Flat memory model or linear memory model refers to a

memory address In computing, a memory address is a reference to a specific memory location in memory used by both software and hardware. These addresses are fixed-length sequences of digits, typically displayed and handled as unsigned integers. This numeric ...

ing paradigm in which "

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...

appears to the program as a single contiguous

address space In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity. For software programs to save and retrieve ...

." The CPU can directly (and linearly)

address An address is a collection of information, presented in a mostly fixed format, used to give the location of a building, apartment, or other structure or a plot of land, generally using border, political boundaries and street names as references, ...

all of the available

locations without having to resort to any sort of

bank switching Bank switching is a technique used in computer design to increase the amount of usable memory beyond the amount directly addressable by the Processor (computing), processor instructions. It can be used to configure a system differently at diffe ...

memory segmentation Memory segmentation is an operating system memory management technique of dividing a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifies ...

paging In computer operating systems, memory paging is a memory management scheme that allows the physical Computer memory, memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compact ...

schemes. Memory management and address translation can still be implemented ''on top of'' a flat memory model in order to facilitate the

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

's functionality, resource protection, multitasking or to increase the memory capacity beyond the limits imposed by the processor's physical address space, but the key feature of a flat memory model is that the entire memory space is linear, sequential and contiguous. In a simple controller, or in a ''single tasking'' embedded application, where memory management is not needed nor desirable, the flat memory model is the most appropriate, because it provides the simplest interface from the programmer's point of view, with direct access to all memory locations and minimum design complexity. In a general purpose computer system, which requires multitasking, resource allocation, and protection, the flat memory system must be augmented by some memory management scheme, which is typically implemented through a combination of dedicated hardware (inside or outside the CPU) and software built into the operating system. The flat memory model (at the physical addressing level) still provides the greatest flexibility for implementing this type of memory management.

Memory models

Most modern memory models fall into one of three categories:

Flat unpaged memory model

*Simple interface for programmers, clean design *Greatest flexibility due to uniform access speed (segmented memory page switches usually incur varied latency due to longer accesses of other pages, either due to extra CPU logic in changing page, or hardware requirements) *Minimum hardware and CPU real estate for simple controller applications *Maximum execution speed, as there is no need to access auxiliary data structures such as a segment or page table in RAM *Not suitable for general computing or multitasking operating systems

Paged memory model

*Suitable for multitasking, general operating system design, resource protection and allocation *Suitable for virtual memory implementation *More CPU real estate, somewhat lower speed *More complex to program *Rigid page boundaries, not always the most memory efficient *This memory model is required when using

Physical Address Extension In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension, is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon process ...

(PAE) in

Pentium Pro The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel and introduced on November 1, 1995. It implements the P6 (microarchitecture), P6 microarchitecture (sometimes termed i686), and was the first x86 Intel C ...

and later x86 CPUs to support 36-bit physical addresses to address more than 4GB of physical memory.

x86 segmented memory model

*Similar to paged memory, but paging is achieved by the implicit addition of two relatively shifted registers: segment:offset *Variable page boundaries, more efficient and flexible than the paged memory model *Quite complex and awkward from a programmer's point of view *More difficult for compilers *Pages can overlap / poor resource protection and isolation *Many to one address translation correspondence: Many segment:offset combinations resolve to the same physical address *Greater chance of programming errors *Implemented in the original Intel 8086, 8088, 80186, 80286, and supported by 80386 and all subsequent

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

machines through to present day

Pentium Pentium is a series of x86 architecture-compatible microprocessors produced by Intel from 1993 to 2023. The Pentium (original), original Pentium was Intel's fifth generation processor, succeeding the i486; Pentium was Intel's flagship proce ...

and Core 2 processors. This memory model has remained ever since in the x86 machines, which now provide multi-mode operation and rarely operate in the compatible segmented mode. See

x86 memory segmentation x86 memory segmentation is a term for the kind of memory segmentation characteristic of the Intel x86 computer instruction set architecture. The x86 architecture has supported memory segmentation since the original Intel 8086 (1978), but ''x86 ...

for details. *saves RAM by moving the segment address, this allows short jumps that require fewer bytes. Within the x86 architectures, when operating in the

real mode Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...

(or emulation), physical address is computed as:General description of Real Mode
"The physical address can be calculated as Value_in_segment_register 16 + Value_in_offset_register." :Address = 16 × segment + offset (I.e., the 16-bit segment register is shifted left by 4 bits and added to a 16-bit offset, resulting in a 20-bit address.)

References

{{Reflist Computer memory

Memory models

Flat unpaged memory model

Paged memory model

x86 segmented memory model

See also

References