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Under MIB addressing, the base and displacement are used to compute an effective address as base + displacement. [ 1 ] : §3.1.1.3 The register specified by the SIB byte's INDEX field does not participate in this effective-address calculation, but is instead treated as a separate input argument to the instructions using this addressing mode.
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.
Base-plus-index and scale-plus-index addressing require the SIB byte, which encodes 2-bit scale factor as well as 3-bit index and 3-bit base registers. Depending on the addressing mode, Disp8/Disp16/Disp32 field may follow with displacement that needs to be added to the address. The EVEX prefix retains fields introduced in the VEX prefix:
Architectures typically allow instructions to include some combination of operand addressing modes: Direct The instruction specifies a complete address Immediate The instruction specifies a value rather than an address Indexed The instruction specifies a register to use as an index. In some architecture the index is scaled by the operand length ...
In contrast to the PDP-11's 3-bit fields, the VAX-11's 4-bit sub-bytes resulted in 16 addressing modes (0–15). 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 ...
Real Address mode, [37] commonly called Real mode, is an operating mode of 8086 and later x86-compatible CPUs. Real mode is characterized by a 20-bit segmented memory address space (meaning that only slightly more than 1 MiB of memory can be addressed [ p ] ), direct software access to peripheral hardware, and no concept of memory protection or ...
Format 4: Only valid on SIC/XE machines, consists of the same elements as format 3, but instead of a 12-bit displacement, stores a 20-bit address. Both format 3 and format 4 have six-bit flag values in them, consisting of the following flag bits: n: Indirect addressing flag; i: Immediate addressing flag; x: Indexed addressing flag
The 16-bit arithmetic operations (ADIW, SBIW) are omitted, as are the load/store with displacement addressing modes (Y+d, Z+d), but the predecrement and postincrement addressing modes are retained. The LPM instruction is omitted; instead program ROM is mapped to the data address space and may be accessed with normal load instructions.