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Control StoreA control store is the part of a CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer. A control store is usually implemented as a diode-array of read-only memory on modern VLSI CPUs. Historically, IBM mainframes loaded their microprograms from floppy disks into a control store consisting of ultra-high speed random-access memory. This permitted IBM to easily repair microprogramming defects in the field. The control store usually has on its outputs a register. It must be obvious that the outputs that go back into the sequencer to determine the next address have to go through some sort of register to prevent a race condition being created. However it turns out that in most designs it pays for all of the other bits to also go through a register. This is because the machine will work faster if the execution of the next microinstruction is delayed by one cycle. This register is known as a pipeline register. The point is that very often the execution of the next microinstruction is dependent on result of the current microinstruction that will not be stable till the end of the current microcycle. It can be seen that either way all of the outputs of the control store go into one big register. In the olden days it used to be possible to buy EPROMS with these register bits on the same chip. The clock signal determining the cycle time of the system primarily clocks this register.
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