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<title>Read-only memory</title>

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Read-only memory

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</style><div role="note" class="hatnote navigation-not-searchable">The concept of read-only data can also refer to <a href="File-system_permissions" title="File-system permissions">file-system permissions</a>.</div>

<div role="note" class="hatnote navigation-not-searchable">"ROM" redirects here. For the country with the country code ROM, see <a href="Romania" title="Romania">Romania</a>. For the museum in Toronto, see <a href="Royal_Ontario_Museum" title="Royal Ontario Museum">Royal Ontario Museum</a>. For other uses, see <a href="ROM_(disambiguation)" class="mw-redirect mw-disambig" title="ROM (disambiguation)">ROM (disambiguation)</a>.</div>

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</style><table class="sidebar sidebar-collapse nomobile nowraplinks hlist"><tbody><tr><th class="sidebar-title"><a href="Computer_memory" title="Computer memory">Computer memory</a> and <a href="Computer_data_storage" title="Computer data storage">data storage</a> types</th></tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">General</div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Memory_cell_(computing)" title="Memory cell (computing)">Memory cell</a></li>

<li><a href="Memory_coherence" title="Memory coherence">Memory coherence</a></li>

<li><a href="Cache_coherence" title="Cache coherence">Cache coherence</a></li>

<li><a href="Memory_hierarchy" title="Memory hierarchy">Memory hierarchy</a></li>

<li><a href="Memory_access_pattern" title="Memory access pattern">Memory access pattern</a></li>

<li><a href="Memory_map" title="Memory map">Memory map</a></li>

<li><a href="Computer_data_storage#Secondary_storage" title="Computer data storage">Secondary storage</a></li>

<li><a href="Semiconductor_memory" title="Semiconductor memory">MOS memory</a>

<ul><li><a href="Floating-gate_MOSFET" title="Floating-gate MOSFET">floating-gate</a></li></ul></li>

<li><a href="Continuous_availability" title="Continuous availability">Continuous availability</a></li>

<li><a href="Areal_density_(computer_storage)" class="mw-redirect" title="Areal density (computer storage)">Areal density (computer storage)</a></li>

<li><a href="Block_(data_storage)" title="Block (data storage)">Block (data storage)</a></li>

<li><a href="Object_storage" title="Object storage">Object storage</a></li>

<li><a href="Direct-attached_storage" title="Direct-attached storage">Direct-attached storage</a></li>

<li><a href="Network-attached_storage" title="Network-attached storage">Network-attached storage</a>

<ul><li><a href="Storage_area_network" title="Storage area network">Storage area network</a></li>

<li><a href="Block-level_storage" title="Block-level storage">Block-level storage</a></li></ul></li>

<li><a href="Single-instance_storage" title="Single-instance storage">Single-instance storage</a></li>

<li><a href="Data_model" title="Data model">Structured data</a></li>

<li><a href="Unstructured_data" title="Unstructured data">Unstructured data</a></li>

<li><a href="Metadata" title="Metadata">Metadata</a></li>

<li><a href="Data_compression" title="Data compression">Data compression</a></li>

<li><a href="Data_corruption" title="Data corruption">Data corruption</a></li>

<li><a href="Data_cleansing" title="Data cleansing">Data cleansing</a></li>

<li><a href="Data_degradation" title="Data degradation">Data degradation</a></li>

<li><a href="Data_integrity" title="Data integrity">Data integrity</a></li>

<li><a href="Data_security" title="Data security">Data security</a></li>

<li><a href="Data_validation" title="Data validation">Data validation</a></li>

<li><a href="Data_validation_and_reconciliation" title="Data validation and reconciliation">Data validation and reconciliation</a></li>

<li><a href="Data_recovery" title="Data recovery">Data recovery</a></li>

<li><a href="Computer_data_storage" title="Computer data storage">Storage</a></li>

<li><a href="Data_cluster" class="mw-redirect" title="Data cluster">Data cluster</a></li>

<li><a href="Directory_(computing)" title="Directory (computing)">Directory</a></li>

<li><a href="Shared_resource" title="Shared resource">Shared resource</a></li>

<li><a href="File_sharing" title="File sharing">File sharing</a></li>

<li><a href="File_system" title="File system">File system</a></li>

<li><a href="Clustered_file_system" title="Clustered file system">Clustered file system</a></li>

<li><a href="Clustered_file_system#Distributed_file_systems" title="Clustered file system">Distributed file system</a></li>

<li><a href="Distributed_file_system_for_cloud" title="Distributed file system for cloud">Distributed file system for cloud</a></li>

<li><a href="Distributed_data_store" title="Distributed data store">Distributed data store</a></li>

<li><a href="Distributed_database" title="Distributed database">Distributed database</a></li>

<li><a href="Database" title="Database">Database</a></li>

<li><a href="Data_storage" title="Data storage">Data storage</a></li>

<li><a href="Data_store" title="Data store">Data store</a></li>

<li><a href="Data_deduplication" title="Data deduplication">Data deduplication</a></li>

<li><a href="Data_structure" title="Data structure">Data structure</a></li>

<li><a href="Data_redundancy" title="Data redundancy">Data redundancy</a></li>

<li><a href="Replication_(computing)" title="Replication (computing)">Replication (computing)</a></li>

