Editing Machine code (section)

==Storing in memory==
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From the point of view of the CPU, machine code is stored in RAM, but is typically also kept in a set of caches for performance reasons. There may be different caches for instructions and data, depending on the architecture.

The CPU knows what machine code to execute, based on its internal program counter. The program counter points to a memory address and is changed based on special instructions which may cause programmatic branches. The program counter is typically set to a hard coded value when the CPU is first powered on, and will hence execute whatever machine code happens to be at this address.

Similarly, the program counter can be set to execute whatever machine code is at some arbitrary address, even if this is not valid machine code. This will typically trigger an architecture specific protection fault.

The CPU is oftentimes told, by page permissions in a paging based system, if the current page actually holds machine code by an execute bit — pages have multiple such permission bits (readable, writable, etc.) for various housekeeping functionality. E.g. on [[Unix-like]] systems memory pages can be toggled to be executable with the {{code|mprotect()}} system call, and on Windows, {{code|VirtualProtect()}} can be used to achieve a similar result. If an attempt is made to execute machine code on a non-executable page, an architecture specific fault will typically occur. Treating [[data as machine code]], or finding new ways to use existing machine code, by various techniques, is the basis of some security vulnerabilities.

Similarly, in a segment based system, segment descriptors can indicate whether a segment can contain executable code and in what [[protection ring|rings]] that code can run.

From the point of view of a [[process (computing)|process]], the ''code space'' is the part of its [[virtual address space|address space]] where the code in execution is stored. In [[computer multitasking|multitasking]] systems this comprises the program's [[code segment]] and usually [[shared libraries]]. In [[Thread (computing)|multi-threading]] environment, different threads of one process share code space along with data space, which reduces the overhead of [[context switching]] considerably as compared to process switching.