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This addressing mode is closely related to the indexed absolute addressing mode. Example 1: Within a subroutine a programmer will mainly be interested in the parameters and the local variables, for which one base register (the frame pointer) usually suffices.
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.
Operations which exceed the bounds of zero or 65535 (0xFFFF) will undergo modulo 64K operation just as any normal 16-bit operation. For example, if the segment register is set to 0x5000 and the offset is being incremented, the moment this counter offset becomes (0x10000), the resulting absolute address will roll over to 0x5000:0000.
The PDP-11 used 3-bit fields for addressing modes (0-7) so there were (electronically) 8 addressing modes. An additional 3-bit field specified the registers (R0–R5, SP, PC). Immediate and absolute address operands applying the two autoincrement modes to the Program Counter (R7), provided a total of 10 conceptual addressing modes.
CISC ISAs like x86-64 offer low register pressure despite having smaller register sets. This is due to the many addressing modes and optimizations (such as sub-register addressing, memory operands in ALU instructions, absolute addressing, PC-relative addressing, and register-to-register spills) that CISC ISAs offer. [13]
Relocation is the process of assigning load addresses for position-dependent code and data of a program and adjusting the code and data to reflect the assigned addresses. [1] [2] Prior to the advent of multiprocess systems, and still in many embedded systems, the addresses for objects are absolute starting at a known location, often zero.
Generally, though, such schemes are a lot of trouble, and for convenience to the programmer absolute addresses (and underlying that, a flat address space) is preferred. A one byte offset, such as the hexadecimal ASCII value of a character (e.g. X'29') can be used to point to an alternative integer value (or index) in an array (e.g., X'01').
If the paging unit is enabled, addresses in a segment are now virtual addresses, rather than physical addresses as they were on the 80286. That is, the segment starting address, the offset, and the final 32-bit address the segmentation unit derived by adding the two are all virtual (or logical) addresses when the paging unit is enabled.