<li><a href="Memory_refresh" title="Memory refresh">Memory refresh</a></li>

<li><a href="Storage_record" title="Storage record">Storage record</a></li>

<li><a href="Information_repository" title="Information repository">Information repository</a></li>

<li><a href="Knowledge_base" title="Knowledge base">Knowledge base</a></li>

<li><a href="Computer_file" title="Computer file">Computer file</a></li>

<li><a href="Object_file" title="Object file">Object file</a></li>

<li><a href="File_deletion" title="File deletion">File deletion</a></li>

<li><a href="File_copying" title="File copying">File copying</a></li>

<li><a href="Backup" title="Backup">Backup</a></li>

<li><a href="Data_communication" title="Data communication">Data communication</a></li>

<li><a href="Information_transfer" title="Information transfer">Information transfer</a></li>

<li><a href="Temporary_file" title="Temporary file">Temporary file</a></li>

<li><a href="Copy_protection" title="Copy protection">Copy protection</a></li>

<li><a href="Digital_rights_management" title="Digital rights management">Digital rights management</a></li>

<li><a href="Volume_(computing)" title="Volume (computing)">Volume (computing)</a></li>

<li><a href="Boot_sector" title="Boot sector">Boot sector</a></li>

<li><a href="Master_boot_record" title="Master boot record">Master boot record</a></li>

<li><a href="Volume_boot_record" title="Volume boot record">Volume boot record</a></li>

<li><a href="GUID_Partition_Table" title="GUID Partition Table">GUID Partition Table</a></li>

<li><a href="Disk_array" title="Disk array">Disk array</a></li>

<li><a href="Disk_image" title="Disk image">Disk image</a></li>

<li><a href="Disk_mirroring" title="Disk mirroring">Disk mirroring</a></li>

<li><a href="Disk_aggregation" title="Disk aggregation">Disk aggregation</a></li>

<li><a href="Disk_partitioning" title="Disk partitioning">Disk partitioning</a></li>

<li><a href="Memory_segmentation" title="Memory segmentation">Memory segmentation</a></li>

<li><a href="Locality_of_reference" title="Locality of reference">Locality of reference</a></li>

<li><a href="Logical_disk" title="Logical disk">Logical disk</a></li>

<li><a href="Storage_virtualization" title="Storage virtualization">Storage virtualization</a></li>

<li><a href="Virtual_memory" title="Virtual memory">Virtual memory</a></li>

<li><a href="Memory-mapped_file" title="Memory-mapped file">Memory-mapped file</a></li>

<li><a href="Software_entropy" class="mw-redirect" title="Software entropy">Software entropy</a></li>

<li><a href="Software_rot" title="Software rot">Software rot</a></li>

<li><a href="In-memory_database" title="In-memory database">In-memory database</a></li>

<li><a href="In-memory_processing" title="In-memory processing">In-memory processing</a></li>

<li><a href="Persistence_(computer_science)" title="Persistence (computer science)">Persistence (computer science)</a></li>

<li><a href="Persistent_data_structure" title="Persistent data structure">Persistent data structure</a></li>

<li><a href="Non-RAID_drive_architectures" title="Non-RAID drive architectures">Non-RAID drive architectures</a></li>

<li><a href="Memory_paging" title="Memory paging">Memory paging</a></li>

<li><a href="Bank_switching" title="Bank switching">Bank switching</a></li>

<li><a href="Grid_computing" title="Grid computing">Grid computing</a></li>

<li><a href="Cloud_computing" title="Cloud computing">Cloud computing</a></li>

<li><a href="Cloud_storage" title="Cloud storage">Cloud storage</a></li>

<li><a href="Fog_computing" title="Fog computing">Fog computing</a></li>

<li><a href="Edge_computing" title="Edge computing">Edge computing</a></li>

<li><a href="Dew_computing" title="Dew computing">Dew computing</a></li>

<li><a href="Amdahl's_law" title="Amdahl's law">Amdahl's law</a></li>

<li><a href="Moore's_law" title="Moore's law">Moore's law</a></li>

<li><a href="Mark_Kryder#Kryder's_law_projection" title="Mark Kryder">Kryder's law</a></li></ul></div></div></td>

</tr><tr><th class="sidebar-heading">

<a href="Volatile_memory" title="Volatile memory">Volatile</a></th></tr><tr><td class="sidebar-content">

<ul><li><a href="Cache_(computing)#HARDWARE" title="Cache (computing)">Hardware cache</a>

<ul><li><a href="CPU_cache" title="CPU cache">CPU cache</a></li>

<li><a href="Scratchpad_memory" title="Scratchpad memory">Scratchpad memory</a></li></ul></li>

<ul><li><a href="EDRAM" title="EDRAM">eDRAM</a></li>

<li><a href="Synchronous_dynamic_random-access_memory" title="Synchronous dynamic random-access memory">SDRAM</a></li>

<li><a href="Synchronous_dynamic_random-access_memory#Synchronous_Graphics_RAM_(SGRAM)" title="Synchronous dynamic random-access memory">SGRAM</a></li>

<li><a href="LPDDR" title="LPDDR">LPDDR</a></li>

<li><a href="Quad_Data_Rate_SRAM" title="Quad Data Rate SRAM">QDRSRAM</a></li>

<li><a href="RDRAM" title="RDRAM">RDRAM</a></li>

<li><a href="Resistive_random-access_memory" title="Resistive random-access memory">ReRAM</a></li>

