Addressing Modes

Addressing modes are the ways how architectures specify the address of an object they want to access. In GPR machines, an addressing mode can specify a constant, a register or a location in memory.

The most common names for addressing modes (names may differ among architectures)
Addressing modes Example Instruction Meaning When used
Register Add R4,R3 R4 <- R4 + R3 When a value is in a register
Immediate Add R4, #3 R4 <- R4 + 3 For constants
Displacement Add R4, 100(R1) R4 <- R4 + M[100+R1] Accessing local variables
Register deffered Add R4,(R1) R4 <- R4 + M[R1] Accessing using a pointer or a computed address
Indexed Add R3, (R1 + R2) R3 <- R3 + M[R1+R2] Useful in array addressing:
R1 - base of array
R2 - index amount
Direct Add R1, (1001) R1 <- R1 + M[1001] Useful in accessing static data
Memory deferred Add R1, @(R3) R1 <- R1 + M[M[R3]] If R3 is the address of a pointer p, then mode yields *p
Add R1, (R2)+ R1 <- R1 +M[R2]
R2 <- R2 + d
Useful for stepping through arrays in a loop.
R2 - start of array
d - size of an element
Add R1,-(R2) R2 <-R2-d
R1 <- R1 + M[R2]
Same as autoincrement.
Both can also be used to implement a stack as push and pop 
Scaled Add R1, 100(R2)[R3] R1<-R1+M[100+R2+R3*d] Used to index arrays. May be applied to any base addressing mode in some machines.
<-  - assignment
M   - the name for memory:
M[R1] refers to contents of memory location whose address is given by the contents of R1

Immediate and displacement addressing modes dominate addressing mode usage. The major question for displacement-style addressing mode is that of the range of displacement used. Choosing the displacement field size is important because it directly affects instruction length. According to measurements taken on the data access on a GPR architecture using SPEC benchmarks displacement values are widely distributed.

Another important instruction set measurement is the range of values for immediates . Small immediate values are used most heavily. However, large immediates are sometimes used, most likely in address calculations.

Encoding of Addressing Modes

How the addressing modes of operands are encoded depends on
the range of addressing modes
the degree of independence between opcodes and modes
For small number of addressing modes or opcode/addressing mode combinations, the addressing mode can be encoded in opcode.
For a larger number of combinations, typically a separate address specifier is needed for each operand.
The architect has to balance several competing forces when encoding the instruction set:
The desire to have as many registers and addressing modes as possible.
The impact of the size of the register and addressing mode fields on the average instruction size and hence on the average program size.
A desire to have instructions encode into lengths that are easy to handle in the implementation (multiples of bytes, fixed-length) with possible sacrificing in average code size.