Microcode: Difference between revisions

Revision as of 07:53, 23 June 2024

Revision as of 12:00, 28 June 2024

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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 than using a custom control store. This changed through the early 1960s with the introduction of mass-produced [[core memory]] and [[core rope]], which was far less expensive thatthan 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, storing the complex series of instructions needed for this task in low cost memory.<ref name=IBM>{{cite web |url=https://www.righto.com/2022/01/ibm360model50.html |title=Simulating the IBM 360/50 mainframe from its microcode |website=Ken Shirriff's blog |first=Ken |last=Shirriff}}</ref>

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/>