<li><a href="Content-addressable_memory" title="Content-addressable memory">Content-addressable memory</a> (CAM)</li>

<li><a href="Computational_RAM" title="Computational RAM">Computational RAM</a></li>

<li><a href="Dual-ported_RAM" title="Dual-ported RAM">Dual-ported RAM</a>

<ul><li><a href="Video_RAM_(dual-ported_DRAM)" class="mw-redirect" title="Video RAM (dual-ported DRAM)">Video RAM (dual-ported DRAM)</a></li></ul></li></ul></div></div></td>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">Historical</div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Williams_tube" title="Williams tube">Williams–Kilburn tube</a> (1946–1947)</li>

<li><a href="Delay-line_memory" title="Delay-line memory">Delay-line memory</a> (1947)</li>

<li><a href="Mellon_optical_memory" title="Mellon optical memory">Mellon optical memory</a> (1951)</li>

<li><a href="Selectron_tube" title="Selectron tube">Selectron tube</a> (1952)</li>

<li><a href="Dekatron" title="Dekatron">Dekatron</a></li>

</tr><tr><th class="sidebar-heading">

<a href="Non-volatile_memory" title="Non-volatile memory">Non-volatile</a></th></tr><tr><td class="sidebar-content">

<ul><li><a href="Diode_matrix" title="Diode matrix">Diode matrix</a></li>

<ul><li><a href="EPROM" title="EPROM">EPROM</a></li>

<li><a href="EEPROM" title="EEPROM">EEPROM</a></li></ul></li>

<li><a href="ROM_cartridge" title="ROM cartridge">ROM cartridge</a></li>

<li><a href="Solid-state_storage" title="Solid-state storage">Solid-state storage</a> (SSS)

<ul><li><a href="Flash_memory" title="Flash memory">Flash memory</a> is used in:</li>

<li><a href="Solid-state_drive" title="Solid-state drive">Solid-state drive</a> (SSD)</li>

<li><a href="Solid-state_hybrid_drive" class="mw-redirect" title="Solid-state hybrid drive">Solid-state hybrid drive</a> (SSHD)</li>

<li><a href="USB_flash_drive" title="USB flash drive">USB flash drive</a></li>

<li><a href="IBM_FlashSystem" title="IBM FlashSystem">IBM FlashSystem</a></li>

<li><a href="Flash_Core_Module" title="Flash Core Module">Flash Core Module</a></li></ul></li>

<li><a href="Memory_card" title="Memory card">Memory card</a>

<ul><li><a href="Memory_Stick" title="Memory Stick">Memory Stick</a></li>

<li><a href="CompactFlash" title="CompactFlash">CompactFlash</a></li>

<li><a href="MultiMediaCard" title="MultiMediaCard">MultiMediaCard</a></li>

<li><a href="SmartMedia" title="SmartMedia">SmartMedia</a></li>

<li><a href="Universal_Flash_Storage" title="Universal Flash Storage">Universal Flash Storage</a></li>

<li><a href="MicroP2" title="MicroP2">MicroP2</a></li>

<li><a href="Programmable_metallization_cell" title="Programmable metallization cell">Programmable metallization cell</a></li></ul></div></div></td>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)"><a href="Non-volatile_random-access_memory" title="Non-volatile random-access memory">NVRAM</a></div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Memistor" title="Memistor">Memistor</a></li>

<li><a href="Memristor" title="Memristor">Memristor</a></li>

<li><a href="Phase-change_memory" title="Phase-change memory">PCM</a> (<a href="3D_XPoint" title="3D XPoint">3D XPoint</a>)</li>

<li><a href="Electrochemical_RAM" title="Electrochemical RAM">Electrochemical RAM</a> (ECRAM)</li>

<li><a href="Programmable_metallization_cell" title="Programmable metallization cell">CBRAM</a></li></ul></div></div></td>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">Early-stage <a href="Non-volatile_random-access_memory" title="Non-volatile random-access memory">NVRAM</a></div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Ferroelectric_RAM" title="Ferroelectric RAM">FeRAM</a></li>

<li><a href="Resistive_random-access_memory" title="Resistive random-access memory">ReRAM</a></li>

<li><a href="Fe_FET" title="Fe FET">FeFET memory</a></li></ul></div></div></td>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)"><a href="Analog_recording" title="Analog recording">Analog recording</a></div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Phonograph_cylinder" title="Phonograph cylinder">Phonograph cylinder</a></li>

<li><a href="Phonograph_record" title="Phonograph record">Phonograph record</a></li>

<li><a href="Quadruplex_videotape" title="Quadruplex videotape">Quadruplex videotape</a></li>

<li><a href="Vision_Electronic_Recording_Apparatus" title="Vision Electronic Recording Apparatus">Vision Electronic Recording Apparatus</a></li>

<li><a href="Magnetic_recording" class="mw-redirect" title="Magnetic recording">Magnetic recording</a>

<ul><li><a href="Magnetic_storage" title="Magnetic storage">Magnetic storage</a></li>

<li><a href="Magnetic_tape" title="Magnetic tape">Magnetic tape</a></li>

<li><a href="Magnetic-tape_data_storage" title="Magnetic-tape data storage">Magnetic-tape data storage</a></li>

