U UNIVAC 1218 - Bitsavers
U UNIVAC 1218 - Bitsavers
U UNIVAC 1218 - Bitsavers
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1. INTRODUCTION<br />
SECTION 1-8-6. JUMP INSTRUCTIONS<br />
Jump instructions are used to transfer program control to other portions of a<br />
program or to other programs. Jump instructions fall into two general categories:<br />
conditional and unconditional. Conditional jumps transfer program control only<br />
if certain specified conditions exist. Unconditional jumps always transfer<br />
program control.<br />
2. UNCONDITIONAL JUMP INSTRUCTIONS<br />
Seven unconditional jump instructions are providedo The use of each instruction<br />
is dependent upon the purpose for transferring program control. The names and<br />
mnemonics of these instructions convey the suitability of each instruction<br />
for a particular application. The three key words used are defined below.<br />
30<br />
1) Direct.<br />
The word direct signifies that control is to be transferred directly to<br />
the address specified by the lower 12 bits of the instruction and the<br />
upper 3 bits of the P register. Since the P register must obviously be<br />
set to the bank in which the jump instruction is stored, direct jumps<br />
are normally used to transfer r.ontrol within a memory bank. However, a<br />
direct jump with B modification can be used to transfer control between<br />
hanks.<br />
2) Indirect.<br />
The word indirect signifies that control is to be transferred to an<br />
address contained in the lower 15 bits of the storage location specified<br />
by the lower 12 bits of the instruction and the upper 3 bits of the P<br />
register. Indirect jumps require an additional memory location and an<br />
extra memory access; however, they permit transfer of program control to<br />
any address in memory, regardless of bank designation.<br />
3) Return.<br />
The word return implies that program control is being transferred temporarily<br />
and that control may be returned to this point in the program<br />
after a specific task has been performed. Therefore, return jumps store<br />
the address of the next sequential instruction in the program before<br />
transferring program control.<br />
INDIRECT RETURN JUMP (IRJP)<br />
Execution time= 12 microseconds<br />
(P) + 1---' (y) ; (y) + 1---. P<br />
Instruction executed from running program:<br />
y = up<br />
Store (P) + 1 at the address given in the low<br />
order 15 bits of (y), then increment that address<br />
by one and enter it into the program address register.<br />
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