Editing Microcode (section)

===Moving up the line===
[[File:Motorola 68000 die.JPG|thumb|The microcode (and "nanocode") of the [[Motorola 68000]] is stored in the two large square blocks in the upper right and controlled by circuitry to the right of it. It takes up a significant amount of the total chip surface.]]
Early [[minicomputer]]s were far too simple to require microcode, and were more similar to earlier mainframes in terms of their instruction sets and the way they were decoded. But it was not long before their designers began using more powerful [[integrated circuit]]s that allowed for more complex ISAs. By the mid-1970s, most new minicomputers and [[superminicomputer]]s were using microcode as well, such as most models of the [[PDP-11]] and, most notably, most models of the [[VAX]], which included high-level instruction not unlike those found in the 360.<ref>{{cite book |date=May 1988 |title=VLSI VAX Micro-Architecture |first=Bob |last=Supnik |publisher=Digital Equipment |url=http://simh.trailing-edge.com/docs/microarch.pdf}}</ref>

The same basic evolution occurred with [[microprocessor]]s as well. Early designs were extremely simple, and even the more powerful 8-bit designs of the mid-1970s like the [[Zilog Z80]] had instruction sets that were simple enough to be implemented in dedicated logic. By this time, the control logic could be patterned into the same die as the CPU, making the difference in cost between ROM and logic less of an issue. However, it was not long before these companies were also facing the problem of introducing higher-performance designs but still wanting to offer [[backward compatibility]]. Among early examples of microcode in micros was the [[Intel 8086]].<ref name=microcode/>

Among the ultimate implementations of microcode in microprocessors is the [[Motorola 68000]]. This offered a highly [[orthogonal instruction set]] with a wide variety of [[addressing mode]]s, all implemented in microcode. This did not come without cost, according to early articles, about 20% of the chip's surface area (and thus cost) is the microcode system<ref>{{cite magazine |magazine=Byte |date= April 1983 |title=Design Philosophy Behind Motorola's MC68000 |first= Thomas |last= Starnes |url=http://www.easy68k.com/paulrsm/doc/dpbm68k1.htm}}</ref> and {{cn|reason=later estimates suggest approximately 23,000|date=December 2024}} of the systems 68,000 transistors were part of the microcode system.