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Instruction Sets: Addressing Modes and Formats - Coggle Diagram
Instruction Sets: Addressing Modes and Formats
Register Addressing
Address field refers to a register rather than a main memory address
EA = R
Advantages:
Only a small address field is needed in the instruction
No time-consuming memory references are required
Disadvantage:
The address space is very limited
Immediate
Simplest form of addressing
Operand = A
Advantage:
no memory reference other than the instruction fetch is required to obtain the operand, thus saving one memory or cache cycle in the instruction cycle
Disadvantage:
The size of the number is restricted to the size of the address field, which, in most instruction sets, is small compared with the word length
This mode can be used to define and use constants or set initial values of variables
Indirect Addressing
Reference to the address of a word in memory which contains a full-length address of the operand
EA = (A)
Advantage
:
For a word length of N an address space of 2N is now available
Disadvantage:
Instruction execution requires two memory references to fetch the operand
A rarely used variant of indirect addressing is multilevel or cascaded indirect addressing
EA = ( . . . (A) . . . )
Disadvantage is that three or more memory references could be required to fetch an operand
Direct Addressing
Address field contains the effective address of the operand
Effective address (EA) = address field (A)
Was common in earlier generations of computers
Requires only one memory reference and no special calculation
Limitation is that it provides only a limited address space
Relative Addressing
The implicitly referenced register is the program counter (PC)
The next instruction address is added to the address field to produce the EA
Typically the address field is treated as a twos complement number for this operation
Thus the effective address is a displacement relative to the address of the instruction
Exploits the concept of locality
Saves address bits in the instruction if most memory references are relatively near to the instruction being executed
Register Indirect Addressing
Analogous to indirect addressing
The only difference is whether the address field refers to a memory location or a register
EA = (R)
Address space limitation of the address field is overcome by having that field refer to a word-length location containing an address
Uses one less memory reference than indirect addressing
Base-Register Addressing
The referenced register contains a main memory address and the address field contains a displacement from that address
The register reference may be explicit or implicit
Exploits the locality of memory references
Convenient means of implementing segmentation
some implementations a single segment base register is employed and is used implicitly
programmer may choose a register to hold the base address of a segment and the instruction must reference it explicitly
Displacement Addressing
Combines the capabilities of direct addressing and register indirect addressing
EA = A + (R)
Requires that the instruction have two address fields, at least one of which is explicit
The value contained in one address field (value = A) is used directly
The other address field refers to a register whose contents are added to A to produce the effective address
Most common uses:
Relative addressing
Base-register addressing
Indexing
Indexed Addressing
The address field references a main memory address and the referenced register contains a positive displacement from that address
The method of calculating the EA is the same as for base-register addressing
An important use is to provide an efficient mechanism for performing iterative operations
Autoindexing
Automatically increment or decrement the index register after each reference to it
EA = A + (R)
(R) (R) + 1
Postindexing
Indexing is performed after the indirection
EA = (A) + (R)
Preindexing
Indexing is performed before the indirection
EA = (A + (R))
Stack Addressing
A stack is a linear array of locations
A stack is a reserved block of locations
Associated with the stack is a pointer whose value is the address of the top of the stack
Is a form of implied addressing
The machine instructions need not include a memory reference but implicitly operate on the top of the stack