Editing Microcode (section)

===Vertical microcode===
In vertical microcode, each microinstruction is significantly encoded, that is, the bit fields generally pass through intermediate combinatory logic that, in turn, generates the control and sequencing signals for internal CPU elements (ALU, registers, etc.). This is in contrast with horizontal microcode, in which the bit fields either directly produce the control and sequencing signals or are only minimally encoded. Consequently, vertical microcode requires smaller instruction lengths and less storage, but requires more time to decode, resulting in a slower CPU clock.<ref>{{cite web
 |url          = http://euler.mat.uson.mx/~havillam/ca/CS323/0708.cs-323003.html
 |title        = CS-323: High Performance Microprocessors – Chapter 1. Microprogramming
 |date         = 2009-10-12
 |access-date  = 2015-08-08
 |author1      = Neal Harman
 |author2      = Andy Gimblett
 |website      = mat.uson.mx
 |archive-date = 2015-04-19
 |archive-url  = https://web.archive.org/web/20150419164703/http://euler.mat.uson.mx/~havillam/ca/CS323/0708.cs-323003.html
 |url-status   = dead
}}</ref>

Some vertical microcode is just the assembly language of a simple conventional computer that is emulating a more complex computer. Some processors, such as [[DEC Alpha]] processors and the CMOS microprocessors on later IBM mainframes [[System/390]] and [[z/Architecture]], use machine code, running in a special mode that gives it access to special instructions, special registers, and other hardware resources unavailable to regular machine code, to implement some instructions and other functions,<ref>{{cite book |last=Vaupel |first=Robert |year=2013 |url=https://books.google.com/books?id=1-dt0ABZQOcC&q=millicode |title=High Availability and Scalability of Mainframe Environments using System z and z/OS as example |page=26 |publisher=KIT Scientific |isbn=978-3-7315-0022-3}}</ref><ref>{{cite journal |last=Rogers |first=Bob |date=September–October 2012 |title=The What and Why of zEnterprise Millicode |journal=IBM Systems Magazine |url=http://www.ibmsystemsmag.com/mainframe/administrator/performance/millicode_rogers/ |access-date=2013-11-07 |url-status=dead |archive-url=https://web.archive.org/web/20131016100828/http://ibmsystemsmag.com/mainframe/administrator/performance/millicode_rogers/ |archive-date=2013-10-16}}</ref> such as page table walks on Alpha processors.<ref>{{cite web |url=http://download.majix.org/dec/palcode_dsgn_gde.pdf|title=PALcode for Alpha Microprocessors System Design Guide |publisher=[[Digital Equipment Corporation]] |date=May 1996 |access-date=November 7, 2013 |url-status=live |archive-url=https://web.archive.org/web/20110815022514/http://download.majix.org/dec/palcode_dsgn_gde.pdf |archive-date=August 15, 2011}}</ref>  This is called [[PALcode]] on Alpha processors and [[millicode]] on IBM mainframe processors.

Another form of vertical microcode has two fields:
{| class="wikitable"
|-
| Field select || Field value
|}

The ''field select'' selects which part of the CPU will be controlled by this word of the control store. The ''field value'' controls that part of the CPU. With this type of microcode, a designer explicitly chooses to make a slower CPU to save money by reducing the unused bits in the control store; however, the reduced complexity may increase the CPU's clock frequency, which lessens the effect of an increased number of cycles per instruction.

As transistors grew cheaper, horizontal microcode came to dominate the design of CPUs using microcode, with vertical microcode being used less often.

When both vertical and horizontal microcode are used, the horizontal microcode may be referred to as ''nanocode'' or ''picocode''.<ref>{{cite book |last=Spruth |first=Wilhelm |date=December 2012 |title=The Design of a Microprocessor |publisher=Springer Science & Business Media |isbn=978-3-642-74916-2 |page=31 |url=https://books.google.com/books?id=0YmqCAAAQBAJ |access-date=Jan 18, 2015 |url-status=live |archive-url=https://web.archive.org/web/20161120195023/https://books.google.com/books?id=0YmqCAAAQBAJ |archive-date=November 20, 2016}}</ref>