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Concept
Bus error
Résumé
In computing, a bus error is a fault raised by hardware, notifying an operating system (OS) that a process is trying to access memory that the CPU cannot physically address: an invalid address for the address bus, hence the name. In modern use on most architectures these are much rarer than segmentation faults, which occur primarily due to memory access violations: problems in the logical address or permissions.
On POSIX-compliant platforms, bus errors usually result in the SIGBUS signal being sent to the process that caused the error. SIGBUS can also be caused by any general device fault that the computer detects, though a bus error rarely means that the computer hardware is physically broken—it is normally caused by a bug in software. Bus errors may also be raised for certain other paging errors; see below.
There are at least three main causes of bus errors:
Software instructs the CPU to read or write a specific physical memory address. Accordingly, the CPU sets this physical address on its address bus and requests all other hardware connected to the CPU to respond with the results, if they answer for this specific address. If no other hardware responds, the CPU raises an exception, stating that the requested physical address is unrecognized by the whole computer system. Note that this only covers physical memory addresses. Trying to access an undefined virtual memory address is generally considered to be a segmentation fault rather than a bus error, though if the MMU is separate, the processor cannot tell the difference.
Most CPUs are byte-addressable, where each unique memory address refers to an 8-bit byte. Most CPUs can access individual bytes from each memory address, but they generally cannot access larger units (16 bits, 32 bits, 64 bits and so on) without these units being "aligned" to a specific boundary (the x86 platform being a notable exception).
For example, if multi-byte accesses must be 16 bit-aligned, addresses (given in bytes) at 0, 2, 4, 6, and so on would be considered aligned and therefore accessible, while addresses 1, 3, 5, and so on would be considered unaligned.
Source officielle
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