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An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere. In computer programming, addressing modes are primarily of interest to those who write in assembly languages and to compiler writers.
The last two components depend on the addressing mode. For example, on the PDP-11/70 (circa 1975), an instruction of the form ADD x (R m ), y (R n ) had a fetch/execute time of 1.35 microseconds plus source and destination times of 0.6 microseconds each, for a total instruction time of 2.55 microseconds.
However, more typical, or frequent, "CISC" instructions merely combine a basic ALU operation, such as "add", with the access of one or more operands in memory (using addressing modes such as direct, indirect, indexed, etc.). Certain architectures may allow two or three operands (including the result) directly in memory or may be able to perform ...
All 16-bit operations are deleted, as are IJMP, ICALL, and all load and store addressing modes except indirect via Z. A second, more successful attempt to subset the AVR instruction set is the "AVR tiny" core. The most significant change is that the AVRtiny core omits registers R0–R15.
What an ISA defines differs between ISAs; in general, ISAs define the supported data types, what state there is (such as the main memory and registers) and their semantics (such as the memory consistency and addressing modes), the instruction set (the set of machine instructions that comprises a computer's machine language), and the input ...
The design of the CPU allows RISC computers few simple addressing modes [2] and predictable instruction times that simplify design of the system as a whole. The conceptual developments of the RISC computer architecture began with the IBM 801 project in the late 1970s, but these were not immediately put into use.
Before the RISC philosophy became prominent, many computer architects tried to bridge the so-called semantic gap, i.e., to design instruction sets that directly support high-level programming constructs such as procedure calls, loop control, and complex addressing modes, allowing data structure and array accesses to be combined into single instructions.
The other advantage is that, because regular memory instructions are used to address devices, all of the CPU's addressing modes are available for the I/O as well as the memory, and instructions that perform an ALU operation directly on a memory operand (loading an operand from a memory location, storing the result to a memory location, or both ...