<li><a href="Tape_drive" title="Tape drive">Tape drive</a></li>

<li><a href="Tape_library" title="Tape library">Tape library</a></li>

<li><a href="Digital_Data_Storage" title="Digital Data Storage">Digital Data Storage</a> (DDS)</li>

<li><a href="Videotape" title="Videotape">Videotape</a></li>

<li><a href="Cassette_tape" title="Cassette tape">Cassette tape</a></li>

<li><a href="Linear_Tape-Open" title="Linear Tape-Open">Linear Tape-Open</a></li>

<li><a href="Betamax" title="Betamax">Betamax</a></li>

<li><a href="8_mm_video_format" title="8 mm video format">8 mm video format</a></li>

<li><a href="MiniDV" class="mw-redirect" title="MiniDV">MiniDV</a></li>

<li><a href="MicroMV" title="MicroMV">MicroMV</a></li>

<li><a href="U-matic" title="U-matic">U-matic</a></li>

<li><a href="Hard_disk_drive" title="Hard disk drive">Hard disk drive</a></li></ul></div></div></td>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)"><a href="Optical_storage" title="Optical storage">Optical</a></div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="3D_optical_data_storage" title="3D optical data storage">3D optical data storage</a>

<ul><li><a href="Optical_disc" title="Optical disc">Optical disc</a></li>

<li><a href="LaserDisc" title="LaserDisc">LaserDisc</a></li>

<li><a href="Compact_Disc_Digital_Audio" title="Compact Disc Digital Audio">Compact Disc Digital Audio</a> (CDDA)</li>

<li><a href="CD_Video" title="CD Video">CD Video</a></li>

<li><a href="Video_CD" title="Video CD">Video CD</a></li>

<li><a href="Super_Video_CD" title="Super Video CD">Super Video CD</a></li>

<li><a href="Nintendo_optical_discs" title="Nintendo optical discs">Nintendo optical discs</a></li>

<li><a href="Hyper_CD-ROM" title="Hyper CD-ROM">Hyper CD-ROM</a></li>

<li><a href="DVD-Video" title="DVD-Video">DVD-Video</a></li>

<li><a href="MiniDVD" title="MiniDVD">MiniDVD</a></li>

<li><a href="Ultra_HD_Blu-ray" title="Ultra HD Blu-ray">Ultra HD Blu-ray</a></li>

<li><a href="Holographic_Versatile_Disc" title="Holographic Versatile Disc">Holographic Versatile Disc</a></li></ul></li>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">In development</div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Programmable_metallization_cell" title="Programmable metallization cell">CBRAM</a></li>

<li><a href="Racetrack_memory" title="Racetrack memory">Racetrack memory</a></li>

<li><a href="Millipede_memory" title="Millipede memory">Millipede memory</a></li>

<li><a href="Electrochemical_RAM" title="Electrochemical RAM">ECRAM</a></li>

<li><a href="Patterned_media" title="Patterned media">Patterned media</a></li>

<li><a href="Holographic_data_storage" title="Holographic data storage">Holographic data storage</a>

<ul><li><a href="Electronic_quantum_holography" title="Electronic quantum holography">Electronic quantum holography</a></li></ul></li>

<li><a href="5D_optical_data_storage" title="5D optical data storage">5D optical data storage</a></li>

<li><a href="DNA_digital_data_storage" title="DNA digital data storage">DNA digital data storage</a></li>

<li><a href="Universal_memory" title="Universal memory">Universal memory</a></li>

<li><a href="Time_crystal" title="Time crystal">Time crystal</a></li>

<li><a href="Quantum_memory" title="Quantum memory">Quantum memory</a></li>

<li><a href="UltraRAM" title="UltraRAM">UltraRAM</a></li></ul></div></div></td>

</tr><tr><td class="sidebar-content">

<div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="color: var(--color-base)">Historical</div><div class="sidebar-list-content mw-collapsible-content">

<ul><li><a href="Paper_data_storage" title="Paper data storage">Paper data storage</a> (1725)</li>

<li><a href="Punched_card" title="Punched card">Punched card</a> (1725)</li>

<li><a href="Punched_tape" title="Punched tape">Punched tape</a> (1725)</li>

<li><a href="Plugboard" title="Plugboard">Plugboard</a></li>

<li><a href="Drum_memory" title="Drum memory">Drum memory</a> (1932)</li>

<li><a href="Magnetic-core_memory" title="Magnetic-core memory">Magnetic-core memory</a> (1949)</li>

<li><a href="Plated-wire_memory" title="Plated-wire memory">Plated-wire memory</a> (1957)</li>

<li><a href="Core_rope_memory" title="Core rope memory">Core rope memory</a> (1960s)</li>

<li><a href="Thin-film_memory" title="Thin-film memory">Thin-film memory</a> (1962)</li>

<li><a href="Twistor_memory" title="Twistor memory">Twistor memory</a> (~1968)</li>

<li><a href="Bubble_memory" title="Bubble memory">Bubble memory</a> (~1970)</li>

<li><a href="Floppy_disk" title="Floppy disk">Floppy disk</a> (1971)</li></ul></div></div></td>

</tr><tr><td class="sidebar-navbar"><style data-mw-deduplicate="TemplateStyles:r1239400231">

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Read-only memory (ROM) is a type of <a href="Non-volatile_memory" title="Non-volatile memory">non-volatile memory</a> used in <a href="Computers" class="mw-redirect" title="Computers">computers</a> and other <a href="Electronic_devices" class="mw-redirect" title="Electronic devices">electronic devices</a>. Data stored in ROM cannot be electronically modified after the manufacture of the <a href="Memory_device" class="mw-redirect" title="Memory device">memory device</a>. Read-only memory is useful for storing <a href="Software" title="Software">software</a> that is rarely changed during the life of the system, also known as <a href="Firmware" title="Firmware">firmware</a>. Software applications, such as <a href="Video_game" title="Video game">video games</a>, for programmable devices can be distributed as <a href="ROM_cartridge" title="ROM cartridge">plug-in cartridges containing ROM</a>.

