Instruction Decoding And Processing; Operation Decoders; Control Circuits; Pulses - IBM 7090 Instruction-Reference

Carry From Adder 9 to Adder 8
A special circuit can prevent or allow a carry from adder 9 to adder 8. The reason
for this is that a floating-point word has two separate parts, a characteristic (positions
1-8) and a fraction (positions 9-35). During some floating-point operations, a carry out
of the fraction must not be allowed to gate to the characteristic.
3.2.00 INSTRUCTION DECODING AND PROCESSING
The output of the program counter is gated to the address register through the address
switches. The output of the address register is then used to locate the address of the
instruction in core storage. The output of core storage is sent to the storage register
and the operation code is also sent to the program register. The output of the program
register then is decoded to instruct the system.
3. 2. 01 Operation Decoders
The output of the program register feeds two decoders. The outputs of positions 1-5
feed the primary operation decoders (POD), and the outputs of positions 6-9 feed the
secondary operation decoders (SOD). The primary operation is used to establish the
basic execution control routing. The secondary operation decoder is used only for the
sense, or primary operation 76 instructions.
Some similar instructions have the same primary operation code. Direct outputs of
the program register or storage register are sent to individual machine circuits to
cause the machine to operate according to the minor differences among these similar
instructions.
3. 2. 02 Control Circuits
The control circuits cause the machine to operate according to the decoding of the
instruction. Specific control circuits cause data to be moved and functions to be per-
formed so the machine gives the desired results of the instruction.
3. 2. 03 Pulses
The control circuits require a great number of pulses and gates. These pulses and
gates, obtained by mixing clock pulses from the multiplexor clock, function in the 7090
as the circuit breakers function in electro-mechanical card machines. They are used
to establish proper sequence of operation in the system.
3.3.00 BASIC CYCLE
The basic cycle in the CPU is 2.1818 microseconds. It is divided into 12 equal times
of the 181. 8 milli-microseconds each. The 12 times ,are sent to the CPU from the
multiplexor.
There are three different types of cycles used by the CPU. These are:
1. Instruction I cycle
2. Execution E cycle
3. Logic L cycle
Every instruction has an I cycle. This cycle brings the instruction from core stor-
age to the CPU, where the instruction is decoded. Some instructions require only the
I cycle for their execution; most instructions require additional cycles.
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