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An instruction set architecture (ISA) is an abstract model of a computer, also referred to as computer architecture.A realization of an ISA is called an implementation.An ISA permits multiple implementations that may vary in performance, physical size, and monetary cost (among other things); because the ISA serves as the interface between software and hardware.
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 "mod" field specifies the addressing mode for the register/memory ("r/m") operand. If the "mod" field is 11 2, the "r/m" field encodes a register in the same manner as the "reg" field. However, if the "mod" field is anything else (00 2, 01 2, or 10 2), the "r/m" field specifies an addressing mode. The interpretation of these five bits ...
The addressing modes were not dramatically changed from 32-bit mode, except that addressing was extended to 64 bits, virtual addresses are now sign extended to 64 bits (in order to disallow mode bits in virtual addresses), and other selector details were dramatically reduced.
In this (original) meaning of offset, only the basic address unit, usually the 8-bit byte, is used to specify the offset's size. In this context an offset is sometimes called a relative address . In IBM System/360 instructions, a 12-bit offset embedded within certain instructions provided a range of between 0 and 4096 bytes.
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.
In some older computer architectures, indirect words supported a variety of more-or-less complicated addressing modes. Another important example is the domain name system which enables names such as en.wikipedia.org to be used in place of network addresses such as 208.80.154.224. The indirection from human-readable names to network addresses ...
A modern x86 CPU may use more than 4 GB of memory, either utilizing the native 64-bit mode of x86-64 CPU, or the Physical Address Extension (PAE), a 36-bit addressing mode. In such a case, a device using DMA with a 32-bit address bus is unable to address memory above the 4 GB line.