Editing Microcode (section)

===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, encoding the complex series of internal steps 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/>

The outcome of this design was that customers could use a low-end model of the family to develop their software, knowing that if more performance was ever needed, they could move to a faster version and nothing else would change. This lowered the barrier to entry and the 360 was a runaway success. By the end of the decade, the use of microcode was ''de rigueur'' across the mainframe industry.