Strictly speaking, read-only memory refers to hard-wired memory, such as <a href="Diode_matrix" title="Diode matrix">diode matrix</a> or a <a href="#Solid-state_ROM">mask ROM</a> <a href="Integrated_circuit" title="Integrated circuit">integrated circuit</a> (IC), that cannot be electronically changed after manufacture. Although discrete circuits can be altered in principle, through the addition of <a href="Jump_wire" title="Jump wire">bodge wires</a> and the removal or replacement of components, ICs cannot. Correction of errors, or updates to the software, require new devices to be manufactured and to replace the installed device.

<a href="Floating-gate" class="mw-redirect" title="Floating-gate">Floating-gate</a> ROM <a href="Semiconductor_memory" title="Semiconductor memory">semiconductor memory</a> in the form of <a href="Erasable_programmable_read-only_memory" class="mw-redirect" title="Erasable programmable read-only memory">erasable programmable read-only memory</a> (EPROM), <a href="Electrically_erasable_programmable_read-only_memory" class="mw-redirect" title="Electrically erasable programmable read-only memory">electrically erasable programmable read-only memory</a> (EEPROM) and <a href="Flash_memory" title="Flash memory">flash memory</a> can be erased and re-programmed. But usually, this can only be done at relatively slow speeds, may require special equipment to achieve, and is typically only possible a certain number of times.<a href="#cite_note-1">[1]</a>

The term "ROM" is sometimes used to refer to a ROM device containing specific software or a file with software to be stored in a writable ROM device. For example, users modifying or replacing the <a href="Android_operating_system" class="mw-redirect" title="Android operating system">Android operating system</a> describe files containing a modified or replacement operating system as "<a href="Custom_ROM" class="mw-redirect" title="Custom ROM">custom ROMs</a>" after the type of storage the file used to be written to, and they may distinguish between ROM (where software and data is stored, usually <a href="Flash_memory" title="Flash memory">Flash memory</a>) and RAM.

ROM and RAM are essential components of a computer, each serving distinct roles. RAM, or Random Access Memory, is a temporary, volatile storage medium that loses data when the system powers down. In contrast, ROM, being non-volatile, preserves its data even after the computer is switched off.<a href="#cite_note-2">[2]</a>

<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Discrete-component_ROM">Discrete-component ROM</h3></div>

IBM used capacitor read-only storage (CROS) and <a href="Transformer_read-only_storage" title="Transformer read-only storage">transformer read-only storage</a> (TROS) to store microcode for the smaller <a href="IBM_System/360" title="IBM System/360">System/360</a> models, the <a href="IBM_System/360_Model_85" title="IBM System/360 Model 85">360/85</a>, and the initial two <a href="IBM_System/370" title="IBM System/370">System/370</a> models (<a href="IBM_System/370_Model_155" title="IBM System/370 Model 155">370/155</a> and <a href="IBM_System/370_Model_165" title="IBM System/370 Model 165">370/165</a>). On some models there was also a <a href="Control_store#Writable_stores" title="Control store">writeable control store</a> (WCS) for additional diagnostics and emulation support. The <a href="Apollo_Guidance_Computer" title="Apollo Guidance Computer">Apollo Guidance Computer</a> used <a href="Core_rope_memory" title="Core rope memory">core rope memory</a>, programmed by threading wires through magnetic cores.

<div class="mw-heading mw-heading3"><h3 id="Solid-state_ROM">Solid-state ROM</h3></div>

<div role="note" class="hatnote navigation-not-searchable">See also: <a href="Semiconductor_memory" title="Semiconductor memory">Semiconductor memory</a></div>

The simplest type of <a href="Solid-state_electronics" title="Solid-state electronics">solid-state</a> ROM is as old as the <a href="Semiconductor" title="Semiconductor">semiconductor</a> technology itself. <a href="Combinational_logic" title="Combinational logic">Combinational</a> <a href="Logic_gate" title="Logic gate">logic gates</a> can be joined manually to map n-bit address input onto arbitrary values of m-bit data output (a <a href="Look-up_table" class="mw-redirect" title="Look-up table">look-up table</a>). With the invention of the <a href="Integrated_circuit" title="Integrated circuit">integrated circuit</a> came <a href="Mask_ROM" class="mw-redirect" title="Mask ROM">mask ROM</a>. Mask ROM consists of a grid of <a href="Word_(data_type)" class="mw-redirect" title="Word (data type)">word</a> lines (the address input) and bit lines (the data output), selectively joined with <a href="Transistor" title="Transistor">transistor</a> switches, and can represent an arbitrary look-up table with a regular physical layout and predictable <a href="Propagation_delay" title="Propagation delay">propagation delay</a>. Mask ROM is programmed with <a href="Photomask" title="Photomask">photomasks</a> in <a href="Photolithography" title="Photolithography">photolithography</a> during <a href="Semiconductor_manufacturing" class="mw-redirect" title="Semiconductor manufacturing">semiconductor manufacturing</a>. The mask defines physical features or structures that will be removed, or added in the ROM chips, and the presence or absence of these features will represent either a 1 or a 0 bit, depending on the ROM design.<a href="#cite_note-umich-3">[3]</a> Thus by design, any attempts to electronically change the data will fail, since the data is defined by the presence or absence of physical features or structures that cannot be electronically changed. For every software program, even for revisions of the same program, the entire mask must be changed, which can be costly.

