position.
Operation
(OP) Register. Two lines of five lights each
display
the bit configuration of the two digits represent -
ing
the operation code of the instruction l ast executed.
Flag bits of these two digits are not displayed.