Editing Addressing mode (section)

==Simple addressing modes for code==

Some simple addressing modes for code are shown below. The nomenclature may vary depending on platform.

===Absolute or direct===
    +----+------------------------------+
    |jump|           address            |
    +----+------------------------------+
 
    (Effective PC address = address)

The effective address for an absolute instruction address is the address parameter itself with no modifications.

===PC-relative===
    +----+------------------------------+
    |jump|           offset             |    jump relative
    +----+------------------------------+
 
    (Effective PC address = next instruction address + offset, offset may be negative)

The effective address for a [[program counter|PC]]-relative instruction address is the offset parameter added to the address of the next instruction. This offset is usually signed to allow reference to code both before and after the instruction.<ref name="maxfield" >
Max Maxfield.
[https://www.eeweb.com/building-a-4-bit-computer-assembly-language-and-assembler-part-1/ "Building a 4-Bit Computer: Assembly Language and Assembler"].
Section "Addressing modes".
2019.
</ref>

This is particularly useful in connection with [[Branch (computer science)|jump instructions]], because typical jumps are to nearby instructions (in a high-level language most '''if''' or '''while''' statements are reasonably short). Measurements of actual programs suggest that an 8 or 10 bit offset is large enough for some 90% of conditional jumps (roughly ±128 or ±512 bytes).<ref>{{cite web|last1=Kong|first1=Shing|last2=Patterson|first2=David|author-link2=David Patterson (computer scientist)|title=Instruction set design|year=1995|at=Slide 27|url=http://www.cs.berkeley.edu/~pattrsn/152/lec3.ps}}</ref>  For jumps to instructions that are not nearby, other addressing modes are used.

Another advantage of PC-relative addressing is that the code may be [[position-independent code|position-independent]], i.e. it can be loaded anywhere in memory without the need to adjust any addresses.

===Register indirect===
    +-------+-----+
    |jumpVia| reg |
    +-------+-----+
 
    (Effective PC address = contents of register 'reg')

The effective address for a register indirect instruction is the address in the specified register. For example, (A7) to access the content of address register A7.

The effect is to transfer control to the instruction whose address is in the specified register.

Many RISC machines, as well as the CISC [[IBM System/360]] and successors, have subroutine call instructions that place the [[return statement|return address]] in an address register—the register indirect addressing mode is used to return from that subroutine call.