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[b] [c] Normally, an addressing mode without an index would simply use a bare ModR/M byte without a SIB byte at all, but this is necessary to encode an ESP-relative address ([ESP+disp0/8/32]). When MOD=00, a BASE of 101, which would specify EBP with zero displacement, instead specifies no base register and a 32-bit displacement.
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.
For example, the top line is an 'H' record, the first 6 hex digits signify its relative starting location, and the last 6 hex digits represent the program's size. The lines throughout are similar, with each 'T' record consisting of 6 hex digits to signify that line's starting location, 2 hex digits to indicate the size (in bytes) of the line ...
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 ...
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.
In real mode, in order to calculate the physical address of a byte of memory, the hardware shifts the contents of the appropriate segment register 4 bits left (effectively multiplying by 16), and then adds the offset. For example, the logical address 7522:F139 yields the 20-bit physical address:
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 addresses: addresses generated from dynamic address translation, and addresses used by code running in real mode; Absolute addresses: physical addresses; On the 360/65, on S/370 models without DAT and when running with translation turned off, there are only a flat real address space and a flat absolute address space.