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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Special function register</span></span>
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<p>
A <b>special function register</b> (SFR) is a <a href="Processor_register" title="Processor register">register</a> within a <a href="Microcontroller" title="Microcontroller">microcontroller</a> that controls or monitors various aspects of the microcontroller's function. Depending on the <a href="Comparison_of_CPU_architectures" class="mw-redirect" title="Comparison of CPU architectures">processor architecture</a>, this can include, but is not limited to:
</p>
<ul><li><a href="Input/output" title="Input/output">I/O</a> and peripheral control (such as serial ports or general-purpose IOs)</li>
<li>timers</li>
<li><a href="Stack_pointer" class="mw-redirect" title="Stack pointer">stack pointer</a></li>
<li>stack limit (to prevent overflows)</li>
<li><a href="Program_counter" title="Program counter">program counter</a></li>
<li><a href="Subroutine" class="mw-redirect" title="Subroutine">subroutine</a> <a href="Return_statement" title="Return statement">return address</a></li>
<li>processor status (servicing an interrupt, running in protected mode, etc.)</li>
<li><a href="Condition_code_register" class="mw-redirect" title="Condition code register">condition codes</a> (result of previous comparisons)</li></ul>
<p>Because special registers are closely tied to some special function or status of the microcontroller, they might not be directly <i>writeable</i> by normal instructions (such as adds, moves, etc.). Instead, some special registers in some microcontroller architectures require special instructions to modify them. For example, the program counter is not directly writeable in many microcontroller architectures. Instead, the programmer uses instructions such as return from subroutine, jump, or branch to modify the program counter. For another example, the condition code register might not be directly writable, instead being updated only by compare instructions.
</p>
<div class="mw-heading mw-heading2"><h2 id="Intel_microcontrollers">Intel microcontrollers</h2></div>
<p>Special function registers are in the upper area of addressable memory, from address 0x80 to 0xFF. This area of memory cannot be used for data or program storage, but is instead a series of memory-mapped ports and registers. All port input and output can therefore be performed by memory move operations on specified addresses in the SFR region. Also, different status registers are mapped into the SFR, for use in checking the status of the 8051, and changing some operational parameters of the 8051.
</p><p>Some SFR bits may be set directly using SETB/LDB instructions on the SFR's address, whereas others may require usage of specific instructions. The <a href="Intel_MCS-96" title="Intel MCS-96">Intel 80196</a> class microcontroller has 24 SFRs, each 1 byte in size; standard <a href="Intel_8051" class="mw-redirect" title="Intel 8051">Intel 8051</a> chips have 21 SFRs.
</p>
<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>
<ul><li><a rel="nofollow" class="external text" href="http://www.hobbyprojects.com/8051_tutorial/special_function_registers.html">i8051 SFRs</a></li>
<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20140129054513/http://www.elec.canterbury.ac.nz:80/PublicArea/Staff/hof/p10-embed/p10-tutorial/p09.html">SPECIAL FUNCTION REGISTERS 1/2</a> at the <a href="Wayback_Machine" title="Wayback Machine">Wayback Machine</a> (archived 2014-01-29)</li>
<li><a rel="nofollow" class="external text" href="http://www.nongnu.org/avr-libc/user-manual/group__avr__sfr.html">SFRs in C programming for AVR</a></li></ul>
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<ul><li><a href="Single-board_microcontroller" title="Single-board microcontroller">Single-board microcontroller</a></li>
</ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Microarchitecture" title="Microarchitecture">Architectures</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="Motorola_68000_series" title="Motorola 68000 series">68000</a></li>
<li><a href="Intel_8051" class="mw-redirect" title="Intel 8051">8051</a></li>
<li><a href="ARC_(processor)" title="ARC (processor)">ARC</a></li>
<li><a href="ARM_architecture_family" title="ARM architecture family">ARM</a></li>
<li><a href="AVR_microcontrollers" title="AVR microcontrollers">AVR</a></li>
<li><a href="MIPS_architecture" title="MIPS architecture">MIPS</a></li>
<li><a href="TI_MSP430" title="TI MSP430">MPS430</a></li>
