IBM 7090 - Skip Instructions

tion 0000. It then traps or transfers the computer to location 0002 where the instruction
sequence is resumed. STR does not place the computer in trapping mode.
5.3. 07 Skip Instructions
The skip instructions allow the programmer to alter the program to meet special
conditions without stopping the computer. These instructions are similar to the condi-
tional transfer instructions but, iIfstead of transferring, they cause one or two instruc-
tions to be skipped. Skipping is accomplished by supplying an extra advance pulse to
the instruction counter.
Most skip instructions cause the computer to skip when the condition being tested is
met. The exceptions are the error testing and 1-0 testing instructions, which cause
skipping when the condition being tested is not met; e. g. , when there are no errors.
This exception (skip on no error) allows a straight-line program until an error is de-
tected. To process an error, an instruction transferring to an error subroutine usually
follows the test instruction. Another exception, the CAS instruction, has three possible
results: it can fail to skip for AC greater, skip once for equal, or skip twice for AC less.
P Bit Test
PBT -0760 ... 0001 (I, L)
Figure 5.3-35
A bit in accumulator (P) position causes the computer to skip one instruction. If
there is no bit in (P), the computer takes the next instruction in sequence.
Low-Order Bit Test
LBT +0760 ... 0001 (I, L) Figure 5.3-35
A bit in accumulator (35) causes the computer to skip one instruction. If there is
no bit in (35), the computer takes the next instruction in sequence.
Storage Zero Test ZET +0520 (I, E) Figure 5.3-36
If the contents of storage location X, except the sign, are zero, the computer will
skip one instruction. If storage is not zero, the computer proceeds to the next instruc-
tion in sequence. Storage is unchanged. The information from storage on the SB is
tested for zero as shown on Systems 2. 12. 52. 1.
Storage Non-Zero Test
NZT -0520 (I, E) Figure 5.3-36
If positions (1-35) of storage location X are not zero, the computer skips the next
instruction. If the contents of storage location X are zero, no skip is taken. Storage
is unchanged.
Compare Accumulator with Storage
CAS +0340 (I, E, L)
Figure 5.3-37
The accumulator is compared with the word at storage location X. Comparison is
accomplished by taking an algebraic difference. If the accumulator is greater than the
word in stor age, no skip is taken. If the accumulator is equal to the word in storage,
one instruction is skipped. If the accumulator is less than the word in storage, the
next two instructions are skipped. Neither the accumulator nor the word in storage is
changed.
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