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Guy Harris (talk | contribs) →The 360: The alternatives were microcode, with control store, and hardwired logic, with no control store. |
Guy Harris (talk | contribs) →The 360: Speak of "internal steps" (control steps or microinstructions) that implement complex operations; speak of "instructions" risks confusing them with machine instructions. |
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===The 360===
{{main|System/360}}
Microcode remained relatively rare in computer design as the cost of the ROM needed to store the code was not significantly different from the cost of custom control logic. This changed through the early 1960s with the introduction of mass-produced [[core memory]] and [[core rope]], which was far less expensive than dedicated logic based on diode arrays or similar solutions. The first to take real advantage of this was [[IBM]] in their 1964 [[System/360]] series. This allowed the machines to have a very complex instruction set, including operations that matched high-level language constructs like formatting binary values as decimal strings,
But the real value in the 360 line was that one could build a series of machines that were completely different internally, yet run the same ISA. For a low-end machine, one might use an 8-bit ALU that requires multiple cycles to complete a single 32-bit addition, while a higher end machine might have a full 32-bit ALU that performs the same addition in a single cycle. These differences could be implemented in control logic, but the cost of implementing a completely different decoder for each machine would be prohibitive. Using microcode meant all that changed was the code in the ROM. For instance, one machine might include a [[floating point unit]] and thus its microcode for multiplying two numbers might be only a few lines line, whereas on the same machine without the FPU this would be a program that did the same using multiple additions, and all that changed was the ROM.<ref name=IBM/>
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