IBM 1620 1 Manual page 35

Nlain
Program
Common
sumb~;';M
4:
~
(j
~
BT -------------
~----------~BB~
Continue
Nlain
Program
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Figure 36. Subroutine Linkage
Branch and Transmit instruction. The last instruction
of the subroutine, Branch Back, returns the program
to the main routine (the address saved in IR-2 when
the Branch and Transmit was executed).
IR-2
IOOOI21---...J
IR-I
I094001 .. ·---...J·
Address
of
Next
Instruction
In Figure 37, the instruction 27 09400 00500 is
executed as follows:
1. The address of the next sequential instruction
is saved. This address will be 00012 if the
Branch and Transmit instruction is at 00000.
2. The Q data is transmitted to the P address
minus one (09399).
3. A branch to 09400 (the P address of the
Branch and Transmit instruction) occurs.
Execution Time. T
==
200
+
40D
q
Il-sec
Branch and Transmit Immediate (BTM-17)
Description. Same as Branch and Transmit (BT-27)
except that the digits in the
Q
part of the instruction
are used as
Q
data.
Execution Time. T
==
200
+
40D/ Il-sec
Branch Back (BB-42)
Description. This instruction causes the computer
to branch unconditionally to either (1) the instruction
at the address saved in IR-2 by the execution of the
last Branch and Transmit or Branch and Transmit
Immediate instruction, or (2) the address saved in
PR-1 by previous depression of the Save key on the
console when the
CPU
was in manual mode.
The Save key function is examined first since it has
priority over a Branch and Transmit instruction. If
Core Storage
Circled numbers refer to
I. Save address of next sequential instruction in register IR-2.
these operations: 2. Transmit data in the field at the Q address to the P address minus one.
3. Branch to the P address.
Figure 3'7. Branch and Transmit - Data Flow
31