IBM 7090 - Page 78

1. Put the word with the smallest characteristic in the AC.
2. Determine the difference between AC and SR characteristics.
a. If the difference is less than 100S) put the difference in the SC.
b. If the difference is greater than 77S, clear the AC.
To determine which characteristic is smaller, the 2's complement of the AC char-
acteristic is added to the SR characteristic. Also, during this time the MQ is reset
and T2 trigger is turned on. These are the initial starting conditions for T2 and the MQ.
Trigger T2 will be used later, during the execution of the instruction, if the MQ con-
tains a zero or not.
A Q carry, because of the complement addition of the AC characteristic to the SR
characteristic, means that the SR characteristic is larger. No Q carry means that the
AC characteristic is larger. Therefore, the word in the SR is moved to the AC and the
word in the AC is moved to the storage register.
Again, the 2's complement of the AC characteristic is added to the SR characteristic
to determine the exact difference between the characteristics. A characteristic differ-
ence of less than 100S causes the difference to be put into the SC and a difference greater
than 77 S causes the AC to be cleared. The AC is cleared for a difference greater than
77 S because, due to this difference, the information eventually would be shifted out of
the AC and MQ anyway. Shifting the MQ and AC fractions right makes possible setting
the characteristics of the SR to the characteristic of the AC. It takes 66
S
shifts to move
a bit from AC fraction position (9) out through MQ fraction position (35). The character-
istic difference is checked for 77 S rather than 66
S
because this is easier to do in the
computer. The AC is cleared rather than let shifting take place because machine time
is saved.
Second Step L Time. Second step L time is used to shift the AC and MQ fractions
right a number of places equal to the difference between the AC and MQ characteristics.
The principle here is the same as moving the point to the left and increasing the expo-
nent by one for each position that the point is moved. The changing of the characteristic
is done later. However, the fraction is adjusted for this change during second step.
The AC and MQ fractions are shifted right until the SC is stepped down to zero.
Third Step L Time. Third step L time occurs when the SC equals 0. The objectives
of the third step are to:
1. Set the AC characteristic equal to the SR characteristic.
2. Add, algebraically, the AC fraction to the SR fraction.
3. Make AC sign equal to the algebraic sign of the result of the addition.
The AC characteristic is set equal to the SR characteristic as a result of the addition
because the AC(9-35) or the l's complement of AC(9-35) is sent to the adders with SR
(1-35). Therefore, when AD(Q-35) are gated back to the AC, the correct characteristic
is set into the AC along with the result of the addition.
When binary fractions with like signs are added, there is a possibility of a carry out
of the high order fraction position. This is the same as a carry across a decimal point.
The 7090 has no register position to hold this carry (1), therefore, the fraction and
characteristic of the sum must be adjusted. This is done by moving the AC and MQ
fractions one position to the right, putting the one in the high order position of the AC
fraction, and increasing the characteristic by one. The characteristic is automatically
increased by one during the addition because the carry out of AD(9) goes to the AD(S).
77