Editing Machine code (section)

==Instruction set==
{{main |Instruction set}}

Every processor or processor family has its own [[instruction set]]. Machine instructions are patterns of [[bit]]s<ref group=nb>On early [[Decimal computer|decimal machines]], patterns of characters, digits and digit sign</ref> that specify some particular action.<ref name="sco-p251">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/251 251]}}</ref> An instruction set is described by its [[UNIVAC_1100/2200_series#Instruction_format|instruction format]]. Some ways in which instruction formats may differ:<ref name="sco-p251"/>
* all instructions may have the same length or instructions may have different lengths;
* the number of instructions may be small or large;
* instructions may or may not align with the architecture's [[Word (computer architecture)|word length]].

A processor's instruction set needs to execute the circuits of a computer's [[Logic level|digital logic level]]. At the digital level, the program needs to control the computer's registers, bus, memory, ALU, and other hardware components.<ref name="sco-p162">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/162 162]}}</ref> To control a computer's [[Computer architecture|architectural]] features, machine instructions are created. Examples of features that are controlled using machine instructions:
* [[memory segmentation|segment registers]]<ref name="sco-p231">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/231 231]}}</ref>
* [[protected mode|protected address mode]]<ref name="sco-p237">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/237 237]}}</ref>
* [[binary-coded decimal]] (BCD) arithmetic<ref name="sco-p236">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/236 236]}}</ref>

The criteria for instruction formats include:
* Instructions most commonly used should be shorter than instructions rarely used.<ref name="sco-p251"/>
* The [[High Bandwidth Memory|memory transfer rate]] of the underlying hardware determines the flexibility of the memory fetch instructions.
* The number of bits in the [[Random-access_memory#Addressing|address field]] requires special consideration.<ref name="sco-p253">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/253 253]}}</ref>

Determining the size of the address field is a choice between space and speed.<ref name="sco-p253"/> On some computers, the number of bits in the address field may be too small to access all of the physical memory. Also, [[virtual address space]] needs to be considered. Another constraint may be a limitation on the size of registers used to construct the address. Whereas a shorter address field allows the instructions to execute more quickly, other physical properties need to be considered when designing the instruction format. 

Instructions can be separated into two types: general-purpose and special-purpose. Special-purpose instructions exploit architectural features that are unique to a computer. General-purpose instructions control architectural features common to all computers.<ref name="sco-p283">{{harvnb|Tanenbaum|1990|p= [https://archive.org/details/structuredcomput00tane/page/283 283]}}</ref>

General-purpose instructions control:
* Data movement from one place to another
* Monadic operations that have one [[operand]] to produce a result
* Dyadic operations that have two operands to produce a result
* Comparisons and conditional jumps
* Procedure calls
* Loop control
* Input/output