IBM 1620 1 Manual page 36

the Save key function is active, the console Save
light is turned off and the branch is to the instruction
whose address was saved in PR-l. If the Save key
function is inactive, the branch is to the address
saved in IR-2 when the last Branch and Transmit
or Branch and Transmit Immediate instruction was
executed. The contents of IR-2 are transfered to IR-l,
and IR-2 is cleared. If a second Branch Back instruc-
tion occurs without an intervening Branch and Trans-
mit instruction or a Save key pepression, a MARS
check results. The P and
Q
addresses of the Branch
Back instruction are not used.
Execution Time. T
==
200 ftsec
Branch on Digit (BD-43)
Description. Branch to the instruction at the P
address if the digit at the
Q
address is not a zero.
If the digit at the Q address is a zero (either a plus
zero or a minus zero), no branch occurs and the next
instruction in sequence is executed.
Execution Time. If the branch occurs (digit is not
zero) T
==
240 ftsec. If the branch does not occur,
T
==
200 ftsec.
Branch No Flag (BNF-44)
Description. Branch to the instruction at the P
address if a flag bit is not present at the
Q address.
If a flag bit is present at the Q address, the next
instruction in sequence is executed.
Execution Time. If the branch occurs (flag bit not
present), T
==
240 ftsec. If the branch does not occur,
T
==
200 ftsec.
Branch No Record Mark (BNR-4S)
Description. Branch to the instruction at the P
address if a record mark character is not present at
the Q address. If a record mark character is present,
the next instruction in sequence is executed.
Last-Record Check. The Branch No Record Mark
instruction can be used with paper tape to perform
a last-record check, similar to the last-card check
used with cards.
Data read from paper tape is transferred to core
storage in records. Each record is distinguished in
storage by a record mark at the end (right-most
position) of the record, resulting from the end-of-line
(EL) punch at the end of the tape record. Reading
and processing of data continues through the last
record, which can be distinguished by two EL
punches at the end. Actually, the. second EL punch
becomes the first and only character of the last record,
i.e., after reading the last data record (terminated
32
by the first of the two successive EL punches), the
next read instruction causes the second EL punch to
read in.
A Branch No Record Mark instruction, which tests
the first (leftmost) character of each record, follows
each read instruction and normally permits data to
be processed. When the first character read in a record
is an EL punch, processing of data is stopped by this
instruction, which causes the program to branch to
the end-of-tape routine.
Execution Time. If the branch occurs (record mark
not present), T
==
240 ftsec. If the branch does not
occur, T
==
200 ftsec.
NOTE: If the 1311 Disk Storage Drive is attached to
the system, an additional special character, a group
mark, is used in disk storage operations. The Branch
No Record Mark instruction treats a group mark in
the same manner as a record mark because· both
contain 8 and 2 bits.
Branch Indicator (BI-46)
Description. The indicator specified by Qs and Q9
of the instruction is interrogated; if the indicator is
ON, a branch to the P address occurs. Indicators are
always in one of two conditions, ON or OFF. The
QT, Ql0, and Qll positions of the instruction are not
used.
Information concerning 1620 indicators is given in
Table 2 and further explained, as follows:
Program Switches (01-04). The status of these four
indicators is determined by the on/off conditions of
their respective Program switches on the 1620 console.
Read and Write Check (06 and 07). The Rd/Wr
Check indicators are turned on when erroneous data
is transferred to or from an input/output device.
Last Card (09). This indicator is turned on when-
ever the data from the last card is correctly trans-
ferred from 1622 input buffer storage to core storage.
Arithmetic (11, 12, 13, and 14). The arithmetic indi-
cators, 11, 12, and 14, are explained under ARITHMETIC
INSTRUCTIONS. The single indicator, High/Positive or
Equal/Zero (13), provides the means of interrogating
both the H/P (11) and E/Z (12) indicators with one
Branch Indicator or Branch No Indicator instruction
- no indicators are turned off by this instruction.
Exponent Check (15) Special Feature. The Expon-
ent Check indicator is turned on by an exponent
underflow or overflow. This indicator is described in
more detail under MANTISSA AND EXPONENT ANALYSIS.
MBR-E and MBR-O (16 and 17). All data entering
and leaving core storage does so via the MBR registers.