In mask ROM, the data is physically encoded in the circuit, so it can only be programmed during fabrication. This leads to a number of serious disadvantages:

<ul><li>It is only economical to buy mask ROM in large quantities, since users must contract with a <a href="Foundry_(electronics)" class="mw-redirect" title="Foundry (electronics)">foundry</a> to produce a custom design for every piece, or revision of software.</li>

<li>The turnaround time between completing the design for a mask ROM and receiving the finished product is long, for the same reason.</li>

<li>Mask ROM is impractical for <a href="R%26D" class="mw-redirect" title="R&D">R&D</a> work since designers frequently need to quickly modify the contents of memory as they refine a design.</li>

<li>If a product is shipped with faulty mask ROM, the only way to fix it is to <a href="Product_recall" title="Product recall">recall</a> the product and physically replace the ROM in every unit shipped. This has happened in the real world with a faulty <a href="Carbon_monoxide_detector" title="Carbon monoxide detector">carbon monoxide detector</a>.<a href="#cite_note-4">[4]</a></li></ul>

Subsequent developments have addressed these shortcomings. <a href="Programmable_read-only_memory" class="mw-redirect" title="Programmable read-only memory">Programmable read-only memory</a> (PROM), invented by <a href="Wen_Tsing_Chow" title="Wen Tsing Chow">Wen Tsing Chow</a> in 1956,<a href="#cite_note-Huang2008-5">[5]</a><a href="#cite_note-AufaureZimányi2013-6">[6]</a> allowed users to program its contents exactly once by physically altering its structure with the application of high-voltage pulses. This addressed problems 1 and 2 above, since a company can simply order a large batch of fresh PROM chips and program them with the desired contents at its designers' convenience.

The advent of the <a href="Metal%E2%80%93oxide%E2%80%93semiconductor_field-effect_transistor" class="mw-redirect" title="Metal–oxide–semiconductor field-effect transistor">metal–oxide–semiconductor field-effect transistor</a> (MOSFET), invented at <a href="Bell_Labs" title="Bell Labs">Bell Labs</a> in 1959,<a href="#cite_note-computerhistory-7">[7]</a> enabled the practical use of <a href="Metal%E2%80%93oxide%E2%80%93semiconductor" class="mw-redirect" title="Metal–oxide–semiconductor">metal–oxide–semiconductor</a> (MOS) transistors as <a href="Memory_cell_(computing)" title="Memory cell (computing)">memory cell</a> storage elements in <a href="Semiconductor_memory" title="Semiconductor memory">semiconductor memory</a>, a function previously served by <a href="Magnetic-core_memory" title="Magnetic-core memory">magnetic cores</a> in <a href="Computer_memory" title="Computer memory">computer memory</a>. In 1967, <a href="Dawon_Kahng" title="Dawon Kahng">Dawon Kahng</a> and <a href="Simon_Sze" title="Simon Sze">Simon Sze</a> of Bell Labs proposed that the <a href="Floating_gate" class="mw-redirect" title="Floating gate">floating gate</a> of a MOS <a href="Semiconductor_device" title="Semiconductor device">semiconductor device</a> could be used for the cell of a reprogrammable ROM, which led to <a href="Dov_Frohman" title="Dov Frohman">Dov Frohman</a> of <a href="Intel" title="Intel">Intel</a> inventing <a href="Erasable_programmable_read-only_memory" class="mw-redirect" title="Erasable programmable read-only memory">erasable programmable read-only memory</a> (EPROM) in 1971.<a href="#cite_note-8">[8]</a><a href="#cite_note-computerhistory1971-9">[9]</a> The 1971 invention of EPROM essentially solved problem 3, since EPROM (unlike PROM) can be repeatedly reset to its unprogrammed state by exposure to strong ultraviolet light.