<li><a href="PIC_microcontrollers" title="PIC microcontrollers">PIC</a></li>
<li><a href="RISC-V" title="RISC-V">RISC-V</a></li>
<li><a href="X86" title="X86">x86</a></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Word_(computer_architecture)" title="Word (computer architecture)">Word length</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%"><a href="4-bit_computing" title="4-bit computing">4-bit</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="AMD_Am2900" title="AMD Am2900">Am2900</a></li>
<li><a href="COP400" title="COP400">COP400</a></li>
<li>MARC4</li>
<li><a href="Rockwell_PPS-4" title="Rockwell PPS-4">PPS-4</a></li>
<li><a href="S1C6x" title="S1C6x">S1C6x</a></li>
<li><a href="Toshiba_TLCS" title="Toshiba TLCS">TLCS-47</a></li>
<li><a href="Texas_Instruments_TMS1000" title="Texas Instruments TMS1000">TMS1000</a></li>
<li><a href="%CE%9CCOM-4" class="mw-redirect" title="ΜCOM-4">μCOM-4</a></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="8-bit_computing" title="8-bit computing">8-bit</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="Motorola_6800" title="Motorola 6800">6800</a>
<ul><li><a href="Motorola_68HC05" title="Motorola 68HC05">68HC05</a></li>
<li><a href="Motorola_68HC08" title="Motorola 68HC08">68HC08</a></li>
<li><a href="Motorola_68HC11" title="Motorola 68HC11">68HC11</a></li>
<li><a href="Freescale_S08" class="mw-redirect" title="Freescale S08">S08</a></li>
<li><a href="Freescale_RS08" title="Freescale RS08">RS08</a></li></ul></li>
<li><a href="MOS_Technology_6502" title="MOS Technology 6502">6502</a>
<ul><li><a href="WDC_65C134" title="WDC 65C134">65C134</a></li>
<li><a href="WDC_65C265" title="WDC 65C265">65C265</a></li>
<li><a href="Mitsubishi_740" title="Mitsubishi 740">MELPS 740</a></li></ul></li>
<li><a href="78K" title="78K">78K</a></li>
<li><a href="Intel_MCS-48" title="Intel MCS-48">8048</a></li>
<li><a href="Intel_8051" class="mw-redirect" title="Intel 8051">8051</a>
<ul><li><a href="XC800_family" title="XC800 family">XC800</a></li></ul></li>
<li><a href="AVR_microcontrollers" title="AVR microcontrollers">AVR</a></li>
<li><a href="COP8" title="COP8">COP8</a></li>
<li><a href="H8_Family" title="H8 Family">H8</a></li>
<li><a href="PIC_microcontroller" class="mw-redirect" title="PIC microcontroller">PIC10/12/16/17/18</a></li>
<li><a href="ST6_and_ST7" title="ST6 and ST7">ST6/ST7</a></li>
<li><a href="STM8" title="STM8">STM8</a></li>
<li><a href="Zilog_Z8" title="Zilog Z8">Z8</a></li>
<li><a href="Zilog_Z80" title="Zilog Z80">Z80</a>
<ul><li><a href="Zilog_eZ80" title="Zilog eZ80">eZ80</a></li>
<li><a href="Rabbit_2000" title="Rabbit 2000">Rabbit 2000</a></li>
<li><a href="Toshiba_TLCS" title="Toshiba TLCS">TLCS-870</a></li></ul></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="16-bit_computing" title="16-bit computing">16-bit</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="WDC_65C816" title="WDC 65C816">65C816</a></li>
<li><a href="Freescale_68HC12" class="mw-redirect" title="Freescale 68HC12">68HC12</a>/<a href="Freescale_68HC16" class="mw-redirect" title="Freescale 68HC16">16</a></li>
<li><a href="Intel_80186" title="Intel 80186">80186</a></li>
<li><a href="C166_family" title="C166 family">C166</a></li>
<li><a href="CompactRISC" title="CompactRISC">CR16/C</a></li>
<li><a href="H8_Family" title="H8 Family">H8S</a></li>
<li><a href="TI_MSP430" title="TI MSP430">MSP430</a></li>
<li><a href="PIC_microcontroller" class="mw-redirect" title="PIC microcontroller">PIC24/dsPIC</a></li>
<li><a href="R8C" title="R8C">R8C</a></li>
<li><a href="RL78" title="RL78">RL78</a></li>
<li><a href="Toshiba_TLCS" title="Toshiba TLCS">TLCS-900</a></li>
<li><a href="Zilog_Z8000" title="Zilog Z8000">Z8000</a></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="32-bit_computing" title="32-bit computing">32-bit</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="AMD_Am29000" title="AMD Am29000">Am29000</a></li>
<li><a href="ARC_(processor)" title="ARC (processor)">ARC</a></li>
<li><a href="List_of_applications_of_ARM_cores" class="mw-redirect" title="List of applications of ARM cores">ARM</a> <a href="ARM_Cortex-M" title="ARM Cortex-M">Cortex-M</a>
<ul><li><a href="EFM32" title="EFM32">EFM32</a></li>
<li><a href="NXP_LPC" title="NXP LPC">LPC</a></li>
<li><a href="Atmel_ARM-based_processors" title="Atmel ARM-based processors">SAM</a></li>
<li><a href="STM32" title="STM32">STM32</a></li>
<li><a href="Infineon_XMC" title="Infineon XMC">XMC</a></li></ul></li>
<li><a href="ARM_Cortex-R" title="ARM Cortex-R">ARM Cortex-R</a></li>
