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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.
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.
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 ...
There are 16 and 32-bit versions of this addressing mode; Immediate mode Data stored in the instruction, e.g. "#400" Quick immediate mode 3-bit unsigned (or 8-bit signed with moveq) with value stored in opcode; In addq and subq, 0 is the equivalent to 8; e.g. moveq #0,d0 was quicker than clr.l d0 (though both made D0 equal to 0)
Most instructions have one or more opcode fields that specify the basic instruction type (such as arithmetic, logical, jump, etc.), the operation (such as add or compare), and other fields that may give the type of the operand(s), the addressing mode(s), the addressing offset(s) or index, or the operand value itself (such constant operands ...
In general, an ISA defines the supported instructions, data types, registers, the hardware support for managing main memory, [clarification needed] fundamental features (such as the memory consistency, addressing modes, virtual memory), and the input/output model of implementations of the ISA.
Q address constants contain not actual addresses but a displacement in the External Dummy Section – similar to the Linux Global Offset Table (see Position-independent code). A J adcon is set by the linkage editor to hold the cumulative length of the External Dummy Section, and does not actually contain an address.
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.