IBM 7090 Instruction-Reference page 54

Enter Keys ENK +0760 ... 0004 (I, L) Figure 5.3-8
The word represented in console keys (S, 1-35) replaces the contents of the MQ. This
is a primary operation 76 instruction. The word in the keys is put into the SR and the
output of the SR is gated into the MQ.
5.3.02 Shifting Instructions
Shift instructions are used to align words, or for fast multiplication or division by a
power of 2. Shifting moves the bits of the AC or MQ, or both, to the right or left within
the registers. Bits shifted out of the end of a register are lost and bits shifted away
from either end of a register are replaced by zeros. Because the shift counter receives
only the last eight positions of the instruction address, the maximum number of shifts
possible is 25510 (11 111 1112).
The number of shifts to be taken is indicated by the address portion of the shift in-
struction. This address is gated to the shift counter at I11(D1). Shifting starts at the
next L1 time and continues to shift at the rate of one position of shift for each clock
pulse until the shift counter has been reduced to zero.
The cyclic makeup of a shifting instruction is an I time followed with as many L times
as required to complete the shifts designated by the address portion of the instruction.
Eleven shifts may be completed during the first L time, and twelve shifts may be com-
pleted during all succeeding L times. The "end operation" condition is Signaled by
having the shift counter at seven or less at any L10 time of a shift instruction. This
means that up to five shifts may be made in the 1 time of the following instruction.
Shifting to the left is the same as multiplying by a power of 2; shifting to the right
reduces or divides by a power of 2. The number of shifts is equal to the exponent.
Accumulator Left Shift ALS +0767 (I, L .•• ) Figure 5.3-9
This instruction causes the contents of the AC (Q-35)* to be shifted left a number of
places equal to the eight low order positions of the address. Zeros replace any bits
shifted away from position (35). Bits shifted past Q are lost. A "1" shifted into the P
position turns on the AC overflow indicator.
Long Left Shift LLS +0763
(I,
L ... )
Figure 5.3-10
For this instruction the contents of the MQ and A C (except the sign positions) are
shifted left the number of places deSignated by the eight low order positions of the ad-
dress. The MQ (1) position is shifted to the AC (35) position. The AC sign is set to
agree with the MQ sign. Bits shifted past Q are lost and bits shifted away from MQ(35)
are replaced by zeros. Bits shifted into P cause the AC overflow indicator to be turned
on.
Logical Left Shift LGL -0763 (I, L ••. ) Figure 5.3-10
This instruction shifts the contents of AC(Q-35) and MQ(S-35) left the number of
places designated by the address. Bits shifted from MQ(l) enter MQ(S) , and from MQ
(S) enter AC(35). Bits shifted into AC(P) cause the AC overflow indicator to be turned
on. Bits shifted past AC(Q) are lost and bits shifted away from MQ(35) are replaced by
zeros. The operation of LGL is similar to LLS except for handling of the MQ sign.
*
This text section uses (Q - 35) to represent (Q, P, 1 - 35).
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