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The Auxiliary Arithmetic Unit i s not a peripheraldevice; it i s an extension of the basic arithmetic
unit of the Central Processor. All peripheral operations may occur concurrently with the use of
this device.
When data in the two-word floating point format enters the Auxiliary Arithmetic Unit, it i s con-
verted into one 40-bit word and stored in the AX register. (Number designations for bits in the
AAU s t a r t with number 1 rather than 0 a s used in memory.) The word exists in the 40-bit AX
register as:
Se
=
Sign of Exponent
Sm
=
Sign of Mantissa
Bits
1
2
9
1 0
2 0
2 1
22
4 0
The QX register i s an extension of the AX register. It also consists of an eight bit exponent with
sign and a thirty bit mantissa with sign. The value of the exponent of the QX register
is
the value
of the AX register minus 30.
When data in the fixed point double precision format enters the Auxiliary Arithmetic Unit, the two
words from memory (Y and Y
+
1) a r e converted into one 40-bit word and stored in the AX
register. The word exists in the 40-bit AX register as:
Mantissa
Se
Exponent
Mantissa
Sm
An instruction for the Auxiliary Arithmetic Unit i s contained in one 20-bit word identical to the
format of Central P r o c e s s o r instructions.
Bits 1
2
2 0
2 1
2 2
4 0
ADDRESS MODIFICATION (INDEXING)
S
The GE-235
is
capable of automatic address modification under program control. This i s
achieved through the use of the special index words in memory locations 0001, 0002, and 0003.
Memory location 0000 i s not used for address modification, but i t can operate a s a counter.
Thirty-one additional index word groups a r e available a s a n optional feature. Each group con-
tains four index words; the first word may be used a s a counter, the other three words may be
used either a s counters o r for automatic address modification. The total of 32 index word groups
(groups a r e numbered 0
-
31), if included in the system, occupy memory locations 0000 through
0127.
Any of these locations may be used for normal storage if not required for address modifi-
cation in a program. Thus, the GE-235 features a total of 96 index words for the address modifi-
cation of instructions and a total of 128 index words which may be used a s counters and incremented
and tested by special index word instructions.
S
=
Sign
Y+l
Y
S
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