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Furthermore, an addressing mode which, in one given architecture, is treated as a single addressing mode may represent functionality that, in another architecture, is covered by two or more addressing modes. For example, some complex instruction set computer (CISC) architectures, such as the Digital Equipment Corporation (DEC) VAX, treat ...
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(Rm),y(Rn) 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. Any case where addressed memory is not in the ...
However, addressing modes 0–3 were "short immediate" for immediate data of 6 bits or less (the 2 low-order bits of the addressing mode being the 2 high-order bits of the immediate data, when prepended to the remaining 4 bits in that data-addressing byte). Since addressing modes 0-3 were identical, this made 13 (electronic) addressing modes ...
(In the examples that follow, a, b, and c are (direct or calculated) addresses referring to memory cells, while reg1 and so on refer to machine registers.) C = A+B 0-operand (zero-address machines), so called stack machines: All arithmetic operations take place using the top one or two positions on the stack: [9] push a, push b, add, pop c.
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.
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 ...
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 ...
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.