Flat memory model or linear memory model refers to a memory addressing paradigm in low-level software design such that the CPU can directly (and sequentially/linearly) address all of the available memory locations without having to resort to any sort of memory segmentation or paging schemes.
Memory management and logical-to-physical address translation can still be implemented on top of a flat memory model in order to facilitate the operating system'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 from address zero to MaxBytes-1.
Memory management is therefore optional; it is neither necessary, nor dictated by the CPU architecture:
- 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.
The vast majority of processor architectures do implement a flat memory design, including all early 8-bit processors, the Motorola 68K series, etc. The original Intel 8086 (Intel's first 16-bit microprocessor) was an exception to this rule which implemented a segmented memory model, because it provided an easy form of memory management with flexible page boundaries for early operating systems such as DOS and allowed access to an address space greater than what would otherwise be available with its 16-bit addressing.
Read more about Flat Memory Model: Von Neumann Vs. Harvard Architectures
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