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Secure Digital (SD) cards have a write-protect tab on the left side. Extensively, media that, by means of design, can't operate outside from this mode: CD-R, DVD-R, Vinyl records, etc. These mechanisms are intended to prevent only accidental data loss or attacks by computer viruses.
The most common form, only available when using a full-size SD card, provides a physical write protection switch which allows the user to advise the host card reader to disallow write access. This does not protect the data on the card if the card reader hardware is not built to respect the write protection switch. [2]
This often increases the speed of image write operations by several times, at the cost of losing the simple sequential read/write semantics of normal memory. Additional bits which are provided on some computer architectures, such as AMD64, allow the shadowing of ROM contents in memory (shadow ROM), and the configuration of memory-mapped I/O.
A reference to a memory location includes a value that identifies a segment and an offset within that segment. A segment descriptor may limit access rights, e.g., read only, only from certain rings. The x86 architecture has multiple segmentation features, which are helpful for using protected memory on this architecture. [1]
To add a data FIFO without breaking compatibility with the 8250 UART's eight configuration registers, the write-only "FIFO control register" was assigned the same port address as the read-only "interrupt identification register". Writes to that address program the FIFO control register, but there is no way to read it back.
Write protect: When set, the CPU cannot write to read-only pages when privilege level is 0 18: AM: Alignment mask: Alignment check enabled if AM set, AC flag (in EFLAGS register) set, and privilege level is 3 29: NW: Not-write through: Globally enables/disable write-through caching: 30: CD: Cache disable: Globally enables/disable the memory ...
A programmable read-only memory (PROM) is a form of digital memory where the contents can be changed once after manufacture of the device. The data is then permanent and cannot be changed. It is one type of read-only memory (ROM).
The position [i.e., setting] of the write protect switch is unknown to the internal circuitry of the card." [121] Some host devices do not support write protection, which is an optional feature of the SD specification, and drivers and devices that do obey a read-only indication may give the user a way to override it. [citation needed]