U UNIVAC 1218 - Bitsavers

1. INTRODUCTION
SECTION 1-8-6. JUMP INSTRUCTIONS
Jump instructions are used to transfer program control to other portions of a
program or to other programs. Jump instructions fall into two general categories:
conditional and unconditional. Conditional jumps transfer program control only
if certain specified conditions exist. Unconditional jumps always transfer
program control.
2. UNCONDITIONAL JUMP INSTRUCTIONS
Seven unconditional jump instructions are providedo The use of each instruction
is dependent upon the purpose for transferring program control. The names and
mnemonics of these instructions convey the suitability of each instruction
for a particular application. The three key words used are defined below.
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1) Direct.
The word direct signifies that control is to be transferred directly to
the address specified by the lower 12 bits of the instruction and the
upper 3 bits of the P register. Since the P register must obviously be
set to the bank in which the jump instruction is stored, direct jumps
are normally used to transfer r.ontrol within a memory bank. However, a
direct jump with B modification can be used to transfer control between
hanks.
2) Indirect.
The word indirect signifies that control is to be transferred to an
address contained in the lower 15 bits of the storage location specified
by the lower 12 bits of the instruction and the upper 3 bits of the P
register. Indirect jumps require an additional memory location and an
extra memory access; however, they permit transfer of program control to
any address in memory, regardless of bank designation.
3) Return.
The word return implies that program control is being transferred temporarily
and that control may be returned to this point in the program
after a specific task has been performed. Therefore, return jumps store
the address of the next sequential instruction in the program before
transferring program control.
INDIRECT RETURN JUMP (IRJP)
Execution time= 12 microseconds
(P) + 1---' (y) ; (y) + 1---. P
Instruction executed from running program:
y = up
Store (P) + 1 at the address given in the low
order 15 bits of (y), then increment that address
by one and enter it into the program address register.
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