<a href="Electrically_erasable_programmable_read-only_memory" class="mw-redirect" title="Electrically erasable programmable read-only memory">Electrically erasable programmable read-only memory</a> (EEPROM), developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at the <a href="Electrotechnical_Laboratory" class="mw-redirect" title="Electrotechnical Laboratory">Electrotechnical Laboratory</a> in 1972,<a href="#cite_note-10">[10]</a> went a long way to solving problem 4, since an EEPROM can be programmed <a href="In-place_programmable" class="mw-redirect" title="In-place programmable">in-place</a> if the containing device provides a means to receive the program contents from an external source (for example, a personal computer via a <a href="Serial_cable" title="Serial cable">serial cable</a>). <a href="Flash_memory" title="Flash memory">Flash memory</a>, invented by <a href="Fujio_Masuoka" title="Fujio Masuoka">Fujio Masuoka</a> at <a href="Toshiba" title="Toshiba">Toshiba</a> in the early 1980s and commercialized in the late 1980s, is a form of EEPROM that makes very efficient use of chip area and can be erased and reprogrammed thousands of times without damage. It permits erasure and programming of only a specific part of the device, instead of the entire device. This can be done at high speed, hence the name "flash".<a href="#cite_note-11">[11]</a><a href="#cite_note-12">[12]</a>

All of these technologies improved the flexibility of ROM, but at a significant cost-per-chip, so that in large quantities mask ROM would remain an economical choice for many years. (Decreasing cost of reprogrammable devices had almost eliminated the market for mask ROM by the year 2000.) Rewriteable technologies were envisioned as replacements for mask ROM.

The most recent development is <a href="NAND_flash" class="mw-redirect" title="NAND flash">NAND flash</a>, also invented at Toshiba. Its designers explicitly broke from past practice, stating plainly that "the aim of NAND flash is to replace <a href="Hard_disk" class="mw-redirect" title="Hard disk">hard disks</a>,"<a href="#cite_note-13">[13]</a> rather than the traditional use of ROM as a form of non-volatile <a href="Primary_storage" class="mw-redirect" title="Primary storage">primary storage</a>. As of 2021, NAND has nearly completely achieved this goal by offering throughput higher than hard disks, lower latency, higher tolerance of physical shock, extreme miniaturization (in the form of <a href="USB_flash_drive" title="USB flash drive">USB flash drives</a> and tiny <a href="MicroSD" class="mw-redirect" title="MicroSD">microSD</a> <a href="Memory_card" title="Memory card">memory cards</a>, for example), and much lower power consumption.

<div class="mw-heading mw-heading3"><h3 id="Use_for_storing_programs">Use for storing programs</h3></div>

Many <a href="Stored-program_computer" title="Stored-program computer">stored-program computers</a> use a form of <a href="Non-volatile_memory" title="Non-volatile memory">non-volatile storage</a> (that is, storage that retains its data when power is removed) to store the initial program that runs when the computer is powered on or otherwise begins execution (a process known<a href="#cite_note-14">[a]</a> as <a href="Bootstrapping_(computing)" class="mw-redirect" title="Bootstrapping (computing)">bootstrapping</a>, often abbreviated to "<a href="Booting" title="Booting">booting</a>" or "booting up"). Likewise, every non-trivial computer needs some form of mutable memory to record changes in its <a href="State_(computer_science)" title="State (computer science)">state</a> as it executes.

Forms of read-only memory were employed as non-volatile storage for programs in most early stored-program computers, such as <a href="ENIAC" title="ENIAC">ENIAC</a> <a href="ENIAC#Improvements" title="ENIAC">after 1948</a>. (Until then it was not a stored-program computer as every program had to be manually wired into the machine, which could take days to weeks.) Read-only memory was simpler to implement since it needed only a mechanism to read stored values, and not to change them in-place, and thus could be implemented with very crude electromechanical devices (see <a href="#Historical_examples">historical examples</a> below). With the advent of <a href="Integrated_circuit" title="Integrated circuit">integrated circuits</a> in the 1960s, both ROM and its mutable counterpart <a href="Static_RAM" class="mw-redirect" title="Static RAM">static RAM</a> were implemented as arrays of <a href="Transistor" title="Transistor">transistors</a> in silicon chips; however, a ROM memory cell could be implemented using fewer transistors than an SRAM memory cell, since the latter needs a <a href="Latch_(electronics)" class="mw-redirect" title="Latch (electronics)">latch</a> (comprising 5-20 transistors) to retain its contents, while a ROM cell might consist of the absence (logical 0) or presence (logical 1) of one transistor connecting a bit line to a word line.<a href="#cite_note-15">[14]</a> Consequently, ROM could be implemented at a lower cost-per-<a href="Bit" title="Bit">bit</a> than RAM for many years.

Most <a href="Home_computer" title="Home computer">home computers</a> of the 1980s stored a <a href="BASIC" title="BASIC">BASIC</a> interpreter or <a href="Operating_system" title="Operating system">operating system</a> in ROM as other forms of non-volatile storage such as <a href="Magnetic_disk" class="mw-redirect" title="Magnetic disk">magnetic disk</a> drives were too costly. For example, the <a href="Commodore_64" title="Commodore 64">Commodore 64</a> included 64 <a href="Kilobyte" title="Kilobyte">KB</a> of RAM and 20 KB of ROM containing a BASIC interpreter and the <a href="KERNAL" title="KERNAL">KERNAL</a> operating system. Later home or office computers such as the <a href="IBM" title="IBM">IBM</a> <a href="PC_XT" class="mw-redirect" title="PC XT">PC XT</a> often included magnetic disk drives, and larger amounts of RAM, allowing them to load their operating systems from disk into RAM, with only a minimal hardware initialization core and <a href="Bootloader" title="Bootloader">bootloader</a> remaining in ROM (known as the <a href="BIOS" title="BIOS">BIOS</a> in <a href="IBM-compatible" class="mw-redirect" title="IBM-compatible">IBM-compatible</a> computers). This arrangement allowed for a more complex and easily upgradeable operating system.