<li><a href="AVR32" title="AVR32">AVR32</a></li>
<li><a href="CompactRISC" title="CompactRISC">CRX</a></li>
<li><a href="Fujitsu_FR" title="Fujitsu FR">FR</a></li>
<li><a href="FR-V_(microprocessor)" title="FR-V (microprocessor)">FR-V</a></li>
<li><a href="H8_Family" title="H8 Family">H8SX</a></li>
<li><a href="M32R" title="M32R">M32R</a></li>
<li><a href="MN103" title="MN103">MN103</a></li>
<li><a href="Motorola_68000" title="Motorola 68000">68000</a>
<ul><li><a href="NXP_ColdFire" title="NXP ColdFire">ColdFire</a></li></ul></li>
<li><a href="PIC_microcontroller" class="mw-redirect" title="PIC microcontroller">PIC32</a></li>
<li><a href="PowerPC" title="PowerPC">PowerPC</a>
<ul><li><a href="MPC5xx" title="MPC5xx">MPC5xx</a></li></ul></li>
<li><a href="Parallax_Propeller" title="Parallax Propeller">Propeller</a></li>
<li><a href="SuperH" title="SuperH">SuperH</a></li>
<li><a href="Toshiba_TLCS" title="Toshiba TLCS">TLCS-900</a></li>
<li><a href="Infineon_TriCore" title="Infineon TriCore">TriCore</a></li>
<li><a href="V850" title="V850">V850</a></li>
<li><a href="RX_microcontroller_family" class="mw-redirect" title="RX microcontroller family">RX</a></li>
<li><a href="Tensilica" title="Tensilica">Xtensa</a></li>
<li><a href="Zilog_Z80000" title="Zilog Z80000">Z80000</a></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="64-bit_computing" title="64-bit computing">64-bit</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="ARC_(processor)" title="ARC (processor)">ARC</a></li>
<li><a href="ARM_Cortex-R" title="ARM Cortex-R">ARM Cortex-R</a></li>
<li><a href="PowerPC#64-bit_PowerPC" title="PowerPC">PowerPC64</a></li></ul>
</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Interfaces</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Programming</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="In-system_programming" title="In-system programming">In-circuit serial programming</a> (ICSP)</li>
<li><a href="In-system_programming" title="In-system programming">In-system programming</a> (ISP)</li>
<li><a href="AVR_microcontrollers#PDI" title="AVR microcontrollers">Program and Debug Interface</a> (PDI)</li>
<li><a href="AVR_microcontrollers#High_voltage_serial" title="AVR microcontrollers">High-voltage serial programming</a> (HVSP)</li>
<li><a href="AVR_microcontrollers#High_voltage_parallel" title="AVR microcontrollers">High voltage parallel programming</a> (HVPP)</li>
<li><a href="AVR_microcontrollers#Bootloader" title="AVR microcontrollers">Bootloader</a></li>
<li><a href="AVR_microcontrollers#ROM" title="AVR microcontrollers">ROM</a></li>
<li><a href="AVR_microcontrollers#aWire" title="AVR microcontrollers">aWire</a></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Debugging</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="Nexus_(standard)" title="Nexus (standard)">Nexus (standard)</a></li>
<li><a href="JTAG" title="JTAG">Joint Test Action Group</a> (JTAG)
<ul><li><a href="DebugWIRE" title="DebugWIRE">debugWIRE</a> (Atmel)</li></ul></li>
<li><a href="PIC_microcontroller" class="mw-redirect" title="PIC microcontroller">In-circuit debugging</a> (ICD)</li>
<li><a href="In-circuit_emulation" title="In-circuit emulation">In-circuit emulator</a> (ICE)</li>
<li><a href="In-target_probe" title="In-target probe">In-target probe</a> (ITP)</li></ul>
</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Lists</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="List_of_common_microcontrollers" title="List of common microcontrollers">List of common microcontrollers</a></li>
<li>By manufacturer
<ul><li><a href="Intel_microprocessor" class="mw-redirect" title="Intel microprocessor">Intel</a></li>
<li><a href="List_of_Freescale_products" class="mw-redirect" title="List of Freescale products">NXP/Freescale</a></li>
<li><a href="List_of_common_microcontrollers#Infineon" title="List of common microcontrollers">Infineon</a></li>
<li><a href="Renesas_Electronics#Products" title="Renesas Electronics">Renesas Electronics</a></li></ul></li>
<li><a href="List_of_Wi-Fi_microcontrollers" title="List of Wi-Fi microcontrollers">List of Wi-Fi microcontrollers</a></li></ul>
</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">
<ul><li><a href="Embedded_system" title="Embedded system">Embedded system</a></li>
<li><a href="Programmable_logic_controller" title="Programmable logic controller">Programmable logic controller</a></li>
<li><a href="List_of_microprocessors" title="List of microprocessors">List of microprocessors</a></li></ul>
</div></td></tr></tbody></table></div>
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