In modern PCs, "ROM" is used to store the basic bootstrapping <a href="Firmware" title="Firmware">firmware</a> for the processor, as well as the various <a href="Firmware" title="Firmware">firmware</a> needed to internally control self-contained devices such as <a href="Graphic_cards" class="mw-redirect" title="Graphic cards">graphic cards</a>, <a href="Hard_disk_drive" title="Hard disk drive">hard disk drives</a>, <a href="Solid-state_drive" title="Solid-state drive">solid-state drives</a>, <a href="Optical_disc_drive" title="Optical disc drive">optical disc drives</a>, <a href="TFT_screen" class="mw-redirect" title="TFT screen">TFT screens</a>, etc., in the system. Today, many of these "read-only" memories – especially the <a href="BIOS" title="BIOS">BIOS</a>/<a href="UEFI" title="UEFI">UEFI</a> – are often replaced with <a href="EEPROM" title="EEPROM">EEPROM</a> or <a href="Flash_memory" title="Flash memory">Flash memory</a> (see below), to permit in-place reprogramming should the need for a firmware upgrade arise. However, simple and mature sub-systems (such as the keyboard or some communication controllers in the integrated circuits on the main board, for example) may employ mask ROM or <a href="Programmable_read-only_memory" class="mw-redirect" title="Programmable read-only memory">OTP</a> (one-time programmable).

ROM and <a href="#Types">successor technologies</a> such as flash are prevalent in <a href="Embedded_system" title="Embedded system">embedded systems</a>. These are in everything from <a href="Industrial_robots" class="mw-redirect" title="Industrial robots">industrial robots</a> to <a href="Home_appliances" class="mw-redirect" title="Home appliances">home appliances</a> and <a href="Consumer_electronics" title="Consumer electronics">consumer electronics</a> (<a href="MP3_player" class="mw-redirect" title="MP3 player">MP3 players</a>, <a href="Set-top_box" title="Set-top box">set-top boxes</a>, etc.) all of which are designed for specific functions, but are based on general-purpose <a href="Microprocessor" title="Microprocessor">microprocessors</a>. With software usually tightly coupled to hardware, program changes are rarely needed in such devices (which typically lack hard disks for reasons of cost, size, or power consumption). As of 2008, most products use Flash rather than mask ROM, and many provide some means for connecting to a PC for <a href="Firmware" title="Firmware">firmware</a> updates; for example, a digital audio player might be updated to support a new <a href="File_format" title="File format">file format</a>. Some hobbyists have taken advantage of this flexibility to reprogram consumer products for new purposes; for example, the <a href="IPodLinux" title="IPodLinux">iPodLinux</a> and <a href="OpenWrt" title="OpenWrt">OpenWrt</a> projects have enabled users to run full-featured <a href="Linux" title="Linux">Linux</a> <a href="Linux_distribution" title="Linux distribution">distributions</a> on their MP3 players and wireless routers, respectively.

ROM is also useful for binary storage of <a href="Cryptographic" class="mw-redirect" title="Cryptographic">cryptographic</a> data, as it makes them difficult to replace, which may be desirable in order to enhance <a href="Information_security" title="Information security">information security</a>.

<div class="mw-heading mw-heading3"><h3 id="Use_for_storing_data">Use for storing data</h3></div>

Since ROM (at least in hard-wired mask form) cannot be modified, it is only suitable for storing data which is not expected to need modification for the life of the device. To that end, ROM has been used in many computers to store <a href="Look-up_table" class="mw-redirect" title="Look-up table">look-up tables</a> for the evaluation of mathematical and logical functions (for example, a <a href="Floating-point_unit" title="Floating-point unit">floating-point unit</a> might <a href="Look-up_table" class="mw-redirect" title="Look-up table">tabulate the sine function</a> in order to facilitate faster computation). This was especially effective when <a href="CPU" class="mw-redirect" title="CPU">CPUs</a> were slow and ROM was cheap compared to RAM.

Notably, the <a href="Display_adapter" class="mw-redirect" title="Display adapter">display adapters</a> of early personal computers stored tables of bitmapped font characters in ROM. This usually meant that the text display <a href="Font" title="Font">font</a> could not be changed interactively. This was the case for both the <a href="Color_Graphics_Adapter" title="Color Graphics Adapter">CGA</a> and <a href="Monochrome_Display_Adapter" class="mw-redirect" title="Monochrome Display Adapter">MDA</a> adapters available with the IBM PC (type 5150).<a href="#cite_note-16">[15]</a>

The use of ROM to store such small amounts of data has disappeared almost completely in modern general-purpose computers. However, <a href="NAND_Flash" class="mw-redirect" title="NAND Flash">NAND Flash</a> has taken over a new role as a medium for <a href="Mass_storage" title="Mass storage">mass storage</a> or <a href="Secondary_storage" class="mw-redirect" title="Secondary storage">secondary storage</a> of files.

<div class="mw-heading mw-heading2"><h2 id="Types">Types</h2></div>

<table class="sidebar sidebar-collapse nomobile nowraplinks hlist"><tbody><tr><th class="sidebar-title"><a href="Computer_memory" title="Computer memory">Computer memory</a> and <a href="Computer_data_storage" title="Computer data storage">data storage</a> types</th></tr><tr><td class="sidebar-content">